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Discussion Starter · #1 ·
Question for everyone. My Battery light and Brake light on the dash are dimly lit. Really dim but on. I changed out a failing alt last week and also changed out the battery. When I first did the swap they were also lit but went out after 5 minitues of idle. Since that time they have been out until this morning. Ran by 2 diferent autopart stores to have them both tested, on the vehicle, and both passed at each different store. Any ideas? Even though it is new and tested well on the vehicle, an alt swap seems the most viable plan but before i do that I thought I would check with you all. :) Also, if the rpm's increase there is not a change in brightness. It stays the same regardless of the driving style so to speak.

Thanks
 

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Discussion Starter · #3 ·
Alright...change alternator. It is a new one, reman at least, so let's assume it was a good one. What could be causing the alternator to fail? I realize it would be simple enough to just chage it out, which i will do, but I want to get to the root of the problem and fix that as well. Any suggestions?
 

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The alternator has one input and TWO outputs.
The main current output is the stud on the back of the alternator case (Pin B) that connects to the fuse and relay panel.
The other output (Pin L) is in the 3 Pin wire harness, it controls the battery/brake lamp, the Shift Lock controller and the AC/Heater relay .
The input (Pin IG) is +12 VDC reference for the voltage regulator from the Ignition Switch.
The third Pin (D) in the connector is not used.

I am reasonably sure when the store tests the alternator it only tests the main output from Pin B and does not test the output from Pin L. There could be a problem in the 3 Pin connector harness or in the alternator. However, if there is no output from Pin L you should have a shift lock problem unless it has been disabled.
 

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Discussion Starter · #5 ·
Thanks Buster. I should receive a new 3-pin harness on monday so I will replace next week. (Thanks again for that source:)) Hopefully that is the culprit. Issue with the alt passing the test is they won't exchange it under warranty since it "passes". I'll let you know how the new harness goes.
 

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On one of the 95s we had I put a new alternator on it when I got it. Always had the dim battery and brake light. I traced wires all over the place, changed a diode on the back of the dash and so on. Nothing would make it stop. Luckily we had more than one Troop at the time so I swapped alternators and problem fixed. The alternator in question worked fine for at least a year and was working fine when I removed it - it just had some sort of dork up inside of it that caused the lights to glow. I never had the questionable unit bench tested but would believe it would have passed as it worked fine.
 

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Discussion Starter · #7 ·
Thanks Van. As long as I am not doing any damage to other electical I don't mind the lights. Just don't want to be frying the ECU.
Appreciate the help guys :D
 

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Just because the alt is testing good, doesn't mean it is.

these trucks are volt hungry eaters, they'll find any weakness in anything.

it's your alternator.
 
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