This is in NO WAY meant as a 'flame' to greatwhite or anyone else, but greatwhite's post got me curious, so I went out to the shop and did some testing on my 93. This is what I found:

greatwhite stated:
If the output from the alternator is 12 - 12.1V, the alternator is faulty.
I pulled the wire apart that goes into the PCM for the regulator function. (I had a similar problem that turned out to be this particular wire being broken, and fixed it with a quick connect/disconnect) When the wire is disconnected from the PCM, (regulator), the alternator has the same voltage as the batteries, 12.4 volts. Hook the wire back up and the alternator charges at 14.1 volts. So that kind of satisfied me that the alternator could NOT be putting out anything and still be good.

13.4 minimum or you are discharging.
I operated my winch and the voltage at the alternator was barely 12.5 volts. Stopped the winch and the voltage came back slightly slowly to the original 14.1 volts. I don't think it was 'discharging', it was working it's little heart out trying to keep up with the demand from the winch. I guess from a technical standpoint it might have been discharging, but for all intents and purposes, it was operating fine. I think to most of us that don't make our living as a professional mechanic or those of us that aren't electrical engineers, 'discharging' is taken to mean 'not working at all'.

A 12v battery is really 13.2, but unlucky 13 and all...
I don't know if this is in practical application or in theory, but I just put a brand new Interstate battery in my truck on May 3rd, and sitting there with the motor not runing and the battery disconnect switch in the off position, (disconnects the entire electrical system), I have 12.8 volts, slightly higher than when the motor is running with the regulator disconnected.

An Alternator Is Not A Battery Charger... and using it for this will kill it dead. It is a battery maintainer.
I can't comment on this because I really don't know. But I DO know that when the voltage drops below a predetermined point, the alternator CHARGES to bring the voltage back up.

If one battery is bad it will take out the second battery in a remarkably short time. The problem is that you likely won't ntice one toasty battery until it takes out the second one and/or the alternator.
This I KNOW is true. A bad battery is just like any other 'load' such as a headlight, blower motor or even a dead short. The good battery and/or the alternator will try and make up the difference and will work their butts off in the process. Add to this the normal electrical load of the system that the alternator and battery was designed for and they just can't keep up with the demand. Something's got to give!

I don't know if any of this will help with the original problem that was posted, but like I said, I was curious because I had the same problem with my 93, and found it to be that broken wire that goes from the alternator to the PCM. It was behind the windshield washer tank and I didn't see it until I had bought a new PCM, (for 600 bucks, thank you very much!), and went to install it. NOW I'm stuck with a PCM I don't need and I'm out the $600 because I had to special order the PCM and they don't take electrical component returns, whether or not they were ever installed.

The moral of this story; Just make sure it really IS the PCM before you buy one!

chaikwa.

 

Drive it to the auto parts store, disconnect BOTH batteries and have them check each battery. It will be a lot cheaper to replace the batteries than the PCM. Besides, that's the next most obvious/easiest thing to check.

 

I agree with the Batteries being the next, most obvious step... and I just did that exact thing. Determination? Both batteries are in good condition.

After all this, I honestly thing the brand spankin' new, Mopar-built, $300 Alternator.... is bad.


Now, here's the big question.... Nobody can test the Alternator off the vehicle, so I'm going to have to take it to the Dealer. Problem? The dealer I bought my Alt. from, is 30 minutes away. I don't want to try driving that far w/o a charging Alt. There's a much closer Dodge dealer... do you guys think I can take my truck there, have them test my recently purchased (bad) Alt & have them exchange it for a new one? Or do I have to go to the dealer I bought it from?

 

Call the parts dept. and ask but any dealer should be able to do the exchange.

 

Is it possible the battery temperature sensor has an impact on this.

The BTS data along with monitored line voltage data is used by the PCM to vary the battery charging rate. You would need to have dealer with the proper diagnostic equipment test this though. Just a wild shot in the dark.

 

Just a hint. Don't go in mad, go in hurt and upset that you spent your hard earned cash on a genuine Mopar replacement part and it seems to be faulty. You expected better from factory parts. You spent the extra money because of the higher quality expected. etc. etc. etc. . This approach will usually get you further along the road to n/c replacement. But if that doesn't work explode on 'em. Most retailers will go further for a person they want for a customer, and those people are reasonable when things go bad. Not happy, just reasonable. Rick

 

My truck starts every morning with the batts at 12.6 or so volts.
With each battery having 6 plates and they are at 2.1 each, this is 12.6 nominal volts. not correct to say 13.6 volts.
We normally think of a battery charger as a metal box that we plug into the wall to charge batteries.
Any device that raises the voltage applied to a battery will charge it. There are better pulse charging units available that are great.
But in this situation, it is not needed.
Have you disconnected the batteries with the engine off, to see it there is a drain on it? The grid heaters can draw more that the alt. can produce at idle.

 

Well, I've given up... I just dropped my truck off at the dealer. I'm currently waiting for a phone call & diagnosis.

 

A freshly charged battery has a surface charge on it and will read up in the 13's but this is a false reading. Letting it sit for a while or pulling a little juice off it will bring a fully charged battery to 12.6 volts.

 

The dealership just called... you know the plug that goes into the back of the Alternator, then into the wiring harness? Well, apparently those wires wore thru, in the middle of the harness. Neither was connected at all, therefore it wasn't getting any charge.


Anyone need a good, used Alternator?

 

Glad you got it going again...

 

Me too, that was turning into a big headache.

Thanks to all for helping!