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

[someday31]

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]

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]

Originally Posted by someday31

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]

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]

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]

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]

Originally Posted by someday31

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]

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]

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]

Originally Posted by someday31

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]

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]

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]

Can I start the truck without the harness hooked up and check voltage at the harness? If so what should my readings be?

[someday31]

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]

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.