I apologize for the double post from HERE, but I don't want to miss out on anybody.



My "GEN" light is on, and it appears the alternator is not charging batteries. Batteries are Optima red tops and measure 11.92 VDC. I have performed voltage leak tests between B+ at alternator and Pos battery terminal AND between GRD at alternator and Neg battery terminal. Voltage difference measures in the hundreths (0.0x) so that is ruled out. 120A fuse at PDC is good. With engine running, I am measuring same voltage between Ground and + alternator field wiring terminal (closest to B+ on alternator) as I am directly at battery.

With engine running and alternator field wiring connected, I installed a fused jumper between - field wiring terminal (furthest from B+ on alternator) and Neg battery terminal, and battery voltage did not increase. With alternator field wiring disconnected, thus completely eliminating PCM, I installed a fused jumper between one of the field wiring studs and Pos battery terminal AND between the other field wiring stud and Neg battery terminal, and battery voltage did not increase.

Is it safe to assume my alternator is at fault?


Thanks.


Roy

 

Wake up dieseltruckresource.

I picked up a replacement alternator, but can return it if I don't open the box. I'd hate to open it up and install it, only to find out it wasn't the culprit. I believe I have covered all the bases, and the problem looks like the alternator, but I want to make sure there is nothing that I have missed.

Thanks.


Roy

 

I had the same symptoms. I was driving down road and my voltmeter dropped to 9, and alt light came on. I installed a rebuilt and solved the problem.

 

If I understand what your diagnosis process was , the alt is bad .
A good parts house should be able to bench test the old alt .
Did you leave the large cable at alt to batt pos connected ?
You should .

 

Replaced alternator and all is well. I just didn't want to open the alternator box until I knew for sure the old alternator was bad. I wish these alternators were the internal regulator type. Then it would be a no-brainer. No charge, replace alternator.


Yes, John Fauighn, of course I left the large cable connected.

 

Sorry I got to you too late on your question but I did just responded to your PM just now before reading this thread and I agreed that the ALT...as you found out...is bad.

It looks like that Alt test you did is a very good way to rule out the PCM from the Alt as a suspect.

It should be clarified for all that you shouldn't trust your local auto parts to correctly test our alt. They frequently say the alt test shows your alt is not charging so its bad when in fact....DUH...our alts don't have a regulator function...its in the pcm (or ecm on some 24v trucks). Beware of having your alt tested to confirm it. This test that was done in this thread has just proven to be a much better alt test...thank you "dozer12216" as shown on his post #7 on this thread:

https://www.dieseltruckresource.com/...lternator+test

Good troubleshooting guys!

Dave

 

Ouch! This stuff hurts my head. Have they changed anything or will this test work on an 03?

https://www.dieseltruckresource.com/...lternator+test

Dave Miller: For some reason your link did not work even though it appears to be the same as mine.

 

To clarify , to check alt in these trucks , disconnect the 2 small wires , these are the field connections , for testing it does not matter , connect one to grd & the other to pos , this is called full fielding the alt , should do for short time only , not more that a minute , with the alt full fielded , it can over heat & burn internally .
The computer makes a variable ground connection depending on battery voltage .
Also having 14-16 volts is not what really counts , it amps that are the true measure of alt , battery and starter , this is called a load test .
Alt should be around 120 amps depending on yr
Batteries are rated by label , but min. [ cold weather is about 800 ea ]
Starter should be about 300 - 400 amps [ this is a ruff guess , do not have specs in front of me ] , if the amps are very low or very high then some thing is wrong with one of , starter - battery - connections .
To help when the high school kid behind the counter is running the tester , also make sure that they know about the external regulator [ not in alt ] , the field needs to be both grounded & energized .

 

Thanks, I re-did the link on my post so maybe now it will work. I don't know for sure if the test will work on the 03. I believe the ECM controls charging at least thru 02. It seems to me they are using the same alt so it should work.

Someone familiar with the 03 charging should step in and verify for you.

Dave

 

This may help the next time the charging system fails. I found if the batteries are low, the 120-amp fuse is OK, and the tach still works, just measure the alternator field voltage with the engine running. If this voltage is greater than about 6 volts the PCM is fine and your alternator is bad. If the voltage is less than about 3 volts the PCM is toast. The field voltage should run around 5 or 6 volts if the batteries are good.

 

Yes, the test works on 03's.
With 2 pin alt connector disconnected and engine running, you could use 12v test light to figure out the feed to alt rotor and consequentlly the pcm control side or other wire.
Guess I'm lucky. Good parts stores with enclosed alt tester have not failed in half dozen tests. Must know what unit is from.

 

Thanks for the info guys.