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

[j cAT]

Originally Posted by someday31

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]

Originally Posted by consultIII

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]

Originally Posted by someday31

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]

Originally Posted by consultIII

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]

Originally Posted by someday31

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]

Originally Posted by someday31

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]

Originally Posted by consultIII

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]

Originally Posted by j cAT

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]

Originally Posted by someday31

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]

Originally Posted by someday31

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]

Originally Posted by consultIII

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]

Originally Posted by consultIII

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]

Originally Posted by j cAT

good idea !

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

[someday31]

Originally Posted by consultIII

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]

Originally Posted by someday31

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!