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Dan
Dan, Technician
Category: Jeep
Satisfied Customers: 24426
Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
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99 Jeep Wrangler 4.0 L with manual trans. Parts store tester

Customer Question

99 Jeep Wrangler 4.0 L with manual trans. Parts store tester says alternator good, battery good, diodes good, voltage regulator bad. Replaced ECM module and voltmeter still tanks after a few minutes. Replaced battery because it was 6 years old, just to see. Problem is the same. Trying to figure out what is going on??
Submitted: 1 year ago.
Category: Jeep
Expert:  Dan replied 1 year ago.

Hello, my name is***** sounds like you probably have a wiring problem going to alternator. Check voltage on both small terminals at the back of the alternator. If both have B+, ground the Dark Green wire at pin 10 of the center Powertrain Control Module (PCM) connector to see if it charges, if not the dark green wire is probably broken. While grounding pin 10 check to see if dark green wire at alternator is grounded, if not run a new wire between alternator and PCM. Let me know how it goes or if you have more questions, thanks.

Customer: replied 1 year ago.
Dan, I have 0 volts between the small alternator terminals (green and green with orange stripe). Also show nearly 0 ohms (continuity) between pin 10 and the green wire at the alternator terminal, meaning the wire is not broken. Also have no connection (O.L. ohm meter reading) between pin 10 and the negative terminal of the battery and between the green terminal of the alternator connection and the negative terminal of the battery. What do you suggest next?
Expert:  Dan replied 1 year ago.

Did alternator charge when grounding dark green wire?

Customer: replied 1 year ago.
Haven't grounded dark green wire as yet. A little confused about the process. Do I cut it loose from Pin 10 and hook it to ground? Or do I splice in another wire (making a "Y") and connect that newly added wire to ground? In that scenario, there would be a connection to Pin 10, to the alternator, and to ground.
Expert:  Dan replied 1 year ago.

You can use a paperclip to backprobe connector at alternator so you can ground wire. Let me know how it goes, thanks.

Customer: replied 1 year ago.
Grounded dark green wire. Problem still there. A little more background, on start-up the voltage is at 12 V on the gauge. After driving about 1/2 mile, the voltage gauge immediately drops as far as it will go and the "check gauges" light comes on. If you shut it off and restart the engine, the process starts all over again. Very rarely, we will get a voltage gauge reading of 14V (where it was before the problem). It will sink to 12 V and then the scenario I already described happens.
Expert:  Dan replied 1 year ago.

Voltage should be closer to 14, alternator does not appear to be working at all. Does large wire going to alternator have battery voltage?

Customer: replied 1 year ago.
Have 12.5 Volts at large wire on alternator.
Customer: replied 1 year ago.
On three occasions, last time on Friday, the auto parts store tester indicated the alternator was good. It was the same store and the same tester all three times.
Expert:  Dan replied 1 year ago.

The dark green/orange wire should have power and the dark green should have ground. Check to see if dark green has voltage at alternator, if not check to see if there is voltage at pin C25 with vehicle running. Let me know, thanks.

Customer: replied 1 year ago.
Zero volts to ground on green/orange wire at alternator terminals with engine not running. To be clear, I unplugged the connector and measured the voltage at the connector terminals. It was zero volts to ground and 12.4 volts to the positive side.Back probed green/orange wire at connector C25. Voltage to ground when first started was around 16.0. Briefly the "check gauges" light came on because volt meter was high. After a short time, dipped to 12.2 volts and stayed there.
Expert:  Dan replied 1 year ago.

If you have voltage at PCM but not alternator it sounds like wiring problem, run new wire between alternator and PCM. Let me know how it goes or if you run into more questions, thanks.

Customer: replied 1 year ago.
With vehicle running, I have 12.2 volts to ground at the PCM C25 and at the green/orange wire at the alternator. I misunderstood your previous instruction and thought the vehicle should be running only when checking the voltage at the PCM Pin C25. Also have continuity on the green/orange wire between pin C25 and the connection at the alternator. The neither the green/orange or the green wire is not broken.
Expert:  Dan replied 1 year ago.

If you have good power on dark green/orange wire but alternator is not charging when grounding dark green wire that points toward alternator. Only other thing it can be is a problem with case ground on alternator. Let me know how it goes or if you have more questions, thanks.

Customer: replied 1 year ago.
I can't find the location of the case ground. There is the large single wire for the alternator output and the two small wires going back to the PCM but no others I can see, nor does there seem to be any place for another connection. Could you tell me where the case ground is located?
Expert:  Dan replied 1 year ago.

Case gets ground through mounts. You can use ground on case and battery + to check voltage, if readings is good you know case ground is good. Let me know how it goes, thanks.

Customer: replied 1 year ago.
I get 12.5 V to the case, so the case ground seems to be OK.I can certainly see your line of thought. Either the new PCM has the same problem with voltage regulation as the old PCM, or the O'Reilly Auto Parts tester is wrong. I don't know which is more likely.
Expert:  Dan replied 1 year ago.

If your sure alternator is getting good power on green/orange wire but grounding dark green wire doesn't help then problem is definitely alternator. For alternator to work it needs good case ground, ground on dark green, and power on green/orange...Then it sends power to battery on large wire. Let me know how it goes, thanks.

Customer: replied 1 year ago.
I have heard that if the alternator is bad then lifting a battery cable from the post with the vehicle running will cause the engine to die. Is that something I should try?
Expert:  Dan replied 1 year ago.

That doesn't tell if the alternator is bad but will tell you if alternator isn't working. But the alternator can stop working for many reasons so that test wouldn't do much. If your 100% sure green/orange has power and jumping dark green to ground doesn't activate alternator then I would replace again. Let me know how it goes, thanks.