77 GMC won't start

Two common suspects are the module bolted in the distributor, and the rotor. If you pull off a spark plug wire to check for spark and you hold it far enough away from metal, and there is spark, it WILL find its way to ground. Once it can't jump that far from the plug wire it will go through the middle of the rotor to the distributor shaft. That is called "punch through" and will leave a trail of carbon behind. That carbon is conductive, and from then on all spark voltage will get shorted to ground through the rotor.

If I understand correctly, some auto parts stores can test the module for you. If you replace it, be sure to use the heat sink grease that comes with it. That will prevent a repeat failure.

Another common problem is an open pickup coil around the shaft of the distributor. It has two wires, (I think they're white and green, but I might be wrong). Use an ohm meter to test it for continuity. They break from the repeated bending when the vacuum advance turns that coil.

Do you have primary voltage at the coil, with key on, pink wire?

I didn't see caradiodoc before I posted. When you say, (yes I think so), I don't know who you are talking to? You use a testlite to check primary voltage at the coil, key on.

There shouldn't be any voltage at the coil with key off, What is the voltage reading, coil positive? Does the battery have a full charge?

Yeah, you need to get the battery charged. You can't test with a down battery.

I agree with what caradiodoc's post.

Thank caradiodoc, I didn't do much. Glad you got it going.

:) They're all easy when you sit real close to the dart board!

LOL

Have a good day.