Dim generator light with brakes on

Could be a sign that your fuseblock has a high resistance at one of the riveted connections. Do you still have the "stock" fuseblock? The tops of fuses #1, #2, and #3 are all supposed to be well connected together, and all be at the battery "+" voltage. As an easy test, use a jumper wire to (also, while everything is still plugged in) directly connect the A (light blue) wire on the top of fuse #2 to the GN (yellow/black) wire on the top of fuse #3 (which is what your fuseblock is supposed to be doing). If the alt light behavior improves, you've found the trouble; if the alternator light behavior is unchanged, at least you didn't have to get greasy for this test ...

 

It's weird only because you don't want the news that your charging circuit is unhealthy. That light is operating as designed but the vr/alternator isn't. The idiot light should completely extinguish above idle rpm. Brakes are only an additional current load like the headlights or turn signals and a pulsating dim light only indicates the inability of the charging circuit to easily maintain full 14.4v to the battery as it formerly did.

The charging circuit is a 3 phase ac system and when one phase drops out, the other 2 much work harder and they soon will fail as well. Current through through the idiot light from the vr to the alternator coils tell it how much voltage to provide and is normally small, hence no visible light. And it takes quite a bit to be dim.

You mention the alternator belt and there is always some possibility that it is slipping should you need some good news but I would think there would be an accompanying squeal. Check tension from topside. I disagree about 1977 alternator being weak (it has lasted 35 years) but that's immaterial to your problem.

 

Yes, most likely OK, but if the resistance is between the A wire bundle and its female terminal (i.e., a bad or corroded swagging) or between the A wire female teminal and the male terminal on the fuseblock, it would give the same symptom (as would any small resistance in the food chain battery -to- H connector -to- A wire -to- top of fuse #2.

Are you saying

1. That with the engine running, headlights "on", and brake lights "off" = no dim generator light present; but, with engine running, headlights "off", and brake lights "on" = dim generator light occurs?

or

2. With the engine running and any current load (headlights "on" alone or brake lights "on" alone) = dim generator light occurs?

 

You seem fixated on the brake light so I would suggest having a inspection of the rear of the car with someone pumping the brakes. Just to make certain the lamps and wiring are normal and not some odd quirky short. Plus doesn't require jacking up the car.

 

You are reaching the completely wrong conclusion IMO. I don't know why you are so resistant to trying this simple test, but just use a jumper wire to connect any of those other terminals that are at the (correct) higher voltage to the top of fuse 1/2/3 -- if the alternator light goes out, the problem is in the food chain "H connector -to- ignition switch -to- A wire -to- top of fuse #2" as I mentioned before (not the alternator). If the connections at your H (four way) connector aren't fried, most likely, you just have a small (unwanted) resistance in your ignition switch.

PS Those "other" terminals, that are at a higher voltage, are also connected to the output of the alternator via the H connector (just as the top of fuses 1/2/3/ should be).

 

If it was squealing (slipping) after cold start-up, then I'd tighten it up some, but if you have no other bad symptom, I wouldn't go in just to do that alone. Sure, if you're already in doing something else, why not, but not something that I'd worry about -- JMO.

13.6V doesn't seem horrible to me. Same sort of thing -- if your stater cranking seems weak (like the alternator isn't charging the battery well), then I'd worry about it more, but if it's all "working" you have nothing to fix

Your alternator doesn't know where the electrons are going, if the headlights (which draw a lot more current than the brake lights) are symptom-free, I can't see how the alternator can be the source of the glowing warning light when just the brake lights are "on".

 

1) If you want a diagram you can read, I have a deal for you.
2) Don't put too much stock in that thread, it has a massive amount of commonly thrown about misinformation about the charging circuit. It's like an alternator 'Wiki' IYKWIM
3) 13.6v is fine under most conditions which you don't specify.
4) The belt deflection is specified around page 63 of your OM

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