The 510 Realm

I have a 78 L20b with matchbox EI in my 68 Wagon and the other day it just quit me (luckily close to home). Cranking did no good. Friend towed me home. I put on another known to be good matchbox control module and tried cranking again....nothing. I pulled #4 plug and set it up on the valve cover so I could look for spark....no luck seeing anything.....so turned ignition on, undid the black/yellow wire from the starter solenoid and jumped with screwdriver.....Be damned if it started instantly pumping air out of #4!!! Turned her off, put #4 plug back in, reattached black/yellow wire to solenoid, vroom, started right up again....and again....and again.....hmmmmmmmmmmmm. Figured I had rid myself of whatever "gremlin" was in there so the next day jumped in started out and made it about 1/4 before the same thing happened again....just shut itself off.

Anybody have this same bugaboo? I'm thinking a hidden broken wife somewhere but before I go replacing "everything" I thought I'd ask.

Thanks, Scott

Not running but headlights work. Not the fusible link...

Crank the engine over and check for spark by holding coil wire near ground or use an old spark plug. Got spark?

Yes.... pull a plug wire off and check for spark. Confirmed no spark. Problem is between the coil wire and plug wire. Distributor cap or rotor carbon tracked or cracked and spark leaking away. Possibly a bad plug wire but not all 4.

No spark ... Check for power on the coil positive terminal with test lamp. Got power???


No power at coil positive terminal when ignition turned or during cranking would indicate a faulty ignition switch or loose plug to the ignition.

Yes there's power to coil. Make a jumper wire and briefly ground the coil negative terminal and release. Got spark???


No spark coil is faulty, probably heats up and quits.

Yes, spark See my next post below

Intermittent electrical issues are tuff to locate.
Did you have power and signal from the EI module itself back to the coil? And was there power at the coil itself? That’s a good place to start.
There is a very good chance you just moved a conection or a connector somewhere in the process. There is always a chance of a broken wire, but unless the harness has been physically damaged it’s notnlokely hidden. Most often it’s a component that fails, or a conection at a plug.

Keep us posted.

Byron

Ah... EI.

If power to the coil on the positive terminal, and a jumper to ground on the negative side produces a spark, then the problem is somewhere in the distributor.



1/ Check for power right at the B terminal on the module.
2/ Check that the wire from the C terminal on the module is solidly connected to the negative side of the coil.
3/ Make damn sure the distributor is solidly GROUNDED to the engine, Even run a ground wire to it to be sure. No amount of parts replacement will fix an intermittent ground.

If everything checks out... Next time it quits pour cold water over the matchbox module to cool it down. If it restarts the module is affected by heat.





Here's a great replacement for the matchbox module using the $10 HEI unit from an early 80's GM mullet mobile.



Some people build elaborate heat sinks but this thing works inside the distributor cap just fine. Outside like the picture will be 5X better.

I had a ignition switch that was causing my die in flight issues.

Thanks for all the help, advise, pictures, wiring diagrams. This site is The Best!!! I ended up finding an open spade on the fuse block and wiring from it to a 20 amp toggle switch I mounted on the drivers side kick panel just below the hood release lever, then back out to the positive side of the coil. The drill is to flip it on, turn the key and away we go. Turn key off, then the toggle off to stop the engine. I guess I "hotwired" around the problem. Thing is my voltage gauge now reads a solid 14.1 volts and will drop some when the electric fan is on and/or the headlights but the drop is less than before this all happened leading me to believe that my old harness had a lot of parasitic draw down to it.

toylet wrote: ↑21 Jul 2018 17:59 I had a ignition switch that was causing my die in flight issues.

Me to! If I pulled down on the key while turning it would fire. Otherwise no deal. Took SO SO long to figure it out!