72 Chevy c20, starts, then dies when key goes to run position
 

Truck ran when I parked it in the barn back in May. Went to start it last week, fired right off, but when I let go of the key, it just dies. I can pump the throttle while the starter is engaged, and it will rev up, but immediately dies when key hits the run position.
I installed a new tumbler, no change in symptoms.
I installed a new ignition switch, no change in symptoms.
Have inspected the wires, cannot find a short anywhere.
My accessories have power when the key is in the run position, and when it is in the accessory position.

I need some help with this. Any ideas will be greatly appreciated.

Re: 72 Chevy c20, starts, then dies when key goes to run position
 

Welcome aboard!
Here is a link to help solve the issue. Basically there are two paths to power the ignition (stock). One from the solenoid when cranking that supplies 12 volts. And on in the run position that through a "Ballast wire" supplies lower voltage to the ignition in the run position. You ballast wire circuit is not working.

http://67-72chevytrucks.com/vboard/s...d.php?t=613916

The link talks about an I6 but other than locations believe the the information will help you solve the problem.

Re: 72 Chevy c20, starts, then dies when key goes to run position
 

Thanks for your help. I just went down to the barn and checked the wire coming from the solenoid to the coil. It is intact, and I do not see any shorts in it. It has 12 volts on it when the key is in the run position. Do you think this circuit could still be the problem?

Re: 72 Chevy c20, starts, then dies when key goes to run position
 

It wouldn't be that wire, it is only hot when the starter solenoid is engaged. it would be the one from the firewall connector to the coil. It should be hot at the coil when the switch is in the run position.

LockDoc

That did it. Someone had spliced a wire into the solenoid wire, then to the distributor. It ran for years like that. After your post, I found the old wire zip tied to the firewall. I jumpered it to the distributor, and now I’m back in business.
I don’t understand why it just quit working the way it WAS hooked up...
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Re: 72 Chevy c20, starts, then dies when key goes to run position
 

The wire from the solenoid delivers 12 volts to the ignition coil when the starter is engaged. After the key is released the coil receives 6-8 volts through the resistor/ballast wire from the firewall connector. If they had a jumper from the wire that goes from the firewall connector to the starter solenoid the coil would have been getting 12 volts at all times which will eventually burn the points. Make sure that the coil is only getting 6-8 volts when the truck is running, and check the points to see if they are burned and pitted..

LockDoc

Re: 72 Chevy c20, starts, then dies when key goes to run position
 

Happy to see you got it. I just came back in from rewiring my own ignition power circuit. I'm glad that LockDoc stepped up and answered you guestions. (He's a good man.)
The wire broke off the connector at the firewall on my Burban Wednesday morning on the way to work and stopped me dead along the highway. Replaced the connector pin and the wire to the ignition. Good to go now.