Hello all I have a 87 2wd pickup and the charge and brake light stay dimly lit at all times. I have 14.3V at the battery all connections at the battery are good as well as engine and frame grounds. I have unplugged the master cylinder level switch and park brake switch. The switches work properly and turn the light on to a full light. Any suggestions on where to go next? Thanks

 

sounds like alternator

 

something is grounding back through that circuit

causing the lights to be dim

brake and tail lights working ??

Any other electric circuits not working ??

 

All brake lights and turn signals work correctly. So I took a peek underneath and found someone T-tapped the holy hell out of the rear end in a poor attempt to add a trailer pig tail. I think that may indeed be causing the issue/ backfeed

Thanks

 

Solved the pig tail issue but the problem remains.

 

Does your brake master have the sensor switch for low fluid? Check that and your alternator plug.

 

It is possible the yellow wire on your alternator plug is either corroded at the plug or the wire might be open someplace .

Still seems like something grounding back via that circuit

 

Yes and I tested it as well as unplugged it. Also the brake switch for the parking brake as well.

 

I'm thinking that sounds like a good next step. I checked the ALT plug for corrosion and it is clean as a whistle. I'm going to ohm out the yellow wire.

 

Quick Check Point...


That should have 12 volts while engine is running. If it doesn't, check other parts of charging system as described in my thread. Search "How Charging System Works"

 

How about 14.27V DC while running? Sounds like the charging system is working pretty good.

 

That's what I cant seem to put my finger on. The charging system is working correctly and proper voltages are present everywhere yet the light is still dimly lit. Thank you RAD4Runner for the help. After I read my previous reply it sounded sarcastic and that's not how it was intended.

 

try hooking it up to another vehicle with jumper cables, both running, but with your alternator disconnected. no funny lights means you need an alternator. or you could take yours in to have it tested, some parts stores can still give them a full test and let you know if a diode is bad.

 

Seems like I remember a similar problem I had years ago with a Delco Remy system.

As I recall, it turned out to be a failed diode in the alternators rectifier bridge.

If possible, substitute another alternator and see what happens.

 

If alternator is disconnected you will not have get charge error light because there is no "L" wire to bring the negative side of charge and brake error lights close to ground.

However, I agree with having alternator tested.

WHEN THERE IS 14.7V at "L" (yellow highlight, charging voltage in normal range) on the negative side of the charge and brake error lights, the only way for charge and brake error lights to come on is
1) IF the positive sides (pink highlight in schematic) connected to 7.5 "IGN" fuse were much greater than 14.7V,
OR
2) If the diodes in combo meter (dash) were bad, and the voltage at IGN switch were much lower than 14.7V. But then, the truck should not run in this scenario.

 

I should have been more specific, and I don't have an EWD to verify if it would work, but if you disconnected the big wire from the alternator that charges the battery while jumped to another running vehicle you may see the charging from the other vehicle and have no warning lite on. leave the control circuits connected to the alternator though. it's still a better test to pull it out and have it bench tested for "diode ripple" I think they call it.

 

Oh we're all cool

Yes, failed rectifier diode may cause odd symptoms like above, even when simple DC measurements show correct voltages.

Best of luck.