Strange Ignition problems...

[Dudeman]

On my '79 620 (L20b/stock matchbox dizzy) I began having spark issues that started as an occasional misfire, but progressed until the truck wouldn't even idle. I'm sure it's spark because all four plugs are getting noticably weak spark and the thing is backfiring a lot too. All connections appear to be fine, distributor shaft isn't wobbly, and cap, rotor, plugs, and wires were replaced within the last 600 miles or so. I bought a new coil, which did absolutely nothing and then a new black box which seemed to solve all problems. Maybe 50 miles after the ignition control module was replaced it started to miss again, and since then the engine has gotten weak, having the same spark issues that it was having before. I'm worried that I am frying my icm somehow, because the problem got better when it was replaced but is now bad again. The problems do not obviously correspond with the parts I replaced, because it ran fine for a while before crapping out on me. I've stopped driving it, and the only thing left to replace before I have an entirely new ignition system is the pickup coil. I was wondering if anybody can help me diagnose the problem or if anyone has experienced anything similar, as I am kind of stumped and tired of throwing money at it without really knowing what's wrong.

 

Thanks!

[DatDoug]

Sounds like you have replaced all the right things for this issue. Check all your battery & alt. connections. Also check your choke wire to your carb. Thats all I can think of rite now. Im sure you will get some more input.

[datzenmike]

Seems to be black box related. This could be the black box but also anything disturbed while changing it too. There are a Red and Green wire from it to the reluctor inside.

 

 

Also the C and B terminals on the upper left. Possibly the plug that fits it is corroded/damaged? Loose or broken wire inside the insulation of the harness?

 

 

Do you have the dizzy fully and firmly grounded???

 

.

[datsunaholic]

Might need a ground lead. The dist originally had a ground lug, not on the module. Sometimes the dist body isn't grounding properly by itself and need that extra wire.

[hang_510]

Seems to be black box related.

 

There are a Red and Green wire from it to the reluctor inside.

 

thats some nice corrosion there...

if yours looks at all like that, problem solved. :)

 

what happens if swap the OG box back in?

 

double check the 2 wires are not corroded as well.

[datzenmike]

The module is screwed to the dizzy body. The dizzy is bolted to the timing cover with a gasket. The timing cover is bolted (also with an insulating gasket) to the block and head which has the negative ground cable bolted to it. It's a long path to ground.

[Dudeman]

Ya I have no separate ground wire from the dizzy, seems to be grounded through the dist. housing like datsunaholic said. Maybe needs a separate ground, something coming from the metal chassis of the black box to the frame perhaps? All connectors look good, but I will check out all my wires again when I get a chance.

 

I actualy disconnected the electric choke wire as it worked only intermitently and she starts fine without it, any chance that could have affected anything?

[Wicked Saint]

This is a long shot and totally separate beast but my 67 Camaro kept blowing point modules in the Mallory distributor. It was a good 19 years ago so I don't recall the exact details but I think it behaved similarly and turned out to be a short somewhere in my wiring harness. Not sure if that helps at all but I hope it may.