95 Grand Cherokee - Electrical Issue

ke4peo · #1 (**permalink**) 11-17-2009, 09:41 AM

Hey all,

Just looking for a little insight here on an issue I'm having. First off, the vehicle specs:

1995 Grand Cherokee Laredo
4.0 I-6 engine
2WD
Automatic
226,491 miles

Back in August the Check Engine light came on and I noticed the volt gauge was reading in a discharge state (about 1/4 up from off position). I ran the codes and only got this one:

41 - An open or shorted condition in the generator field control circuit.

I had the charging system tested at both Autozone and Advance Auto. Autozone found a bad battery. Since it was an Advanced Auto battery, I went there to replace it. They tested it and confirmed. Replaced the battery and it appeared fine for an hour or so. Then the Check Engine light came back on. Ran the codes. Again, 41.

I tested the voltage myself using a multimeter and each time the check engine light comes on, it seems as though there is a drop in voltage on the alternator. But both Autozone and Advance say the alternator is fine.

I'm beginning to wonder if maybe the battery cables need to be replaced, or if the alternator is really failing. Another forum suggested I check the 'battery temperature sensor' that sits under the battery in the holding tray. My holding tray /is/ broken, so maybe that's the culprit.

Since this happened in August, I purchased I replacement vehicle but find I like my Jeep more. I've been going out and starting it up every day, putting it on a battery charger once a week. One thing I noticed is that sometimes, when it's started, the check engine light is off and the voltage is normal. Shut it off, start it up, light may come back on and voltage may go low.

I know there's a problem somewhere since if it's not charged every so often it will go completely dead. Anyone got suggestions?

SteelHeadz · #2 (**permalink**) 11-17-2009, 10:16 AM

Process of elimination suggest that the battery temp sensor needs to be where it's measuring the surface temp of the battery not the exaust or any other surface. The sensor delivers a signal to the PCM that regulates the charging output from the alternator. Yes, definitely check the cables for corrosion and the electrical connectors to the PDC(power distribution center).

MudderChuck · #3 (**permalink**) 11-17-2009, 10:29 AM

I'm kind of Grand ignorant, I've only had XJ's. They are often similar.
The battery tray temperature sensor sounds like a likely culprit. It imputs the regulator for the alternator, one function is to tone it down a bit, when it senses the battery is getting too hot (garbage in, garbage out).
The regulator for the alternator is in the computer (PCM or whatever they call it in the Grand). The path for the field control is from the ASD relay to the field winding in the alternator then to the vehicle computer, through the regulator circuit in the computer, to ground. It probably regulates on the ground side of the field winding. Though that time frame was a transition period and they eventually changed it to regulating the input side of the field winding rather than the ground side.
The volt gages in most vehicles can be liars, the gage taps the system someplace other than right at the battery and only really tells you the voltage, plus or minus the gage error, at the exact spot it taps into the electrical system. You really have to check the voltage at the battery itself with a volt meter, with the vehicle running, to get the big picture.
Cleaning up all of the battery clamps, battery poles and the major ground from the battery to where ever is always a good place to start. I always spray them with solvent, clean them with a 3M pad or sandpaper until they shine and then spray them again just in case I smeared a little overlooked grease or oil around while sanding. The system is basically low voltage and fairly high amperage while charging the battery. The larger the (clean) contact area the better, 12-14 volts isn't high enough to push through even a mild coating of oil or corrosion.

ke4peo · #4 (**permalink**) 11-20-2009, 02:05 PM

Quote:

Originally Posted by SteelHeadz

Process of elimination suggest that the battery temp sensor needs to be where it's measuring the surface temp of the battery not the exaust or any other surface. The sensor delivers a signal to the PCM that regulates the charging output from the alternator. Yes, definitely check the cables for corrosion and the electrical connectors to the PDC(power distribution center).

Ok, so maybe my Grand Cherokee Laredo doesn't have a battery temp sensor. I removed the battery and there's nothing under it at all. Just the plastic tray. I wonder if it was an optional addon for my model and year.

I checked the cables for corrosion, and used a steel brush on all the ones I could locate to clean them off. I got the following connectors cleaned:

Terminal connectors
Body ground point
Engine Block ground point
Alternator Positive connection
PDC (Power Distribution Center) positive connection

I couldn't locate the connectors to the PCM, but it's only been a 8 months since I replaced the Crankshaft Sensor and cleaned/greased the PCM connector (the one that plugs into the PCM with all the wires.

The first few starts, the Check Engine light was on and the gauge was reading low (though higher than it was before). On about the 5th shut off/start up, the gauge returned to normal and the Check Engine light went out. Currently I have the battery on a fast charger to charge up to full.

ke4peo · #5 (**permalink**) 11-20-2009, 04:53 PM

Ok....so that didn't fix the issue. I think I'm going to go to Pull-A-Part tomorrow and yank an alternator and PCM from a Jeep there.

SteelHeadz · #6 (**permalink**) 11-20-2009, 05:17 PM

Make sure you take note of the PCM part number you currently have and try to match the numbers. The last 2 letter designations of the part number will indicate the software version. For example if the PCM part number is 521XXXXAA and the one you find on the junkyard is a 521XXXXAB, the AB is the newer software. Another thing to look for when your swapping PCM and hunting down the correct part number is if the front clip or anywhere close to the PCM has a gray and white decal from Chrysler with a handwritten TSB# or a recall # , take note of the new PCM software Part number ..

ke4peo · #7 (**permalink**) 11-20-2009, 05:47 PM

Quote:

Originally Posted by SteelHeadz

Make sure you take note of the PCM part number you currently have and try to match the numbers. The last 2 letter designations of the part number will indicate the software version. For example if the PCM part number is 521XXXXAA and the one you find on the junkyard is a 521XXXXAB, the AB is the newer software. Another thing to look for when your swapping PCM and hunting down the correct part number is if the front clip or anywhere close to the PCM has a gray and white decal from Chrysler with a handwritten TSB# or a recall # , take note of the new PCM software Part number ..

So.....I need to make sure the last 2 digits match? Or are you saying that I can use an AB PCM in place of my AA PCM? I always thought that so long as it was a 4x2 I-6 automatic (same as mine) then the PCM /should/ work.

SteelHeadz · #8 (**permalink**) 11-21-2009, 12:02 AM

Quote:

Originally Posted by ke4peo

So.....I need to make sure the last 2 digits match? Or are you saying that I can use an AB PCM in place of my AA PCM? I always thought that so long as it was a 4x2 I-6 automatic (same as mine) then the PCM /should/ work.

No, the last 2 alpha digits can go from an AA to an AB , they dont need to match. IF you find one that's an AA or AB and the one you have is an AC, then the software on that PCM your replacing it with is an older version but it can be re-programmed with the latest software from a Dealer. The numerical digits has to match. From the late build 04 models to the Mid 98 models, there were several changes in the software, and one will not work well with the other. Also, if your Jeep is a Federal emmision vehicle, do not put a california emmision PCM on it or else you will have several erroneus DTC's flashing at you.

ke4peo · #9 (**permalink**) 11-21-2009, 12:28 AM

Quote:

Originally Posted by SteelHeadz

No, the last 2 alpha digits can go from an AA to an AB , they dont need to match. IF you find one that's an AA or AB and the one you have is an AC, then the software on that PCM your replacing it with is an older version but it can be re-programmed with the latest software from a Dealer. The numerical digits has to match. From the late build 04 models to the Mid 98 models, there were several changes in the software, and one will not work well with the other. Also, if your Jeep is a Federal emmision vehicle, do not put a california emmision PCM on it or else you will have several erroneus DTC's flashing at you.

Ahhhh.....gotcha. I can go forward in alpha sequence, but not backwards. Makes sense now. I'd probably be pulling from either an 1994/1995/1996 model as those are the ones on the salvage yard lot. With luck I'll find one that matches or is higher sequence. I don't think I could pull one from a 97 or 98 model as those had newer interior designs and dash systems.

Thanks again for the clarification!