There is no fuse that would cause it.

Do the voltage checks in post #12.

Then, connect the meter between the big stud on the back of the alternator and the case and do the same tests.

Then, connect the meter to the brown wire by back-probing the connector and check the voltage with the key on with the engine both off and running.

 

This is correct, I didn't think there was a fuse. I was trying to remember the color of the wire and thought it was brown.
Good post.

 

Good morning my friends.
Problem solved!
It was the battery.
I measured it and it was 12.4 so according to your post too low.
I charged one night and the the light was on again but ....... When I start driving after 2 km the light start to off and on and at the end definitively off.

 

Keep an eye on it. Doesn't seem quite right. You really ought to get that battery load tested and that alternator tested also. Autozone and those places will do it for free.

AND if you do need an alternator, then I'd upgrade from that stock 63 amp unit (per autozone it looks like the stock alternator for a 77 is 63 amps). You can get a 94-amp 12-Si that will bolt right on.

 

The 12.4 was with the engine running at idle or just battery voltage?

 

I'm doubting that fixed it. The amount the battery is being charged has no effect on the light circuit. There is a circuit in the alternator that switches the light off once the alternator starts to charge and that circuit has no load sensing or voltage sensing in it at all.

 

IvanoMan,
There is voltage flowing through the light when the key is turned on and grounds in the circuit for the voltage regulator. The light is on because of this voltage difference.
When the alternator is producing electricity, it equalizes the voltage and the light goes out because it is no longer grounded.
There is a good explanation on this site which has been posted before. Scroll down to the "Alternator Warning Light"
http://www.pirate4x4.com/tech/billavista/Wiring/Part2/

 

That is a great explanation/article of alternators(probably the best I have ever read) and a nice summary of all the GM alternators!!!!^^^^^ Thank You.

Glad to see my memory is still pretty good concerning physics and the varying charging voltages that are possible depending on load/charging variables in my earlier Post #15. The summary is depending on load and charging the alternator voltage must be more than the battery voltage of 12.6v but realistically could be anything north of 13 volts ( which I believe you mentioned earlier as well) up to 14.6 volts, which I have seen in certain cars. A fairly normal range is 14.0-14.6 with an average of 14.2 but it is never just one number, but somewhere in these ranges......

 

I agree with all the above posts, it's not that simple.

As I stated in some earlier posts, it was intermittent. I had plenty of Sunday afternoons thinking I had "solved" the problem. Only to find out within a day or two that light came back on.
I made some mods within the last few years that have caused gremlins in my electrical system. I will probably make a new post about what I have learned and try as best I can to give credit where I found it. But yeah, anyway and for now, this forum rocks and I thank everyone for every post that they make. I haven't given back like I should, I'm trying, starting with this thread.

 

That's got some good info, but also some completely wrong info when talking about GM SI or CS alternators. The part explaining - "2) the alternator output is higher than the battery voltage." is complete BS. The light never indicates a weak or discharged battery. The explanation about the battery likely being weak if the light getting brighter when the engine is revved is fabricated BS.

The light might glow if the diode trio has failed or the main rectifier diodes have failed. It could also light if the wiring between the alternator stud and the ignition switch is damaged or broken. In these cases, the voltage between the diode trio pair and the ignition switch could be different enough to cause it to glow. The voltages should never be different enough to make the light come on with only a weak battery. So, the light would glow if the alternator output was greater than the ignition switch voltage, but never because of a discharged battery. It would do it because the electrical system somewhere between the alternator and ignition switch has issues.

 

What part of this is incorrect?
IvanoMan,
There is voltage flowing through the light when the key is turned on and grounds in the circuit for the voltage regulator. The light is on because of this voltage difference.
When the alternator is producing electricity, it equalizes the voltage and the light goes out because it is no longer grounded.

 

What part of what is incorrect????

 

You quoted me, so the question was and still is what part of my statement is incorrect. I didn't author the article you attacked, my post was for the OP in an attempt to give him a simplistic explanation to the alternator light function in the first part of the paragraph, nothing more. In the future, do not quote me.

 

2 choices guys....

1) you clean up the mess you created

2) I clean the mess up

if I do it you won't like it.....take the bickering offline or to PM but take it out of this thread
I am sure you both know how to use the edit post feature

 

 

...and yet, after warnings issued and 2 cautions posted within this thread by 2 different mod/admins, we still have people continuing the conflict. It was addressed.

Just to be absolutely clear in case Pewter and I were not earlier, do not continue this conflict and bickering further. Christmas Vacations are not the only vacations people take in December.

 

My first attempt to diagnose the issue after the usual voltage checks engine running and with engine off is to pull the alternator off, rip it open and check the diodes. They should all flow in one direction only when checked with an Ohm meter. This is not hard at all. If you have a popped diode that flows electricity in both directions, the light will come on. Diodes are easy to break if an electrical wire gets shorted accidentally. Diodes are cheap and easy to replace if they have popped.