cluckmaster

Hey guys- was driving to Hurlock, MD this morning, and noticed my GEN (i guess generator/alternator) light was on. Dash voltmeter was on the notch below 14 (so I guess somewhere around 10 or 12). Stayed like that all the way, and then went on and off several times between Hurlock and home (about an hour). Shut her off for about ten minutes, then started back up to drive in to work, and the light was off and the meter read just over 14 the whole way in. Just walked out and started her up and she went up to 10 or 12, sat that way for a minute, then started slowly bouncing back and forth between 10 and 14.5 or so. I don't want to go buy a $144 alternator unless it's actually bad. Any ideas whats going on?

Nopt sure if this is related, but I filled my tank a few days ago, and when I started up, the fuel gauge went crazy, bouncing all over. Shut it down, started back up ,and it climbed to full and has been good since.

Any info would be great. Hate for the alternator to die tonight after dark. THANKS! -Josh

zeeew

I have the exact same question. Mine did the same thing this morning, wife was on way to work, she brought it back home and took the car. I drove the truck to work and it was fine. It has done this once before. The first time the battery connections were a bit dirty, I cleaned all the connections and thought that might have been the problem, except for this morning.
Both times the truck was not plugged in and it was quite cold both times. Both times worked ok after shut off for a while and started back up.
1996 Cummins 2500 5 speed

Tate

Just throwing this out, but the grid heaters will pull the voltage gauge down to around 12V from your usual 14. Completely normal as it cycles. If it does this after the truck is warm, or at speed, then you actually have a problem.

TIMMY22

I had a similar situation a while back, turned out to be salt in the trailer plug terminals. sprayed it with a garden hose and it was fine.

TxDiesel007

Agreed here

Grid heaters suck alot outta the battery, If warm tho, one battery might be bad, alternator is shot, or worst case senario, u lost charging function on the PCM, and ull need a voltage regulator, external, or at worse to replace a PCM..

Rick

zeeew

This is abnormally low, even for the cold. Needle is WAY down and stays there, doesnt cycle back up. At speed, stays down. Then gen light comes on.
Shut it off, restart later, works ok.

Tate

I'd say get your batteries load tested, and your alternator tested. Free things that will help rule it out. I don't know if a bad battery temp sensor would throw a code on the older trucks, but thats another option. The temp of the batteries is what regulates the voltage.

cluckmaster

Hasn't come back on since Wednesday morning- pretty odd. So..a bad battery could throw the GEN warning light on? I've still got the original batteries (262k mi) and I just replaced the contacts in the starter recently so was thinking about possibly putting in some new Interstates.

Also, how would I know if it's the PCM? Can I test that somehow?

Where is the battery temp sensor located? Thanks!! -Josh

zeeew

I think battery temp sensor started with the 24v trucks? I may be wrong here though, not sure my 96 has one. If it does its something else to look into.

Tate

Check under the driverside battery. Looks like a round plug in the tray, and theres a harness running into it.

cluckmaster

Update- after work, went out, and everything was fine. No light, meter read 14.5 or so. Yesterday, same stupid thing happened....GEN light came on, meter read 10 or 12. Shut it down, got back in later and the light was gone and the meter read 14.5 again. Then it happened again this morning. Lights and dash never dim or anything like that. The really funny thing is..that I didn't put together until the second and third time it happened..my pyro goes crazy whenever this happens, bouncing all over the place, but it always goes back to normal when the GEN light goes off and the meter reads normal again. Sooo...to sum up, I turn on the truck, GEN light comes on and meter reads low volts, and the pyro acts crazy. Shut it down for awhile, start it back up, and everything is normal. ???????

cluckmaster

Is there a temp sensor that can go bad on the grid heater that would cause it to stay on or come on even after the truck is up to normal operating temps?

Also, what's the voltage regulator? Thanks! -Josh

zeeew

cluck I believe the voltage regulator is in the computer and automatic. Yesterday I took both batteries out of the truck and scrubbed them with a toothrush with baking soda, rinsed them well. Cleaned all wire connectors with soda as well. I am waiting for a report from the wife tonite to see if it helped. Also installed relay kit at headlights, so if headlights arent brighter now, something is still wrong.
I can hear the idle change slightly when the grid heater cycles. Also the wife said the lights/dash didnt dim when the gen light was on.

When the gen light is on does it mean the alternator stator is not getting any voltage from the computer?

cluckmaster

Well, volt meter was WAAAYYY down this morning, to about 7 or so. Sooo.. quick trip to Auto Zone, had the alt and batteries tested. Said all were bad. Bought a new alt, stuck her in, and started her up. GEN light was off, and after a few minutes, the voltmeter was back up to 14.5 and steady. Dad is an Interstate dealer, so I opted to keep the batteries til I can get to his shop for a pair at dealer cost. Pyro is acting normal again so far.

Just didn't think the symptoms sounded like a dying alt, but guess I was wrong. Not sure what was causing the pyro to act up. THanks for the help guys. -Josh