jbellingham Posted October 16, 2018 Share Posted October 16, 2018 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! Link to comment Share on other sites More sharing options...
Ωhm Posted October 16, 2018 Share Posted October 16, 2018 Clusters with voltmeter do not use the brown/tan (88 Elect Manual call it as tan w/tr) wire. Yellow wire is hot with KEY ON and during CRANK. Link to comment Share on other sites More sharing options...
jbellingham Posted October 16, 2018 Author Share Posted October 16, 2018 ok thanks - do you know what the tan wire is for? Is it supposed to have current coming through, because i'm getting nothing on my multimeter Link to comment Share on other sites More sharing options...
Ωhm Posted October 16, 2018 Share Posted October 16, 2018 It would connect the ground side of the idiot (alternator) lamp. In your case its just an extra wire. Link to comment Share on other sites More sharing options...
Gene Posted October 17, 2018 Share Posted October 17, 2018 (edited) *******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 October 18, 2018 by Gene Correction based on info from Ohm Link to comment Share on other sites More sharing options...
Gene Posted October 17, 2018 Share Posted October 17, 2018 (edited) *****EDIT The following applies only to vehicles with an indicator light NOT vehicles with a voltmeter**** Similar discussion Edited October 18, 2018 by Gene Corrected due to info from Ohm Link to comment Share on other sites More sharing options...
Ωhm Posted October 18, 2018 Share Posted October 18, 2018 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). Link to comment Share on other sites More sharing options...
jbellingham Posted October 18, 2018 Author Share Posted October 18, 2018 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.... Link to comment Share on other sites More sharing options...
Jeep Driver Posted October 18, 2018 Share Posted October 18, 2018 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. Link to comment Share on other sites More sharing options...
Gene Posted October 18, 2018 Share Posted October 18, 2018 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 Link to comment Share on other sites More sharing options...
Gene Posted October 18, 2018 Share Posted October 18, 2018 Actually looks like this is addressed in post by Ohm above...sorry....thanks Gene Link to comment Share on other sites More sharing options...
Gene Posted October 18, 2018 Share Posted October 18, 2018 I probably should have realized that you do not question the electrical knowledge of someone named Ohm... Gene Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now