1980 GMC K20 5.7L No Start After Trans Swap

I'll start by saying I don't like working on vehicles not from this millennia ESPECIALLY if it's 80s or older... I'm not used to working on them, it's hard to get good information, wiring diagrams are terrible and they are usually in AWFUL condition when they come into the shop.

Me and Old-Timer rushed to get this tranny swap done before the long weekend. He has a bad habit of removing the rear crossmember and letting the engine and trans hang unsupported on just the front engine mounts. So I'm blaming this one on him He let the drivetrain hang really low and we both heard a crunch! We finish bolting everything up and he's gotta run. Boss-Man is out of town, Old-Timer has to leave, it'll be just me by myself, end of the day Friday on a long weekend... What could possibly go wrong?? :blink:

I go to fire it up and it won't start. Cranks nice but won't fire. I smell fuel and think that I've flooded it. The vehicle has been sitting for two days. Again, I know very little about carburetors and like to keep it that way! So I call the guy and ask if it's difficult to start when totally cold. He says no and that it fires up instantly.

When I'm cranking it I can hear a loud popping/snapping sound. Just like a coil firing into air. I inspect the distributor and find it kinda loose. Then I feel a big crack right where a holddown is. Well that sucks. It's just a cap and potentially a rotor. No big deal. I'll come in on Saturday (New Year's Eve) and finish it. Should only take 15 minutes... Boy was I wrong...

The forum complained about too many links so I'll just do this post in multiple parts


I transfer all the new stuff over but still doesn't start. This is one of those annoying ones where the coil is in the cap. I really hate the connector/terminal design on these.






I check the terminals at the top and can't find a power. I then check the vehicle side of the harness and it has good power. I discovered on this particular one you have to hold the connectors and use a screwdriver to push those terminals down into the connectors if you want a nice tight connection. It still doesn't start! But hey I have power at the terminal now. The picture above has insulated wire from the terminal all the way into the coil. Mine has a section of uninsulated wire just after the terminal. I check there and don't find power. Must be a bad crimp or something.



The crimps looked good though. That's when I realized the "bare" wire I was touching was actually coated in an insulating varnish like in motor windings. Boy do I feel S-M-R-T!

I was totally expecting this to work. I was pretty sure I heard the snapping everytime I cranked. Which would indicate that power & control should be fine. Other than the power feed I wasn't really sure what wires were which. There's a 3 wire connector going to the inside of the distributor and a 2 wire connector going to the rest of the truck. The outer pins of each connector attach to the same wire and the middle terminal of the coil only goes to the connector from the distributor. It's easy to find out which is the power feed. I'm guessing the middle pin from the distributor must be the coil control wire and the other pin is probably some kind of tach signal wire. So I hook up an amp clamp to the power feed and probe each of the other wires. I see no current draw and no control signals of any kind. I manually tested the coil with jumper wires and I could get it to spark.

I quickly give up trying to find any useful information for inside the distributor so I try and figure it out.. There's only three wires going into the distributor. Only two of those wires connect to anything outside of the distributor/coil.. There is no external computer. So one external wire is a power feed and the other is probably some kind of output signal, like a tach, that can be ignored for now. The three wires going into the distributor go into a little module. The other end of the module has a two wire connector going to something else inside the distributor.




One of the three wires is a power feed. The middle wire that just goes to the coil must be the coil control wire. This leads me to believe that the other wire that goes out to the rest of the vehicle must be a tach output signal wire. So the module must be case grounded. Since there's nothing else, those two wires on the other side of that module must go to some kind of A/C self generating speed sensor. I did a trace on them and got a signal. I don't know what it's supposed to look like but I got a nice, clean and consistent pattern so good enough to move on. I then pull up the other channels and trace the other three wires. I don't care which wire is which so I just grab them all. One should be a constant power and one should be a control wire that has a primary ignition voltage pattern.



Ignoring the noise and not caring what wire is what I can clearly see there is no primary voltage pattern. It has a clean input signal, has a power, is on a clean ground surface so that module must be garbage. Must have got fried when the coil was firing with no place for the spark to go. I put a new one in and it started sparking good.




Again, I'm really not concerned with what's what. I can see there's a consistent input, has power and some kind of control. Here's a closeup of the ignition primary after the new module.

That sucks! I have broken a Chevy distributor cap doing that in the past, lesson learned!
You'd think the fan shroud would have stopped it from damaging the distributor so badly though...
Nice work figuring what wire did what in there

Thanks for posting the waveforms and the labeled pic of the distributor too.