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Alternator Wiring


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Hello - having some charging issues that I've traced back to the alternator for my 1988 MJ.   I'm want to make sure wiring is ok before I go out and rebuild my alternator.   The harness on the alternator has 2 wires - yellow and brown/tan.   The yellow wire has voltage when ignition is on, but I'm not getting any voltage reading for the brown/tan wire, and I'm wondering if that's a problem.    I've looked through a number of online forums, but haven't had much success in figuring out what these wires are for, and how to test them.

 

I'm assuming that one of the two wires (brown??) goes to the gauges.   My battery gauge is working properly, and no idiot light has come on (not sure whether there is one...)

 

Any help is appreciated!

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*******EDIT: The below post only applies to vehicles with a battery light NOT to vehicles with a voltmeter******

 

Take this of the grain of salt… This is from memory, decades ago…

 

Your 88 uses a GM alternator. With the ignition switch on, BOTH the yellow, and the tan/white wires, should show voltage. The yellow comes directly from the ignition switch.

 

However, the tan/white passes through the voltmeter, or idiot light, and then goes down to the alternator.

 

If this wire has been disconnected, or the idiot light burns out, this wire will show no voltage. The alternator will not work.

 

For test purposes, or even as a long-term repair, you can put a 10 ohm resistor in series to mimic the light/gauge.

 

So tap into the yellow wire, connect a 10 ohm resistor, and then connect this to the tan/white going to the alternator plug.

 

I forget all the details, but I remember "back in the day" in GMs that if the idiot light burned out, the dash would have to be pulled to replace the light for the alternator to work.

 

See page 24 of this manual for details   http://www.bteventures.com//mj1988electricalmanual.pdf

 

Now, having said all this, the more I look at the diagram, the more confused I get. I am not sure if everything I said above is true, but I am reasonably sure that if the tan/white wire does not have voltage the alternator will not work.

 

Hope this helps!

 

Gene
 

Edited by Gene
Correction based on info from Ohm
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*****EDIT The following applies only to vehicles with an indicator light NOT vehicles with a voltmeter****

 

Similar discussion

 

 

 

Edited by Gene
Corrected due to info from Ohm
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Cluster voltmeter uses C203_6 (12vdc) and C203_10 for ground, while the ALT lamp use C203_6 (12vdc) and C203_3 (Tan w/tr) for path back to alternator. When alternator is faulty, C203_3 (Tan w/tr) should supply ground leg and turn lamp ON. However this vehicle is voltmeter equipped. The reason I mentioned "its just an extra wire" is because it terminates nowhere with GAUGES/HI LINE clusters. Refer to 88 Elect Manual pg 96 (Instruments Clusters).

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Appreciate your help - I installed the new alternator and it seems to be working and charging  fine with no voltage coming through the tan wire.  

 

Seems like like a really stupid idea to wire a vehicle so alternator quits working when dash light burns out....   

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30 minutes ago, jbellingham said:

Appreciate your help - I installed the new alternator and it seems to be working and charging  fine with no voltage coming through the tan wire.  

 

Seems like like a really stupid idea to wire a vehicle so alternator quits working when dash light burns out....   

That's not how it works. 

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1 hour ago, Jeep Driver said:

That's not how it works. 

 

Hi Jeep Driver,

 

Not sure about our Jeeps with a gauge, but at least some applications must have a working dash light. Please see page 87 of this link.

 

https://books.google.com/books?id=3q85p56_PxIC&pg=PA87&lpg=PA87&dq=OLDER+GM+NOT+CHARGING+BURNED+OUT+INDICATOR+BULB&source=bl&ots=nhJHY8UN0K&sig=qn_1iNBXbtp9oTwxuaVQpIatXhM&hl=en&sa=X&ved=2ahUKEwivgZygmI_eAhVMjqQKHdRmCDg4ChDoATADegQIBxAB#v=onepage&q=OLDER GM NOT CHARGING BURNED OUT INDICATOR BULB&f=false

 

Your thoughts?

 

Thanks

 

Gene

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