ECM/ Grounding issues

[moposcar]

My 72 Challenger will not start consistently. 1.5 weeks ago it was starting fine (after I loosened and tightened the bolt that attaches the ECM to the firewall). If the car sits for very long at all (in a shop/garage) you have to do that or it will not start. It basically tries to fire after you let off the key which always tells me it is an ecm or bad ground on the ECM. Are there any solutions to this? It used to never do this 20 years ago, but now it is always acting up. I've changed ecms before and the one that is on it is new.

What do you guys recommend or do to keep the old electronic ignition system in good shape?

 

[Steve340]

It could be the ignition key wiring or electrical switch. They have cranking and running 12 volt supplies to the ignition system.

 

[moposcar]

I have 12 volts coming out of the ignition in both run and start modes. It has always been the ecm and like it is not getting a good ground. Has anyone else seen this or what do you do to keep good ground contacts with the body?

 

[Steve340]

Perhaps add a separate earth wire then?

 

[Challenger RTA]

Electronic ignition system will always work or not. Only on time I have seen a erratic Electronic ignition system act up. It was the pickup in the distributor. I'm talking about 50 years.

 

[Challenger RTA]

As Steve340 suggest a ground wire to a good ground., like the wiper motor stud.
Do it as a temporary measure. If it fixes it. I would say your hole is wallowed out. to fix it put washes on each side and use a nut and bold to collapse the hole.
If that doesn't work bolt it on or place a piece of metal with a hole in it behind the cowl and run it through. Once you do that get back to us.

 

[sixpactogo]

My 72 Challenger will not start consistently. 1.5 weeks ago it was starting fine (after I loosened and tightened the bolt that attaches the ECM to the firewall). If the car sits for very long at all (in a shop/garage) you have to do that or it will not start. It basically tries to fire after you let off the key

When it tries to fire when you let off the key, That usually indicates the ballast resistor took a dump or you have a bad connection in your start circuit at the firewall. (or bad ignition switch)
If you have a bad ground on your ECM, it will still start and run but open the transistor in the ECM after a short time. I'm voting for a bad connection at the firewall since the ballast resistor is either good or bad. No intermittent failures. A wiring diagram and a digital meter are your friends here.

 

[moposcar]

Finally had time to work on the Challenger today. I took the ecm and ecm bracket off the firewall, sanded the holes and wire brushed the bolts. Then the one bolt that had a loose connection on the firewall I bolted it with a nut and bolt. I put dielectric grease on all the bolts. Car started right up after that. Hopefully it will stay for a while.

Thanks for all the replies and the wiring schematic.