Hey guys, I'm across the state for work and my truck just stopped charging. Last night the Gen light came on and the volts read around 12. This morning, after a weak start, the volts dropped to 8. My headlights and dash lights are all but out (still a faint glow) and my radio is off. Seatbelt light is on.

Also, when Gen light came on I tried the A/C and it still works fine (clutch engages) and tach still works fine. So I'm thinking alternator, not PCM. Sound right? Thanks in advance

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Edit: the voltage is now below 8. The brake/ABS lights are barely glowing and the seatbelts are locked.

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Ok now speedometer is dead. Tach still works

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Probable Alternator, be advised, it doesn't have to come from a Cummins, 136 Amp was used for different vehicles including regular Ram, Jeep Grand Cherokee, etc. All you have to do is change pulley.

 

If I was closer I would bring ya one man........ I keep a spare in the back.

 

Thanks a ton.

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Well, alternator tested fine. Guess it's the PCM. Sure didn't seem that way from the symptoms. Guess I'll try to hunt one down.

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2nd part to the problem. I am looking at PCMs and I must have the VIN. I have a 1998 cab on a 1997 truck. Is there anywhere else that the actual truck VIN would be labeled other than the cab? Thanks.

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Side note: I disconnected both battery negatives to remove the alternator when I tested it. They were disconnected for around an hour which must have reset the PCM because I jump started it and its now running/driving/charging just fine.

So I have definitely narrowed the problem down to a faulty PCM, it would seem.

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That's not the usual PCM failure mode. Not sure, but look for corrosion that you might have moved when you removed the negative cables. Since it is a 98, wiring is different than my 97, do you have a FSM for it? Before I went changing a PCM, I would seriously inspect all the wiring pertaining to the charging circuit first. Since you never lost the tach, I doubt it is in the PCM input, but would be from the PCM back out.

 

It actually is a 1997; it just has a 1998 cab ( homemade holy grail dually, haha) I could have possibly knocked some corrosion loose when I was wrenching. Everything looked pretty clean, but it doesn't take much, I guess.


Since the all the other PCM functions seem fine, would an external voltage regulator be a probable fix? Or not, because its actually the PCM that tells the alternator to charge the system in the first place?

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Ok, going thru my 97 FSM, this is the first year charging voltage apply did not go thru the ASD relay, should help us a bit.
First thing they do is ask for a physical inspection, which you have done, next is to see if any codes were thrown, did the check engine light come on? Might not hurt to take it to AutoZone and have it scanned.

They have you test the cables, engine running at 2400 RPM, touch negative of your VOM to the + side terminal of the batt, and then the + side of the VOM to the B+ terminal of the ALt., if voltage is below .6V, you are good, if above, check for corrosion or bad connection or wiring. Then they check the - side Touch the - side of the VOM to the negative side battery post and the + side lead to the GROUND TERMINAL of the alt., this is the nut that attaches the field terminal block to the Alternator case ( NOT the field terminals themselves ) You are looking for less than .3V, if more, figure out the problem with the ends or cable. Looking at this, and going back over your problem, I am not so sure you didn't fix this thing right when you took the negative terminals apart......... Anyway, if all that checks out, it gets more complicated to test further, I would probably get the 35 dollar FSM on disc from genos for a 97, it will more than pay for itself........
I will look for the wiring from the fields to the PCM for ya......

On edit, here ya go.

 

Absolutely awesome. Helpful does does not begin to describe that post. Thanks a ton.

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