royta

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

royta

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

Dartmouth 12V

I would suspect that there is a local autoparts store like napa or something that can bench test an alternator. They only take about 10 minutes to remove and most places will test them for free. Unfotrunately, there is enough stuff going on in the charging circuit that it is hard to definitively test everything.

royta

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.

Dave Miller

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