How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask George H. Your Own Question
George H.
George H., ASE Certified Technician
Category: Jeep
Satisfied Customers: 18668
Experience:  ASE Master Tech 15+ years
Type Your Jeep Question Here...
George H. is online now
A new question is answered every 9 seconds

1994 Jeep Grand cherokee se: Replaced alternator..wiring harness

This answer was rated:

My 1994 Jeep Grand Cherokee won't charge the battery. Replaced alternator/battery wiring harness. Bench tested the alternator, OK. Just bought a reconditioned PCM and replaced the original. Still get a P1682 code from the PCM. With engine running, get same voltage at battery and alternator ends of the positive wire. Vehicle starts fine, just won't charge. Would check the two smaller wires: green and white/blue that go from the alternator somewhere but don't know what I'm looking for, either the other end in a plug, voltages, or continuity.

Hello I will help you with your question,


The green wire runs to the PCM and where it passes under the battery tray is a connector that is oftem corroded. This wire provides ground through the PCM which controls the current of the field coil regulating the output of the alternator.


Check continuity between the alternator connector and the PCM connector C2 pin #10 if this is a 2004 as posted above or PIN 20 if this is a 1994


Let me know what year and what I can do to help you with this





Customer: replied 7 years ago.
Got continuity, and resistance of just 0.3 ohms. This is a 2004 Grand Cherokee Laredo.

Ok the connector that I referred to is for the 1994.


If you attach a jumper wire to the green wire at the connector for the alternator and temporally ground it does the alternator charge? You can run a straigh pin in the back of the connector from the wire side and attach a jumper to it. Just ground it long enough to see if the alternator is charging


The white red wire should have battery voltage while the engine is running


Have you cleared the code?



Customer: replied 7 years ago.
I read the code on the odometer as I don't have an ODBC-2. Is there a manual way to do it. I have a 1996 Ford truck and had to buy an old fashioned ODBC for it and remember that you could short some pins at the ODBC plug to clear codes, but have'n researched how to do it manually with an "approved" ODBC!

I attached a jumper wire to the green wire while it was plugged into the alternator via an unfolded paperclig through the back of the connector and had my daughter start the car. She did not observe the gague come up, and I with my voltage meter, did not see any voltage change at the battery.
Your 2004 needs a scantool to reset the codes. Check for battery voltage on the white/red wire at the alternator with the engine running. If you don't have power there check the wire to the PCM connector C3 pin #25
Customer: replied 7 years ago.
OK. No voltage at the alternator. Then did a continuity check. Not knowing which plug was C3, I tried both the "outside plugs. No continuity. Just for my edification, how are the plugs numbered as you view them looking from the engine compartment to the firewall?

But, going on, if it's the wire, what's the best way to replace it? And, are there more tests I can do - I suppose with pins through the back of the plug and jumper wires?

The connectors are 1,2,3 black, white, grey


This is the pinout of the grey connector


You want #25, if you have to replace the wire I would check for power at the computer, clear the codes and then if there is power at the PCM but not at the alternator overlay a new wire cutting the old wire close to the connectors and soldering them in.


Check the battery temp sensor under the battery tray. There should be a continuious circuit between pin #15 in this same connector and #4 in the black #1 connector. About 1500 ohms would be good or at least a complete circuit.





Customer: replied 7 years ago.
Open circuit through the temp sensor. Of course, I would have been looking for a Corrosion problem" as about a year ago her battery began failing (chalked it off to normal aging), but the wire harness and positive cable were badly corroded. So told her (my daughter) to get someone to replace the battery wiring harness. After that, seemed to work OK for 9 months or so when she began to have intermittent charging problems. She had the alternator replaced then, but things never worked quite right after that.

If the temp sensor under that black plastic "button" on the bottom of the battery tray? Is there a way to temporarily short it to see if things work right instead of taking the risk of mistouching a terminal on the PCM when checking for voltage?

The sensor will "tell" the PCM if the battery is too hot to charge or may be frozen and then it will not charge either. The sensor may be as much as 7000 ohms so be sure it really is open.





Customer: replied 7 years ago.
You're right. 12,000 ohms

They don't give me a range so I have to think that the connection is poor and the PCM is reading too cold to charge the battery.


I would need a scantool to see what the PCM thinks the battery temp is.


If you had a resistor to put about 5000 ohms across the sensor connector that would tell if that is the problem.


I will look a little deeper to see if I can find a spec for the sensor but all the info is written with you having a scantool as the basis.


Customer: replied 7 years ago.
Let me do some digging in my electronic parts cabinet. I'd rather diagnose, but I guess a sensor wouldn't cost that much if I just wanted to be a "fitter" which is what prompted me to go back to being a home mechanic. I'm 65 and am not as limber as I used to be so I try to give this type of work to the "shop" but oftentimes one gets a mechanic who just wants to replace part after part, and that's what has happened with this project. I bought a Hayne's manual, hoping it would do what you're doing for me, but the manual does not have wiring diagrams and stops short of diagnosing the PCM, telling the reader to "go to the "dealer." Unfoirtunately, my independent mechanics ask friends at the dealership too as the general shop manuals are about 3 years behind the models.

I see you worked at a Hyundai dealership. When my 2009 gets old enough to start giving me troubles I'll want to contact you for advice!

Anyway, give me some time to look for the resistor. I have to unpack some boxes in the garage.

OK I will wait for your Hyundai to age!!


You can use a rheostat if you have one that you can adjust so it reads 5000 ohms or a 4700 ohm resistor is pretty common. I would just want to be about half of what the circuit reads right now.


I don't have a price for the temp sensor but it can't be that much, just be sure there is no corosion in the connector.



Customer: replied 7 years ago.
Well. I didn't find a resistor, but I did find a POY which I "tuned" to 5K ohms. I just haven't figured out how to pull the connector out of the bottom of the sensor without taking out the battery tray (something which I did last week because I "lost" my 12mm wrench down the fenderwell below the headlight!)

I'll study the pic some more befor getting out the sockets again.

If I do bridge the sensor with the POT, then do I go back to starting the car and looking at voltages at the battery?

I think you are going to have to clear the code to tell the PCM that it is OK to charge the battery again, you can "borrow" a code reader from the autoparts store on a buy/return basis. Most of the stores have a tool program.


With a code reader you should be able to clear the codes and check to see if the field voltage is back so it will charge again. The PCM supplies both the voltage and the ground for the field of the alternator so it has to be convinced that it is OK to charge the battery.


Not the best design but it hasn't changed in 20 years.



Customer: replied 7 years ago.
OK, but that will have to wait until tomorrow as the parts stores are probably closed now. Can I keep this case open? If so, how do I do that (in case my daughter - or I - accidently close the browser window.

When you login to the JA site you will see this question so you can post to it and I will respond as soon as I see your post. IT will be open as long as you want it to be. You should have a ton of e-mails about replies and posts and such, all will have a link to this post so you can get to the site from there.


Let me know what you find and what I can do to help





Customer: replied 7 years ago.
It's "tomorrow" and I got a scantool. I cleared the codes and went back to checking things. First, I started the car and observed the voltmeter on the dash. As I charged the batter last night, before I started the car it was reading 12-13V. After the car started, it read 10.5-11V. I turned the ignition off and put the scantool back on. It reported there were no codes in the DTC.

Next, I checked the resistance of the battery temperature sensor by placing a couple of insulated alligator clips on the pins of the sensor and checking impedance. My VoM read 11K Ohms.

Next, rereading last night's posts, I saw you mentioned checking the voltage at both ends of the ree/white wire (it's blue/white on this car). As there is no way to push a pin into the back of the C3 connector, I took the same insulated alligator clips, pulled plug C3 and put the clip on pin 25 of the PCM (I didn't want to just touch the prod to the pin last night because I was worried I might short it against an adjacent pin). With the battery sensor disconected, and with the engine running, I got 0.2V at the pin. Next, I put the temp sensor back in the circuit, repeated the test and got 0.02V at the pin.

So, with an ODBII device ready I am now ready for more instructions.

#25 is the power to the field of the alternator so if there is no power the PCM has failed. I know you replaced it, I'm hoping you still have the original.


Install the original and see if you have a change in the voltage. Have you had any luck getting a resistor to substitute for the temp sensor? Something about 5K should be about right.


Does the scanner do anything more than read codes?


Maybe you have a charging voltage or battery voltage or en=ven better battery temperature data point?



Customer: replied 7 years ago.
I have the original still. I can put it back in.

I'm taking it I did the correct proceedure to test the voltage at pin 25? Actually, I didn't start the car when I had that jumper on the PCM. All I did was turn on the ignition.

While you're reading this, I'll put the original PCM back in and put a jumper on pin 25 of C3 (and clear its scan codes first).

The scanner in an INNOVA Can ODBw & 1. Quickly, there are instructions on D/: scan codes, erasing the scan codes, and doing an I/M readiness check.

You will have to have the engine running as the PCM does not turn the generator on until the engine is started and the idle is stable.


I see that the connector pinout lists a white/blue wire at pin#25 and the wiring diagram shows a white/red so that is the difference.


You will not be able to see data with that scanner but it is OK to get and clear codes with.


You can check the voltage at the alternator if you want, as long as you checked to be sure there is a wire between pin #25 and the alternator.


Let me know if you have voltage with the engine running then you can check for ground (one lead to the positive battery terminal, look for voltatge with the other lead) at the dark green wire



Customer: replied 7 years ago.
Ok, I'll do what you said.

But, after I reinstalled the original PCM, I checked for scan codes. There were none.

Next, I pulled plug C3 and checked the voltage with the ignition switch on, not starter (as I don't know how the vehicle would run, or even start, with plug C3 disconnected. I got 2.11V and then it started dropping. I quit looking after it dropped to 2.07 V.

So now, I'll plug C3 back in, check voltage at the battery terminals, start the car, and check voltages again at the battery terminals and report back to you. I'll also push a pin through the back of the socket on the alternator at the blue/shite wire and give you that voltage too.

The charging system codes are two ignition codes so you will have to cycle it twice to have the code set.


OK the alternator is the only reading that you really need. If you have battery voltage there then you will be sure there is a complete wire between the PCM and the alternator and the PCM is supplying voltage. Next you would check for ground to the dark green wire.



Did you have any luck finding a resistor for the temp sensor?





Customer: replied 7 years ago.

The voltage at the battery before starting is 12.87V

After starting is 12.30V.

With the engine running, the voltage from the jumper pushed in the back of the alternator at the blue/white wire was 0.05V.

Yesterday, I had a POT set to aroung 5K. Shall I pull the temp sensor, replace it with the POT at 5K, then do these same tests?

I would install the new PCM plug in all the connectors and check the voltage at the alternator with the engine running and the pot in the temp sensor connector.


If you don't get voltatge to the alternator the PCM is no good.


Sorry but it does look like the PCM is bad.



Customer: replied 7 years ago.
I still get the same readings with the POT in place.

While waiting, I put a pin to the green wire and checked resistance to ground. It seems to have infinite resistance with the plug pulled and high resistance with the pin pushed in to the alternator. Is this right?

If it's the PCM, and I've had two failures, then could there be something elso that is causing the PCM to fail. I don't know what these guys do at autocomputer exchange (Davie, FL) to really rebuild these things. The tabs on the back of the refurb unit don't show any sign of having been bent.

I've read about the circuit board ground being different from vehicle ground and of insulation rubbing thin on the PCM mounts. Are the mounting screws supposted to be insulated from the PCM by paint, or should the tabs be filed for good contact?

I do know the source that you got your PCM from and I won't do business with them again. If you can get your money back I would.


The ground for the PCM is hard wired onto the circuit board and brought out on the grounds of the connector so the case will not affect this. I think you have a bad PCM and your original had failed as well. A new PCM from a reputable supplier should repair this issue.


I should have asked where you got it from first - sorry.


It is late and I need to be up early tomorrow so please excuse me.



George H. and 2 other Jeep Specialists are ready to help you
Customer: replied 7 years ago.
I theory we're finished.

But, now the rest of the story.

I was going to put together a narrative before I took the car to the dealership, but as I was writing, I recalled the step where I was to check the continuity of the white/red (in my white/blue) wire from the back of the alternator to C3 #25. If you go back to our notes, you'll see I said I got no continuity (the circuit was open.) Having procured a box of butt join splices - usually I use these on old truck marker lights on the farm - I spliced another wire running parallel to the blue/white wire. When I did. I got continuity pin to pin. Next, I plugged the connectors back in, started the car, and viola, the generator showed charge. I think we got sidetracked into checking the battery thermal sensor switch. Now, this white/blue wire does not appear to go anywhere in betwen the alternator and plug C3, but, somehow, it got an open somewhere in one of those cable runs.

Glad you stuck to it, I thought you found no power to the white/blue wire from the PCM so that is why I only mentioned checking the continuity and being sure the voltage made it from the PCM to the generator a couple of times.


I am glad you got it handled but I would use your original PCM if possible and return the junk (reman) one to Davie. I have seen the inside of those and I was an electronic tech in an eariler life so I would not trust it to last


Thanks for filling me in



Customer: replied 7 years ago.
See my 8:04 and 8:30 replies. Must have been a miscomminication. Perhaps I sidetracked you when I asked for the plug number vs. color definitions. At 8:30 I stated got no voltage, then did a continuity check. I see I didn't explicitly state the results of my continuity check. I guess because I implied that since I got no voltage I was confirning it with no continuity.

Yes, I kept the original PCM. The rebuilt case looked dirtier than the original, and there just didn't seem to be any indication that the case had really ever been opened.

A long time ago I rebuilt a Triumph spitfire transmission. Finding there were few transmission parts I learned how the "rebuilt transmission" worked: get transmissions out of the junkyard, do little to nothing to them, and price them according to how they "looked and sounded on the bench" and the likelyhood of getting a return during the length of warantee. It takes a while for most people to put a car back together then drive it long enough to detect another failure. Hope by then the waraantee time is over! Yes, I could get a few mail bearings, maybe some gears, but forget about bushings, shafts, forks and synchronizers.

Thanks again for giiving me the circuit information that I couldn't seem to find elsewhere. My daughter is estatic - now if she'd only move out of the house!

With the transmissions they get power washed and a coat of silver paint to raise the price!


I heard on a financial planning show that 2 million have moved back in with their parents so be careful what you wish for.


Thanks again for letting me know the outcome



Customer: replied 7 years ago.
After being out for 8 years, she moved back in 10 years ago!
Maybe she is going to take care of you in your dotage!
Customer: replied 7 years ago.
I think that's what my wife is hoping for, but then, when sees how lazy she's gotten, she wonders. In the meantime, my married daughter, who lives on the East Coast gets all jealous about how her sister is "freeloading." It must be a woman thing. Perhaps that's why I look for things to fix rather than sit around the house and get all caught up in the emotional discussion - even if I occasionally get accused of being cold and detached. That part must be a man thing. (As we were chatting I was reordering a lawnmower part rather than buy a new one.

I know what you mean, I bought a junk house because it had a garage large enough to put a lift in because I drive junk and fix it rather than buy new!


Let me know if there is anything I can do for you in the future