77 GMC won't start

Tiny
DRRLUMBERJACK
  • MEMBER
  • 1977 GMC C3500
  • V8
  • 4WD
  • AUTOMATIC
  • 10,000 MILES
77 GMC won't start. Getting fuel but no spark. Replaced coil and checked ignition lead - still no start or spark.
Do you
have the same problem?
Yes
No
Monday, April 11th, 2011 AT 7:21 PM

15 Replies

Tiny
CARADIODOC
  • EXPERT
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.
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 7:34 PM
Tiny
JDL
  • EXPERT
Do you have primary voltage at the coil, with key on, pink wire?
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 7:35 PM
Tiny
DRRLUMBERJACK
  • MEMBER
Yes, I think so.
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 7:39 PM
Tiny
JDL
  • EXPERT
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.
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 7:53 PM
Tiny
DRRLUMBERJACK
  • MEMBER
I used a volt meter and it showed a change in voltage when the ignition was turned on.
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 8:03 PM
Tiny
JDL
  • EXPERT
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?
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 8:10 PM
Tiny
DRRLUMBERJACK
  • MEMBER
It has voltage with the key on, not off. Battery is now not charged from all the attempted starting.
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 8:12 PM
Tiny
JDL
  • EXPERT
Yeah, you need to get the battery charged. You can't test with a down battery.

I agree with what caradiodoc's post.
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 8:18 PM
Tiny
DRRLUMBERJACK
  • MEMBER
Thanks, I'll give it a try.
Was this
answer
helpful?
Yes
No
Monday, April 11th, 2011 AT 8:20 PM
Tiny
DRRLUMBERJACK
  • MEMBER
It was the module. Purs like a kitten now. A very old ugly 1977 kitten, but purring none-the-less. Thank you!
Was this
answer
helpful?
Yes
No
Tuesday, April 12th, 2011 AT 1:35 PM
Tiny
JDL
  • EXPERT
Thank caradiodoc, I didn't do much. Glad you got it going.
Was this
answer
helpful?
Yes
No
Tuesday, April 12th, 2011 AT 1:57 PM
Tiny
CARADIODOC
  • EXPERT
:) They're all easy when you sit real close to the dart board!
Was this
answer
helpful?
Yes
No
Tuesday, April 12th, 2011 AT 7:29 PM
Tiny
JDL
  • EXPERT
LOL
Was this
answer
helpful?
Yes
No
Tuesday, April 12th, 2011 AT 7:37 PM
Tiny
DRRLUMBERJACK
  • MEMBER
I'll bookmark your page. You were both helpful.
Was this
answer
helpful?
Yes
No
Tuesday, April 12th, 2011 AT 8:42 PM
Tiny
JDL
  • EXPERT
Have a good day.
Was this
answer
helpful?
Yes
No
Sunday, April 24th, 2011 AT 7:18 PM

Please login or register to post a reply.

Recommended Guides