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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 29523
Experience:  GM Grand Master Technician 2007. 14 years experience.
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2000 GMC Safari: intermittent..the alternator, serpentine belt

Customer Question

I have a 2000 GMC Safari. I am getting an intermittent battery charging light. I have replaced the alternator, serpentine belt, and belt tensioner. It is still doing it. I have also check all of my battery connections, including the one at the starter. All good.
Submitted: 9 years ago.
Category: Chevy
Expert:  GM Tech (Cam) replied 9 years ago.
WelcomeCustomer Your van's charge indicator system is as follows. The single small wire that is at the plug on the alternator goes to the VCM (engine computer). This wire is used to activate the field coil and provides charge feedback. The VCM sends a message to the cluster regarding the state of charging. With out a scan tool and it acting up, the only thing left that you can check is the condition of that wire to the VCM. If you have access to a scan tool, what you want to do is find the charging data in the VCM and see what the VCM is seeing at the time of the trouble. Please feel free to ask if I can assist you with this charging issue.
Customer: replied 9 years ago.

Where is the VCM located in the van?


How would the scan tool trouble shoot the cause if this problem?


If you had a scan tool and say it was acting up, what would the data point you too?


Where could I get a scan tool? Do you think Auto zone or Advance auto would carry one - even as a rental or something?


Expert:  GM Tech (Cam) replied 9 years ago.
Very good questions. The VCM is on the left side of the engine compartment on the lower side of the battery. Because the VCM transmittes the charging status over the data line, it, the VCM, will be responsible for monitoring the alternator. Lets say the charge light is on, the data you can read from the VCM may show that the alternator is operating properly, but if the light is on, this would mean the cluster is not displaying the proper charge status. Another scenario, in the charge light is on and the data from the VCM show the alternator is not charging, but a test of the alternator shows it is charging properly, this would point to the VCM or the wire that goes from the VCM to the alternator. Scan tools are very costly to buy, $3000 or so. As for Auto Zone or a parts store like that having one as a rental, I cannot say, but it would be worth asking. My suggestion, check that wire that goes from the alternator to the VCM. Based on the information you have provided, I would first check the VCM for any codes. Then check that wire. Code readers are inexpensive, $30 or so, and will retrieve any codes from the power train, and clear them.
Customer: replied 9 years ago.

I know the altenator is good because I had it tested at Autozone. So, the alternator is not the cause. I had the alternator replaced with another one just as a precaution, and it was still doing it.


So what would that tell you based on your different proposed scenarios if we rule out the alternator?


As it relates to the location of the VCM, I know the PCM on my van is next to the battery within the engine compartment. ( bottom left side of the battery (looking into the engine compartment). We are not confusing the PCM and the VCM are we?

Expert:  GM Tech (Cam) replied 9 years ago.
Ok. If we have ruled out the alternator, that leaves the wiring to the alternator, the VCM, (which stand for vehicle control module and is the same as a PCM, but it is called a VCM in your van) or the instrument panel cluster.
Customer: replied 9 years ago.

I did inspect the PLIS connector and the brown wire at the alternator, it looked fine to me. Since I initially replaced the alternator about 6 months ago and the this issue has started slowly and now ramping up, I could have a partial broken wire at the PLIS connector. That is the only thing that would see some fatigue and stress over time.


Anyway to trick out the PCM/VCM to think the alternator is OK? If I disconnected the connector from the alternator, could I connect the brown wire to gnd or to 12V to trick the PCM/VCM??


I want to rule out the PCM/VCM as my last possible cause of the problem.


Based on your experience, how would you check that the brown wire/connector was good?

Expert:  GM Tech (Cam) replied 9 years ago.
The way to check the wire is to first, visually inspect it for chaffing, simply because it is when this wire get grounded that the VCM commands the charge light on. Second, test the resistance of the wire at each end, and then to the ground. I think you might be able to trick the VCM into thinking all is good by connecting the brown wire to 12v, but I have never tried this. I agree with your wanting to know 100% before replacing the VCM .
Customer: replied 9 years ago.

OK, last question and we can go to bed and you can get paid... I have looked at my PCM/VCM. There are four big connectors/wire harnesses. Two on the top and two on the bottom.


Do you know which one contains the battery charging (brown wire)?


I think there is more than one brown wire in that big clump of connectors..

Expert:  GM Tech (Cam) replied 9 years ago.

There is a zoo of wires. I will tell you the wire and connector you need. Of the 4 connectors, you need C3, which is clear. This is the colour of the connector portion that goes into the VCM, and if I remember, the colours are marked by position on the case of the VCM to aid in ID and installation. The terminal you need is #5. Here is a picture of the connector. It is looking into the VCM side of the connector.

Object Number: 358779  Size: CF

Please ask if you need more assistance.

GM Tech (Cam) and 3 other Chevy Specialists are ready to help you
Customer: replied 9 years ago.


I checked the connectors. They are marked red, black, blue and white. Which one do you think it is?

Expert:  GM Tech (Cam) replied 9 years ago.
White is considered clear.
Customer: replied 9 years ago.
Thank you for your help. If I need your assistance again on this issue, how can I get you for my expert again?
Expert:  GM Tech (Cam) replied 9 years ago.
You can sign in to this website, go to reports, and this topic will be displayed. You can reply to it and it will re-list it. You can also create a new question, and specify in the first portion that you want me to be your expert. Thanks for the accept and the bonus. I appreciate them.