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Ok, Im stumped, i recently picked up this 5.2l v8 grand cherokee. i bought it knowing it needed a new computer. i replaced the computer, started it up and drove it almost all the way home before the battery died on it. i didn’t realize on my way home it wasn’t charging. i switched batteries with my other vehicle and it did the same thing. so i know battery is good becasue i put the jeep battery in my wifes car and it charges fine. So i moved on to the alternator, took it to orileys had it tested and it was bad. luckly the previous owner gave me the receipt, it was only 6 months old. They replaced the alternator under warranty, but it still isnt charging. well i noticed when i took it off that the tensioner wasn’t springing back like it should so i replaced that. Still no luck. i read up on these jeeps and found out that the voltage regulator is in the computer not on the alternator. so i thought that maybe i bought a computer (off ebay) with a defect with the voltage regulator which i hear is common for these jeeps. I saw where you can add a external voltage regulator to bypass the computer which i did, but still not charging. i took the new alternator back twice to different stores to test, both times it tested good. i also ordered a second voltage regulator and same results. i have cleaned all connections at the battery and directly grounded the voltage regulator to the negative terminal of the battery and ran positive directly to the battery for a quick test of the voltage regulator. what can i be missing? hooking the voltage regulator should bypass all ecu control what else can i check? any help is much appreciated. thanks
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The alternator will need stator field voltage. Modern GM alternators can have as few as one terminal with the regulator built into the unit. Some alternators need to have voltage applied to the stator before it will produce current at the output terminal.
The external voltage regulator that I hooked up had 2 terminals the center one has two wires coming off of it . One goes positive ignition switch the other to one field post of alternator. The second post of the voltage regulator goes directly to the second field post of the alternator. The body of the regulator is grounded to body ground and to the alternator. So If I understand you correctly in my senario I do have power going to the alternator via the wire coming off the center post of the voltage regulator.
Not sure what the wiring for your regulator is but here is a sketch on how your Cherokee should be wired. If you measure between the two field terminals with your voltmeter there should be voltage present in order to generate current at the output terminal.
i will check this morning for voltage between the two field connections for voltage. let say there is voltage but battery doesn’t charge..would that mean alternator is bad? Then what about the opposite scenario, no voltage between two? bad ground, bad voltage regulator?
Are the field wires DarkGreen and DarkGreen / Violet as depicted in the drawing? If would be useful to measure the voltage (with the engine running) between each wire and the NEGATIVE terminal of the battery and then again between each wire and the POSITIVE terminal of the battery. What you want to determine is which wire is faulty and what positive or negative potential is being expressed on each wire by the computer. Even if you are not currently using the wires from the PCM. It would be best to eventually prove whether the PCM is defective or you have a wiring fault.
Now back to your bailing wire solution. Please supply the part number of the regulator you used or the vehicle it came from and I will attempt to determine how it should be wired to make your generator work as an alternator. I have done stuff like this before and can tell you the results will be somewhat less than those originally provided by the Detroit engineers.
First Thanks for working with me…here are my results
the wires are green and green/violet and the model number of the voltage regulator is Tru-Tech VR125T.this first set is with the car off and external voltage regulator hooked up.
B+ to B+ on alt 1.6v
bat neg to alt neg post 0 v
B+ to field 1 10.8v
B+ to field 2 10.8v
B+ to Bat Neg 12.3vCar On / Voltage Regulator
B+ to field 1 10.6v
b+ to field 2 .46 v
B+ to B+ 18.37v
Bat Neg to alternator case = 0
B+ to alt Case = 11.87
Bat Neg to field 1 = 1.23 v
bat neg to field 2 = 11.51 v
Bat + to bat neg = 11.8 v
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Test 2 with computer hooked up (stock) Car Off
B+ to alt B+ =.04v
bat neg to alt neg post = 0
B+ to field 1 = 12.3
B+ to field 2 = 12.3
bat neg to field 1 = 0
bat neg to field 2 = 0
B+ to bat neg = 12.3Car on with computer hooked up.
B+ to field 1 = 11.3 v
B+ to field 2 = .7 v
Bat neg to field 1 = .7
Bat Neg to field 2 = 11.3 v
B+ to alt B+ = 14.4 v (looks like alt working?)
bat neg to alt B+ = 0v
B+ to alternator case 11.78 v
bat neg to alternator case = 0 v
B+ to bat neg = 11.6 V (not charging battery)I hope this all makes sense but to me it seems like the computer is working correctly but the charge isn’t getting to the battery for some reason? alt is putting out 14.4 at the reg bat + terminal on the alt. what do you think? Am i looking at the results correctly?
Again thanks for your help.Was the ebay seller that you bought the computer from based in miami florida?
If so its probably bad.
There are many over at jeepforum that have bought remaned pcm,s from a company down in florida and they always fails.I think that this is your problem.
Fast forward to 2 minutes and he will start to talk about the voltage regulator and common problems with it.i belive i found the problem or attest a problem. i found a bad fuseable link from the alternator to the pdc. i am headed to the autoparts store to pick one up.. ill keep ya posted.
I was gonna say check voltage at alt charging post and then check voltage at battery. If alt is over 13 volts but battery is under then check fuse. I can’t tell you how many times I’ve seen “professional” mechanics misdiagnose this problem. Spend 200 on an alternator and it does the exact same thing.
OK, the Fusible link was definitely the issue, so its fixed now sort of….the old fusible link was a 10 gauge wire, the auto parts stores around here only have 12 Guage fusible links. so i was stuck replacing with a 12 gauge. now i have the car running off the computer and its putting out at the battery 13.4 V. which i think is too low but at least its charging. do you think the lower charging is due to the thinner gauge fusible link? or do you think i should just replace all the positive and negative wiring on the alternator circuit?
GOOD JOB! Found it on your own! 13.4 is OK. Actually 13.6 to 14.4 is proper depending on the condition of the battery. Check the voltage at the Output of the Alternator/Generator and compare to the Plus battery terminal with everything on: Lights, AC, Fans, Etc. Remember the Computer can tell what the voltage is and controls the amount of charge. If the voltage on the battery maintains 13.4 I think you are good. If the difference in voltage at the Alternator/Generator and the Plus Terminal of the battery is greater than a volt or two I would order the proper link on-line and replace the 12 guage with 10.
Good that you found it.
You should try to find one thats the same size as the old one and replace it later to avoid any future problems.
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