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Ask Ron Your Own Question
Ron
Ron, ASE Certified Technician
Category: Jeep
Satisfied Customers: 34757
Experience:  34 year working in the automotive field as a Tech.
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Waiting for response from jeep mechanic

Customer Question

Waiting for response from jeep mechanic
Submitted: 3 months ago.
Category: Jeep
Expert:  Ron replied 3 months ago.

Hello. My name is ***** ***** I am happy to assist you today. Give me just a moment to review your post, and I'll be right back with the help you need.

Expert:  Ron replied 3 months ago.

What is your question ? Please explain with complete details.

Customer: replied 3 months ago.
I have a 94 Jeep Grand Cherokee 5.2 liter that has voltage issues when it warms up.
Customer: replied 3 months ago.
Changed the battery and alternator still same problem
Expert:  Ron replied 3 months ago.

What is the engine running voltage from the alternator at the battery when the problem is occurring?

Customer: replied 3 months ago.
I had the auto parts shop check it and they said it was not consistent but around 14 volts
Customer: replied 3 months ago.
The fan rear defrost lights all make the voltage drop more
Expert:  Ron replied 3 months ago.

The normal charge rate on a properly charging alternator should be 13.6 to 14.2 depending on the amount of load. Is your staying in this range?

Customer: replied 3 months ago.
I'll wait for your phone call that I just requested
Expert:  Ron replied 3 months ago.

The normal charge rate on a properly charging alternator should be 13.6 to 14.2 depending on the amount of load. Is your staying in this range?

Customer: replied 3 months ago.
Not sure if it is in that exact range but it stays around 14 on the gauge then for some reason once the car warms up the voltage drops. Sometimes it starts to stall
Expert:  Ron replied 3 months ago.

Your charging rate is controlled by the PCM (computer) and will go up or down depending on load. Are you able to do some testing when the voltage drops if I send you you steps to do so?

Customer: replied 3 months ago.
Turn the fan on and it drops more yet. If I give it gas the voltage comes back to 14 volts
Expert:  Ron replied 3 months ago.

Your charging rate is controlled by the PCM (computer) and will go up or down depending on load. Are you able to do some testing when the voltage drops if I send you you steps to do so?

Customer: replied 3 months ago.
I'd love to but don't have a meter
Expert:  Ron replied 3 months ago.

All you will need a a volt meter. With out testing the circuit there is not way to know what is failing. The problem may very likely be with the PCM. Can you get your hands on a volt meter?

Customer: replied 3 months ago.
i can try is changing the pcm difficult? I do have mechanical skills so I'm not totally lost
Customer: replied 3 months ago.
I'll make a quick call in search for a meter. Should I stay on this site and text you if I find one
Expert:  Ron replied 3 months ago.

If in fact the PCM is the cause , no its not hard to change but before you change any parts you should do the testing.

See the steps in the link below to test it as well as the factory wiring diagram for testing reference.

https://dl.dropboxusercontent.com/u/104923897/Alternator%20Jeep%20Jan%2010.pdf

https://dl.dropboxusercontent.com/u/104923897/Jeep%20wiring%20diagram%20Jan%2010.pdf

Customer: replied 3 months ago.
Working on getting a volt meter . Will you still be available?
Expert:  Ron replied 3 months ago.

Yes Im here for a while yet today and all day tomorrow as well.

Customer: replied 3 months ago.
Ok I'm working on it now I'll shoot you a text when I get one
Expert:  Ron replied 3 months ago.

Okay . Im here if need be.

Customer: replied 3 months ago.
Heading to my work will you be available in 20 min
Expert:  Ron replied 3 months ago.

Yes, Ill be here.

Customer: replied 3 months ago.
Ok got a volt meter but the voltage isn't acting weird yet
Expert:  Ron replied 3 months ago.

Okay then you may want to wait this the problem is occurring to start your testing if in fact you actually have a problem.

Customer: replied 3 months ago.
It won't start acting up until the car warms up sounds weird
Customer: replied 3 months ago.
Something is definetly wrong I'll let it idle for a minute hopefully it will do it while I'm still talking to you
Expert:  Ron replied 3 months ago.

Okay try that and let me know.

Customer: replied 3 months ago.
Ok
Customer: replied 3 months ago.
While I'm waiting for this to happen one more thing . Once the car does warm up and it starts to act up things like the fan speed drops in corilation with the voltage dropping on the gauge.
Customer: replied 3 months ago.
I wouldn't be concerned about it but it makes the car almost stall.
Customer: replied 3 months ago.
Ok now when I turn on the fan the voltage is starting to drop. What do I do next
Expert:  Ron replied 3 months ago.

Its normal for the voltage to dope some when you put a load on the system like the blower motor, how far dose it drop?

Customer: replied 3 months ago.
Should I check that at the battery? On the gauge it drops close to the red line and then the fan speed changes. I understand there will be a drop in voltage when you run an accessories
Expert:  Ron replied 3 months ago.

Yes connect you volt meter to the battery and read the voltage while its dropping.

Customer: replied 3 months ago.
I'll be sitting at a stop sign with the fan running the gauge will rapidly drop and and the fan speed drops with it
Expert:  Ron replied 3 months ago.

Okay connect you volt meter to the battery and let see if the voltage reading there match your gauge.

Customer: replied 3 months ago.
The voltage started out at 14.5 with nothing on. Now it's at 14 and the gauge has dropped with it
Expert:  Ron replied 3 months ago.

That is totally normal, as long as it dose not drop below 13.6 volts.

Customer: replied 3 months ago.
Even a rapid drop in voltage that makes the fan speed drop dramatcally
Customer: replied 3 months ago.
Its starting with the fan on high and the rear defroster on it goes to 12.9 at the battery
Customer: replied 3 months ago.
Once I turned on the rear defrost the voltage dropped off
Expert:  Ron replied 3 months ago.

The fan speed is going to change normally as the voltage decreases and increases but if your voltage at the battery is not dropping below 13.6 volts with the engine running then you do not have a problem. If its dropping to 12.9 then take a ground wire with the engine running and probe the green wire on the alternator and see if the voltage come back up as it should . If it dose climb back up then your PCM is faulty.

Customer: replied 3 months ago.
Are u still available
Expert:  Ron replied 3 months ago.

Yes Im still here, did you ground the green wire with the engine running?

Customer: replied 3 months ago.
Checked from what I believe was the green wire with the other wire on the ground and it was 12.6. Brought it back to the auto parts store to check the alternator with the rear defrost off it passed once I turned on the rear defrost it failed
Customer: replied 3 months ago.
What do ya think the new alternator is bad?
Expert:  Ron replied 3 months ago.

What I need you to do is with the alternator on the engine and running back probe the green wire at the alternator with a ground wire and see if the voltage starts to climb. Did you do that?

Customer: replied 3 months ago.
With the car running I put a probe on the green wire on the alternator and the other probe to ground and it read 12.4 and didn't climb. With the rear defrost off it read 13.1 and stayed there
Expert:  Ron replied 3 months ago.

Thats is not what I need you to do. I need you to ground this wire. Jump a wire from the negative battery post and probe the green wire with the ground.

Customer: replied 3 months ago.
Got it
Expert:  Ron replied 3 months ago.

With this wire ground the alternator should full field and charge at the full rate. If it dose then the problem is with your PCM. Let get it grounded with the engine running and see if the voltage climbs up as it should.

Customer: replied 3 months ago.
Makes sense sorry for the miscommunication
Expert:  Ron replied 3 months ago.

No problem, lets try this and let me know the results.

Customer: replied 3 months ago.
One more question there is three wires going to the alternator two are smaller ones and one bigger one . When I was screwing up I had one meter probe on the battery ground and poked one of the smaller wires and read voltage is that the wire I should probe to ground or the other small one
Customer: replied 3 months ago.
Have patients please
Expert:  Ron replied 3 months ago.

Let me know when you readyl

Customer: replied 3 months ago.
Should I be probing the small wire that had voltage to ground or the other one
Expert:  Ron replied 3 months ago.

You need to probe and ground the dark green wire at the alternator. It is the only dark green wire.

Customer: replied 3 months ago.
Ok Ron I finally rigged something to ground the green wire. The voltage was sitting at 13 volts then when I jumped the ground it went to 15.1
Customer: replied 3 months ago.
Hope your still there so does that mean replace the pcm
Customer: replied 3 months ago.
Thanks for your help and patience with me. If I had the right tools for the job it would have been a little easier for me.
Expert:  Ron replied 3 months ago.

That dose point towards a problem with the PCM , for some reason its not allowing the computer to keep up with the load on the charging system. Your very welcome and thank you as well.