97 JGC 6 cyl AT: Bad PCM or Alternator?

[jackie-boy]

Today I was driving and the battery voltage dropped suddenly as indicated by the gauge. The "check gauges" light came on, but the vehicle did not stall. I got the jeep home. A little while later I started it up. I tested the voltage at the battery when it was running and when it was off. Both voltages were measured at about 25 volts. This is pretty high considering the voltage is not supposed to exceed 12.5 volts (Haynes manual). Could it be the PCM? Any help??

[bringselpup]

I'd mebbe take a hard look at whatever you're using to get that voltage reading. 25 volts is enough off the scale to suggest something ain't right with the test equip.

No codes? Mine threw a 41. Run the engine, when you see the needle dip to the low side give that alternator a good thwack on the body. If the needle goes back into the charging zone you're shopping for a new alternator. My needle jumped back after giving it a love tap with a 12" galvanized fence bolt. Alternator swap made the code go away and the needle stays on the charge side again.

[jackie-boy]

Well it's a brand new voltmeter. I think that the ground clamp is bad (I can see a crack in it). There was no code. It would make sense because the battery would not be able to dump the extra voltage to ground. It stays running once it's started, but then won't start back up.

[bringselpup]

If you have no code I wonder about your battery. Can you scare up a known good battery from somewhere and put it in for a bit?

Then maybe get the one you took out to a conventional auto parts store. Most have some way to test things like alternators and batteries these days to see if they are bad. But we'll get some fast feedback from seeing how the system is with another battery in there.

[oljeeptek]

You should have 14.5 volts running. The battery cant cause 25 volt output from the alternator. Your pcm monitors the difference from its positive and negative terminals to determine how much voltage to put out. I suspect you had a bad ground cable and the pcm couldn't read the actual battery voltage. This would also cause the battery not to take a charge even at 25v. (This voltage should normally have boiled the battery real good.)

You could have a battery with an intermittant open circuit internally, but it's not very likely. Besides, that kind of failure it usually accompanied by a battery explosion.

Be very carefull working around it until it's fixed.