Silverado Alternator not putting out proper volts - Already changed the alternator...


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someday31
02-14-2010, 12:46 PM
I have a 2003 1500 Silverado 5.3. Since I have owned the truck the volt gauge would always be over 14 when running, and with a meter I would get about 14.77 volts. Now my gauge is under 14 and I get 14.10 out of the alternator. My battery now does not get fully charged.
I have replaced the alternator and even upgraded to the 150amp. I bought a 100% new not a rebuilt. I put a brand new battery in. I have added new grounds and cleaned the existing grounds.
The truck passes all the tests for the alternator, but the battery comes back "good battery low charge".... this is the same results before and after all the replaced parts mentioned. Its effecting my lights that I wound up buying brighter bulbs. Been to chevy dealer $103 later no answers....was told I "have a ghost" and to "live with it"! Their Tech 2 scanner showed the same results mentioned.
Please help...what do you guys get out of your alternator, where is your gauge...after the 14? Thanks in advance!

consultIII
02-14-2010, 01:52 PM
check for voltage drops across alternator grd and insulated circuits.

With vehicle running turn on all electriacal loads; put voltmeter put red lead on alternator output terminal and black lead on positive battery post.

test ground circuit by putting red lead on alt case and black lead on battery neg post.

should read not more than .1 volts per connection or more than 0.3v total. If detect voltage drop higher than this you have bad connection or high resistance in the circuit.

j cAT
02-14-2010, 07:38 PM
I have a 2003 1500 Silverado 5.3. Since I have owned the truck the volt gauge would always be over 14 when running, and with a meter I would get about 14.77 volts. Now my gauge is under 14 and I get 14.10 out of the alternator. My battery now does not get fully charged.
I have replaced the alternator and even upgraded to the 150amp. I bought a 100% new not a rebuilt. I put a brand new battery in. I have added new grounds and cleaned the existing grounds.
The truck passes all the tests for the alternator, but the battery comes back "good battery low charge".... this is the same results before and after all the replaced parts mentioned. Its effecting my lights that I wound up buying brighter bulbs. Been to chevy dealer $103 later no answers....was told I "have a ghost" and to "live with it"! Their Tech 2 scanner showed the same results mentioned.
Please help...what do you guys get out of your alternator, where is your gauge...after the 14? Thanks in advance!

measure the volts at the large red altenator wire to the battery ground terminal ..then measure the voltage across the battery terminals ...this should be the same...with the engine fully at operating temp the voltage should be 13.75 volts dc...this is the mid point voltage..

example if it where 10deg f air temp the voltage would be 15 volts...

example if the air temp were 110deg f the voltage would be 13.25 volts...

the colder the alternator the higher the voltage...

your problem is not an alternator issue...with your detailed desciption of what you have found so far...

I believe you have a leakage issue...or you are operating a aftermarket electrical device that draws more current than the alternator can produce...

with the vehicle off place an ammeter in series with the negative battery connection and measure the amperage with the vehicle off all doors closed ...this current after a few minutes should drop to 50MA - 200 milli amperes...thats .2AMPS at most..see how much leakage current you have ..

j cAT
02-14-2010, 07:41 PM
forgot to mention..... on my 2000 sil LT 4X4 the alternator amp output is 66amps with all lights on doors open a/c at maximum...

so I have about 40amps of reserve current...obviously if I were operating at 90-100amps with aftermarket devices my alternator would not last very long...

someday31
02-14-2010, 08:16 PM
Thanks for all the help thus far....I will try all the suggestions tomorrow and come back with the results.
I forgot to mention that I have done a parasitic draw test and initially drew 2.50 amps, but then within 30 seconds it dropped to under .15miliamps. So nothing is drawing on it with the key off. I know the problem is lack of output of the alternator. Just today I put a brand new battery in, 1000CA 825CCA, it load tested perfectly at the store....I hooked it up and my gauges still didn't go back to where they should be. I'm sure within a day or 2 the battery will fail the load test.
I will check the amps tomorrow. One place I took it to told me that I get 88 amps output with lights, blower on....so a little more than half of the 150 amp alt I have.
Oh yeah I have even changed the fusible link to no avail. I have no aftermarket items hooked up. I had added driving enhancement lights but have disconnected them, I added them AFTER this problem began with the charging issue, just to help see at night!
Do guys get 14.5+ volts out of your silverado alternator?

rhandwor
02-14-2010, 09:22 PM
Check all electrical connections at the alternator. Look at the plug for smaller wires. A loose connector could be your problem.
http://www.alldata.com/ Buy a years subscription cost around $29.05
You may have to check some items in the dash.
http://www.nationsautoelectric.com/regulators.html
They sell a self excited voltage regulator as a last case option.
I've had to replace both a bulb or the ammeter to get them to charge. This is why you need a wiring diagram.

j cAT
02-15-2010, 09:01 AM
Thanks for all the help thus far....I will try all the suggestions tomorrow and come back with the results.
I forgot to mention that I have done a parasitic draw test and initially drew 2.50 amps, but then within 30 seconds it dropped to under .15miliamps. So nothing is drawing on it with the key off. I know the problem is lack of output of the alternator. Just today I put a brand new battery in, 1000CA 825CCA, it load tested perfectly at the store....I hooked it up and my gauges still didn't go back to where they should be. I'm sure within a day or 2 the battery will fail the load test.
I will check the amps tomorrow. One place I took it to told me that I get 88 amps output with lights, blower on....so a little more than half of the 150 amp alt I have.
Oh yeah I have even changed the fusible link to no avail. I have no aftermarket items hooked up. I had added driving enhancement lights but have disconnected them, I added them AFTER this problem began with the charging issue, just to help see at night!
Do guys get 14.5+ volts out of your silverado alternator?

with this detailed info your problem is the fact your using the dash volt meter to indictate to you that you have a problem...

the dash guages on your year vehicle are defective many have had to be replaced ...the testing of your alternator current output although higher than I believe to be correct [but that could be because of the battery low on charge] is putting out good amperage...

with a meter [not the dash meter ] the voltage when the engine is hot should be 13.75 volts..not 14.75 volts....If the voltage were always that high you would damage the battery ..

If you have a problem it is the defective dash volt meter..

j cAT
02-15-2010, 09:09 AM
the only other reason for the battery to loose charge besides the leakage current being too high at rest is driving very short distances in cold weather with all electrical equiptment on..

when the battery is cold it cannot absorb the charging current it takes about 20 minutes for the battery to get warm enough to charge correctly..

THE INTERMITTENT INTERIOR LIGHTS OR OTHER EQUITMENT COMING ON , WILL CAUSE THIS BATTERY TO FAIL...ALSO.

when shutting down the vehicle disconnect the battery ground , then see if the battery holds charge...if it does you need to find out whats draining the battery...

someday31
02-15-2010, 09:26 PM
I have done all of the tests mentioned and they came back perfect. I am using a volt meter, not relying on my gauge. I am leaning towards a PCM issue. Does anyone know what the 3 wires in the alternator harness is? I know one is the PCM signal to the alternator, one is alternator output...not sure what that third wire is.
Also, I plan on unplugging the harness and then seeing if I get volts to the harness from the PCM..... I think its the red wire. I read somewhere that should only be 5 volts.
I had also wonder about applying 5 volts to that pin and then seeing what the alternator puts out?
What do you guys think?

j cAT
02-15-2010, 10:18 PM
I have done all of the tests mentioned and they came back perfect. I am using a volt meter, not relying on my gauge. I am leaning towards a PCM issue. Does anyone know what the 3 wires in the alternator harness is? I know one is the PCM signal to the alternator, one is alternator output...not sure what that third wire is.
Also, I plan on unplugging the harness and then seeing if I get volts to the harness from the PCM..... I think its the red wire. I read somewhere that should only be 5 volts.
I had also wonder about applying 5 volts to that pin and then seeing what the alternator puts out?
What do you guys think?

you are ignoring my input on this. I have clearly stated what I believe is your issue given your extensive test data...

I think you are not thinking correctly ,,,possibly OCD .

old_master
02-15-2010, 10:41 PM
Alternator output voltage and amperage is varied and controlled by the PCM.

Wiring at the alternator:

The red wire comes from the battery and through a fuse link and then to the output terminal of the alternator. It should show battery voltage at all times.

The dark blue wire goes to supplemental brake assist.

The brown wire goes to the PCM for the charge indicator control.

The grey wire comes from the PCM and goes to the voltage regulator inside the alternator. The PCM monitors voltage and load on the battery and determines what the charge voltage should be. It then sends a signal, using a duty cycle, to the voltage regulator. The greater the load on the battery, the longer the duty cycle.

"Old School".... ideal voltage, in a perfect world, was 14.2 volts. Now that the PCM is in the equation, "Old School" is out!

someday31
02-15-2010, 10:47 PM
I read your input and I am using a volt meter, not relying on my gauges. I have left the battery unhooked for the night and done parasitic draw tests as well. My issue isnt a battery drain, it is lack of output from my alternator. I use to always get 14.50+ out of my alternator and now it only puts out 14.10 maximum. The voltage regulator setpoint on these alternators is 14.80 when I look online. Plus I have measure other 5.3 silverados '03 and they put out over 14.50+ like mine use to do. This is why I am leaning towards the pcm.

someday31
02-15-2010, 11:02 PM
Can I start the truck without the harness hooked up and check voltage at the harness? If so what should my readings be?

someday31
02-16-2010, 05:32 PM
So I put a meter on the wire harness with the truck running, not connected to the alternator. I got 9.77 volts out of one of the terminals. I think thats my problem...the PCM is putting out too many volts? I thought I should be getting 5 volts out of it? I didnt get volts out of the other 2 wires on the harness, running or not......PLEASE DISCUSS.....

consultIII
02-16-2010, 10:30 PM
description of what you did is too vague. I'm looking at the charging system diagram so post wire colours and where you put your meter leads and voltages read for each wire.

Are you testing output at idle or 2500rpm?

Was the replacement battery a lead/acid battery, gel cell or absorbed glass matt battery all which have slightly different charging voltage requirements.

someday31
02-16-2010, 10:37 PM
I took my reading at idle, 500rpm. I hooked the negative wire of the meter to an alternator bolt for ground and the positive I touched to the middle prong I believe, I am under the assumption that that wire comes from the pcm?
I believe my battery was an acid battery - Duralast Gold 800CCA 1000CA 78 Size.
If the pcm is putting out too much voltage 9.77 versus the 5 that I think it should could that cause the volt regulator to not put out enough volts?
Thanks.

consultIII
02-17-2010, 12:02 AM
I'll check a similar truck tommorow.

This system should be charging the battery reguardless of 14.5 volts or 14.1 volts ...both are over 12.6 so it should be taking a charge. Stranger things have happened though.

Just to make sure we are not chasing zebra's...
You should be testing at 2500 rpm because ecm controlled alternators don't put out much at idle...and aren't designed to charge dead batteries but rather maintain the system...asking todays compact alternators to charge dead batteries can overheat the stator windings. I've seem it before when people don't drive enough or their driving habits don't allow the car to charge the batteries.

New batteries are not fully charged so charge it for 8 hours with a battery charger and see how long it goes.

someday31
02-17-2010, 08:37 AM
consultIII Could you also check your wire harness going into the alternator...how many volts do you get coming into the harness with the truck at idle? I want to see if me getting 9.77 volts is too high. Thanks!

someday31
02-17-2010, 08:59 PM
I took the harness off the alternator and then applied 12 volts to the voltage reg terminal that gets energized from the pcm. I still got 14.10 out of the alternator, so I think the pcm is not my problem. I get 55amps out of the alternator with the lights and blower on at idle and about 13.90volts. That seems low???

old_master
02-17-2010, 09:07 PM
Checking alternator output at idle tells you nothing. The engine needs to be at 2500RPM for the alternator to produce maximum output.

someday31
02-17-2010, 09:12 PM
What output should I expect at 2500rpm on a 150amp alternator?

old_master
02-17-2010, 09:18 PM
With all accessories turned off, battery fully charged, engine at 2,500 RPM, max alternator output should be approximately 130 to 140 amps, and voltage, ideally, should be at 14.2 volts.

someday31
02-17-2010, 09:26 PM
Ok...is there a way I can test this by myself, its electronic throttle :(

old_master
02-17-2010, 09:31 PM
If your ammeter is inductive, you don't need anyone else. Put the inductive probe on the alternator output, (red wire), apply B+ to the field, gas her up and watch the ammeter.

j cAT
02-18-2010, 08:55 AM
I took the harness off the alternator and then applied 12 volts to the voltage reg terminal that gets energized from the pcm. I still got 14.10 out of the alternator, so I think the pcm is not my problem. I get 55amps out of the alternator with the lights and blower on at idle and about 13.90volts. That seems low???


once again you display data that shows your charging system is running correctly...this is what I have observed on my vehicle with a fully charged battery ....volts and current are correct...


with your other questions on alternator output , the alternator will not output max current unless you place a load on the electrical system to force this to occur..

the pcm does not control alternator current.. this only monitors the alternator..the internal regulator controls the alternator current ..the voltage generated is also controlled by the internal regulator of the alternator..


your problem of the battery losing charge is , because at rest no engine on , the battery is being discharged..

someday31
02-18-2010, 03:02 PM
What do you guys think of this......

http://www.zzperformance.com/grand_prix/products1.php?id=517&catid=107 (http://www.zzperformance.com/grand_prix/products1.php?id=517&catid=107)

old_master
02-18-2010, 03:48 PM
I'd be skeptical... If charging voltage is higher than 14.2 volts, it will shorten the life of the light bulbs, the battery, any motor, including the fuel pump, and other electrical devices along with possibly setting a DTC. I trust the factory engineers more than I trust.... what was the name of the company?? But, it's your vehicle, and they sell it! Just my opinion... and some facts ;)

j cAT
02-18-2010, 05:21 PM
What do you guys think of this......

http://www.zzperformance.com/grand_prix/products1.php?id=517&catid=107 (http://www.zzperformance.com/grand_prix/products1.php?id=517&catid=107)


Only a fool would buy that crap ! like I said your charging system is working correctly...

you have a battery leakage issue , not a charging issue...find the componet that is sucking the power out of the battery when the engine is OFF .

someday31
02-18-2010, 08:02 PM
Only a fool would buy that crap ! like I said your charging system is working correctly...

you have a battery leakage issue , not a charging issue...find the componet that is sucking the power out of the battery when the engine is OFF .


I have done a parisitic draw test and I get 2 milliamps with the key off. Its not a draw issue based on this test.

consultIII
02-18-2010, 08:45 PM
here are the numbers someday31

tested a 2003 6.0l 2500hd. Has the same charging system wiring and controls.

checked voltages at alternator connector (backprobed terminals with connectors connected) with truck off, koeo and koer at idle; results as follows:

terminal c at alternator which is gry wire...goes to pcm... and is the off/run/crank volts field duty cycle signal

key off eng off 0v, koeo 2.074v, keor 3.98v


terminal b at alternator which is brown wire...goes to pcm... and is the serial data charge indicator control

key off eng off 0v, koeo 1.311v, koer 11.22v

of course forgot to check alt output amp and volts...but I know the charging system is good on this truck.

j cAT
02-18-2010, 09:44 PM
I have done a parisitic draw test and I get 2 milliamps with the key off. Its not a draw issue based on this test.

just because you for the moment your testing see this leakage of 2 MA does not mean it is constantly at this level...disconnect the battery when the engine is off and see if the battery holds full charge...

many have had these leakages occur randomly..

also as others have stated no charger / alternator will output more the 14 volts constantly because if so the battery will get cooked..

once the battery is at normal temp the charging voltage on a charged battery would be 13.75 max....

someday31
02-18-2010, 09:51 PM
here are the numbers someday31

tested a 2003 6.0l 2500hd. Has the same charging system wiring and controls.

checked voltages at alternator connector (backprobed terminals with connectors connected) with truck off, koeo and koer at idle; results as follows:

terminal c at alternator which is gry wire...goes to pcm... and is the off/run/crank volts field duty cycle signal

key off eng off 0v, koeo 2.074v, keor 3.98v


terminal b at alternator which is brown wire...goes to pcm... and is the serial data charge indicator control

key off eng off 0v, koeo 1.311v, koer 11.22v

of course forgot to check alt output amp and volts...but I know the charging system is good on this truck.

Thanks for all the readings...I will back probe my alternator hooked up and come back with readings. Can you tell me what volts you get out of the alternator and amps at idle with no lights/blower on?

consultIII
02-19-2010, 07:55 PM
Thanks for all the readings...I will back probe my alternator hooked up and come back with readings. Can you tell me what volts you get out of the alternator and amps at idle with no lights/blower on?

readings taken 1 min after start up, engine idling, no load other than dtrl on and fan blower fan at speed 1.

14.8v 43amps

what are your numbers?

someday31
02-19-2010, 10:33 PM
readings taken 1 min after start up, engine idling, no load other than dtrl on and fan blower fan at speed 1.

14.8v 43amps

what are your numbers?


Thanks...my volts use to be 14.77....now are 14.05-14.10 and amps are around 40-45 at idle, but that is with the larger 150 amp alternator so should be higher than your stock 105 if that is what you have? I just checked another silverado around here today, got 14.78volts and about 58amps at idle!

consultIII
02-20-2010, 10:53 AM
Thanks...my volts use to be 14.77....now are 14.05-14.10 and amps are around 40-45 at idle, but that is with the larger 150 amp alternator so should be higher than your stock 105 if that is what you have? I just checked another silverado around here today, got 14.78volts and about 58amps at idle!

what were the numbers from the alternator field signal and charge control wires?

j cAT
02-20-2010, 10:57 AM
Thanks...my volts use to be 14.77....now are 14.05-14.10 and amps are around 40-45 at idle, but that is with the larger 150 amp alternator so should be higher than your stock 105 if that is what you have? I just checked another silverado around here today, got 14.78volts and about 58amps at idle!

Did you read his reply that on cold start up the volts where 14.8 ...

temperature effects the alternator output....
SO IF YOUR ENGINE IS HOT AT THE NORMAL TEMP YOU WILL GET 13.75 VOLTS +/- .25VOLTS .

The alternator size does not effect the voltage generated...this is once again controlled by the internal alternator regulator....a 105 amp or 400amp alternator on your vehicle will output the same VOLTS ....

someday31
02-20-2010, 11:43 AM
what were the numbers from the alternator field signal and charge control wires?

Ok here are my numbers at the harness, just checked them today.......

This is the harness setup (hard to tell colors), but this is the order and Im sure blue is last.... Gray / Brown/ Blue (Note the "blue" wire is the one closest to the driver side front tire....the one on the end of the harness the other end doesnt have a pin its 4 prong, 3 pins used)

In run engine off.... 2.03V / 1.30V / 0V

Running.... 2.30V / 10.98V / 7.03

What do you think...PCM problem?

someday31
02-20-2010, 11:46 AM
Did you read his reply that on cold start up the volts where 14.8 ...

temperature effects the alternator output....
SO IF YOUR ENGINE IS HOT AT THE NORMAL TEMP YOU WILL GET 13.75 VOLTS +/- .25VOLTS .

The alternator size does not effect the voltage generated...this is once again controlled by the internal alternator regulator....a 105 amp or 400amp alternator on your vehicle will output the same VOLTS ....

Yeah what I meant was higher amp output the volts should be the same. Im saying that my alternator should put out more amps at idle than stock, especially with my battery not being fully charged due to the problem.

j cAT
02-20-2010, 03:52 PM
Yeah what I meant was higher amp output the volts should be the same. Im saying that my alternator should put out more amps at idle than stock, especially with my battery not being fully charged due to the problem.

the battery can only take in a set amount of current ...when the battery is cold the battery will absorb less current , a hot battery will absorb more current...that also applies to the battery suppling current..so when a battery is cold you do not get the maximum battery output...this is why in the cold areas most get the larger sized batteries...like the 875 CCA / 1000 amp cap at optimum temp...

if you had a 400amp alternator the most the discharged battery could take in would be about 40amps...on a charge...with the 14.75 volts..

now if you placed a 37 volt dc 200amp charger on it you could ram in 100amps into it , but the battery would soon be boiling ....

whenever the battery is low on a charge the current supplied is limited to about 10amps...this is to protect the battery plates from damage...this is why these alternators are designed as you have thoughly tested and found out..

my 105 amp alternator puts out 66amps with every thing on .when hot ..at the 13.75 volts ..this is the same as your system..this test was done when the vehicle was new in 2000...I have in 10 yrs , only installed one battery..the reason I replaced the OEM battery , was because it was 7yrs old...

If you have no parasitic loads draining the battery , then the only other reason would be a 1000watt sound system is drawing more current than the alternator can supply...or some other device like a large inverter power supply ...these items can draw 50-80 amps ..

consultIII
02-20-2010, 04:14 PM
Ok here are my numbers at the harness, just checked them today.......

This is the harness setup (hard to tell colors), but this is the order and Im sure blue is last.... Gray / Brown/ Blue (Note the "blue" wire is the one closest to the driver side front tire....the one on the end of the harness the other end doesnt have a pin its 4 prong, 3 pins used)

In run engine off.... 2.03V / 1.30V / 0V

Running.... 2.30V / 10.98V / 7.03

What do you think...PCM problem?


A 12.6 v battery is going to charge as long as the volts are 0.5v over 12.6v. The electrical pressure is there to push the current into the battery.
Based on the 14.1 volts at the battery and 40amps at the battery (as long as there is no insulated circuit voltage drop)...I think the problem is elsewhere.

think we should all meet at jcat's, fix the truck, then get pizza at the lynwood in randolph.

j cAT
02-20-2010, 04:34 PM
A 12.6 v battery is going to charge as long as the volts are 0.5v over 12.6v. The electrical pressure is there to push the current into the battery.
Based on the 14.1 volts at the battery and 40amps at the battery (as long as there is no insulated circuit voltage drop)...I think the problem is elsewhere.

think we should all meet at jcat's, fix the truck, then get pizza at the lynwood in randolph.

good idea !

someday31
02-20-2010, 04:34 PM
A 12.6 v battery is going to charge as long as the volts are 0.5v over 12.6v. The electrical pressure is there to push the current into the battery.
Based on the 14.1 volts at the battery and 40amps at the battery (as long as there is no insulated circuit voltage drop)...I think the problem is elsewhere.

think we should all meet at jcat's, fix the truck, then get pizza at the lynwood in randolph.

So you think my numbers at the harness are within range?

someday31
02-20-2010, 05:11 PM
good idea !

That would be cool! Im in Southeastern NY, so thats a far drive out there!

someday31
02-20-2010, 07:55 PM
A 12.6 v battery is going to charge as long as the volts are 0.5v over 12.6v. The electrical pressure is there to push the current into the battery.
Based on the 14.1 volts at the battery and 40amps at the battery (as long as there is no insulated circuit voltage drop)...I think the problem is elsewhere.

think we should all meet at jcat's, fix the truck, then get pizza at the lynwood in randolph.


lol...I will bring the beer! So do you think my numbers at the harness seem correct?

consultIII
02-20-2010, 08:24 PM
lol...I will bring the beer! So do you think my numbers at the harness seem correct?

they seem reasonable from truck to truck...honestly, have never checked there before (gm doesn't have spec's at these terminals that I have found). Was as interested as you to see what they would be.

If I leave now, I'll be in NY by tesday...keep that beer cold!

someday31
02-20-2010, 08:36 PM
they seem reasonable from truck to truck...honestly, have never checked there before (gm doesn't have spec's at these terminals that I have found). Was as interested as you to see what they would be.

If I leave now, I'll be in NY by tesday...keep that beer cold!

Ok guys heres a thought, my plan for Sunday....I am going get another battery out of another vehicle and then hook it up to the output of my truck alternator. I then would just ground it to the frame so nothing else is hooked up to the alternator. I want to see what volts I get...I will do that with the harness hooked up and with me just powering the terminal that the ecm powers. If this gets me my 14.77 volts with the harness hooked up then that would mean to me that my problem is after the alternator...ie the starter, fuse box, etc.

What do you think? Is there any reason the alternator shouldnt charge that other battery...it would be like bench testing it, but would allow me to see voltage straight from the alternator!

Nothing will please me more than to go back to the chevy dealer one day and tell them that we figured it out and that their $93/hr techs couldnt! They just tell me I have a ghost and have to live with it! lol

someday31
02-21-2010, 08:56 PM
I did my test as mentioned above...I actually got lower voltage than when hooked to my truck. I think thats because there wasnt enough load and the alternator sensed it so didnt put out as much, I got 13.85 volts. Anyways....im out of ideas at this point guys!

Time for a beer and throw up the flag, I give up!

rhandwor
02-22-2010, 04:09 PM
I had the same problem on my Silverado. I didn't do the testing you did but the problem was a crack in the battery leaking battery acid. I originally cleaned the terminal and put it back on it worked fine for a week or two.
I pulled the battery saw the problem and purchased a new battery. I dipped the terminal end in baking soda and water. After the acid neutralized I cleaned the end thoroughly. The bolt was pitted bad and I put in another good bolt.
The charge rate on the ammeter went from 14.2 to 14.5 and dropped after the initial drain was replaced.
If you recently changed anti freeze this is an insulator. Clean with contact cleaner brush and reinstall the terminals.
You can take the alternator to a shop and have it bench tested.

someday31
02-22-2010, 07:46 PM
I had the same problem on my Silverado. I didn't do the testing you did but the problem was a crack in the battery leaking battery acid. I originally cleaned the terminal and put it back on it worked fine for a week or two.
I pulled the battery saw the problem and purchased a new battery. I dipped the terminal end in baking soda and water. After the acid neutralized I cleaned the end thoroughly. The bolt was pitted bad and I put in another good bolt.
The charge rate on the ammeter went from 14.2 to 14.5 and dropped after the initial drain was replaced.
If you recently changed anti freeze this is an insulator. Clean with contact cleaner brush and reinstall the terminals.
You can take the alternator to a shop and have it bench tested.


I actually have put a brand new battery in, have changed it 3 times in the past few weeks. I have changed the battery terminals tonight and no change. I really dont know whats left to do!

98chev
02-22-2010, 10:33 PM
Hmm this is wierd. I have a similiar problem with my truck. It is a 2003 Z-71 5.3. with all accessories on the voltages drops to 12.4 volts (checked with meter) as i turn off stuff the voltages comes back a little for each one when everything is off voltage is around 14.4 this is with truck idle or reved up hot or cold but worse when hot. now that it is winter i dont notice the problem as much. took truck to alternator shop benched tested and tested on truck amperage output was good just voltage drops. tried different alt. same thing new battery same thing. The truck is never dead so i am living with it but just doesnt seem right.

j cAT
02-23-2010, 09:24 AM
Hmm this is wierd. I have a similiar problem with my truck. It is a 2003 Z-71 5.3. with all accessories on the voltages drops to 12.4 volts (checked with meter) as i turn off stuff the voltages comes back a little for each one when everything is off voltage is around 14.4 this is with truck idle or reved up hot or cold but worse when hot. now that it is winter i dont notice the problem as much. took truck to alternator shop benched tested and tested on truck amperage output was good just voltage drops. tried different alt. same thing new battery same thing. The truck is never dead so i am living with it but just doesnt seem right.

you must check this out with the alternator in the vehicle using an amprobe to measure the alternator current...with the engine running for about a half hour and the engine is at normal temp measure the current . it should be around 60-70amps with all electricals on.. if it is above this turn the electrical devices off one at a time to find out which one is drawing the excessive current....

now if the voltage drops with this amount of current [60 AMPS] check the voltage at the alternator terminal as this alternator wire may be defective..

normally when the alternator cannot supply the correct output current with OEM DEVICES , this is because the current is not full wave rectifcation...it is with pulses not a solid current stream..

since you just get 12.4volts at battery and the battery and alternator have been replaced with new you need more testing to determine whats drawing excessive current..or defective wiring/grounds..

someday31
02-23-2010, 07:09 PM
Hmm this is wierd. I have a similiar problem with my truck. It is a 2003 Z-71 5.3. with all accessories on the voltages drops to 12.4 volts (checked with meter) as i turn off stuff the voltages comes back a little for each one when everything is off voltage is around 14.4 this is with truck idle or reved up hot or cold but worse when hot. now that it is winter i dont notice the problem as much. took truck to alternator shop benched tested and tested on truck amperage output was good just voltage drops. tried different alt. same thing new battery same thing. The truck is never dead so i am living with it but just doesnt seem right.


Yeah this is the first time I havent been able to solve a car issue like this! I am planning on going to alternator repair shop this weekend to see what they have to say about everything!

consultIII
02-23-2010, 10:20 PM
re. battery low state of charge!! Does this apply??

when you shut your truck off at night are you leaving key in the igniton? This will keep the bcm active waiting for your next input and drain the battery overnight. Even if the key is in the off position but inserted into the cylinder.

someday31
02-23-2010, 11:03 PM
re. battery low state of charge!! Does this apply??

when you shut your truck off at night are you leaving key in the igniton? This will keep the bcm active waiting for your next input and drain the battery overnight. Even if the key is in the off position but inserted into the cylinder.


I dont leave the key in mine ever. I live in NY afterall. lol

RahX
02-24-2010, 01:52 AM
Ok, so like everyone else has said, the alternator is behaving normally. You've replaced it so that eliminates the alternator (mostly) You've replaced the battery, that eliminates that. Have you done a voltage drop on the ground wire to the body/frame/engine? From the positive battery terminal to the positive on the alternator? I would agree with the thinking that you have an intermittent drain on the battery. The only way you're going to catch it is if you get lucky or watch it for a long time. A higher amp alternator isn't going to do anything for you. Like others have said, your battery can only take so much of a charge. Usually 14v@20-30 amps is a very heavy charge. If it stays that high for too long, the battery will boil the electrolyte and burn itself up.

rhandwor
02-24-2010, 08:44 AM
Some items to check red fuse able link at starter solenoid.
10 amp fuse left bussed electrical center lower left side of dash.
Ground left radiator support bracket.
Ground 4.3L right front of engine.
Ground 5.3L left front of engine.
Ground mid bussed electrical center{center under left side of dash} right rear of engine.
Look at connections in splice pack plastic cover close to battery on positive cable.

j cAT
02-24-2010, 09:34 AM
Ok, so like everyone else has said, the alternator is behaving normally. You've replaced it so that eliminates the alternator (mostly) You've replaced the battery, that eliminates that. Have you done a voltage drop on the ground wire to the body/frame/engine? From the positive battery terminal to the positive on the alternator? I would agree with the thinking that you have an intermittent drain on the battery. The only way you're going to catch it is if you get lucky or watch it for a long time. A higher amp alternator isn't going to do anything for you. Like others have said, your battery can only take so much of a charge. Usually 14v@20-30 amps is a very heavy charge. If it stays that high for too long, the battery will boil the electrolyte and burn itself up.

although this guy still has battery charge loss ,,,, this post has very detailed info that is great reference data for others , that have issues or need training on how this all works.............

rhandwor
02-26-2010, 06:41 AM
Alternator output voltage and amperage is varied and controlled by the PCM.

Wiring at the alternator:

The red wire comes from the battery and through a fuse link and then to the output terminal of the alternator. It should show battery voltage at all times.

The dark blue wire goes to supplemental brake assist.

The brown wire goes to the PCM for the charge indicator control.

The grey wire comes from the PCM and goes to the voltage regulator inside the alternator. The PCM monitors voltage and load on the battery and determines what the charge voltage should be. It then sends a signal, using a duty cycle, to the voltage regulator. The greater the load on the battery, the longer the duty cycle.

"Old School".... ideal voltage, in a perfect world, was 14.2 volts. Now that the PCM is in the equation, "Old School" is out!
While I don't have a 2003 wiring diagram. I looked at a 1999 Mitchell and a 2000 Alldata wiring diagram. Mitchell says the battery light bulb if burnt out will cause problems. Apparently it is still used to excite the field. On my 2000 this bulb is the message center. It goes on stating to ohm wires going to the IPC {instrument panel cluster} check the ground for the circuit and fuse for this item. Mitchell says to replace after all checks are done. Apparently 13 to 16 Volt at 2500 rpm lights,defroster,close to a full load. Alldata shows a logic and states the computer monitors this circuit to regulate voltage. Is this still true for the newer models?

someday31
03-05-2010, 08:43 PM
I took a chance bought this harness

http://www.zzperformance.com/grand_prix/products1.php?id=517&catid=107

This resulted in no change of output, not sure what that means. Tomorrow I'm heading to a alternator repair place, will let you guys know what that results in.

someday31
03-07-2010, 09:31 PM
Ok so I am finally making a little progress in figuring out my issue. I brought one of the alternators from my truck and had the shop bench test it. They were only able to get the 14.10 volts out of it like I am getting! Then they proceeded to test one of their rebuilts that fits my truck and they get 14.77, which I use to get! So the guy told me he thinks I have the newest style that fits the late model trucks with a lower voltage setpoint.
Anyways I then decided to buy a different alternator to try from advanced auto. I put that in and I finally got the 14.80volts...at first... then as i drove it slowly retracted down to the 14.10 volts. So what does that mean, whats going on.... is it temperature sensor dropping volts as it gets warmed up? I would think my volts would stay the same, just amps would drop as warmed up.
Btw brought the truck for a test and its still getting good battery, low charge!
Does anyone know the difference between the AD244 and DR244 alternators....

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