1993 TBI 5.7 No spark

[Zeke's Garage]

Hello all, I mostly hang out in the 67-72 board, but I have a '93 that now has zero spark after I swapped out transmissions. Yes, I pulled the truck into the garage bay, jacked it up, swapped transmissions (4l60e auto), dropped it back down, went to start it, and zero spark. Getting fuel, no PCM codes. Before the transmission went out (lost 3-4 gears, had to drive it home in second), I have been chasing a stumble and rich mixture issue, so I had replaced the entire ignition system before the trans issue.

Here's what I checked/replaced

1. Power to the pink wire going to the coil with the key on.
2. Cap, rotor and wires are new <200 miles on them, even swapped in the old ones that had less <1k miles on them, just to make sure
3. Swapped in two other ignition modules that tested good (3 modules overall)
4. The distributor is a new parts house unit, as the old one had rusted pole pieces that caused an initial rough idling issue.
5. Tested and swapped in two other coils (again 3 overall units)
6. The engine is a 'new' long block that I put in about 9k miles ago. The old one had eaten the cam.
7. Swapped in a stock PCM, the one in the truck was a replacement unit and the PROM was suspect to me as I couldn't tell what unit number it was.
8. Tested/OHM'd out the pair of PNK and WHT wires going from the coil to the ignition module
9. When I say no spark, I mean nothing from the coil to ground as well as from the dist to plugs.. just to clarify.

From what I understand the ignition system will provide spark even without the PCM, the PCM just sends a signal to alter timing is necessary.

So, I'm thinking perhaps I'm not getting signal from the reluctor pickup on the dist.. I'm sitting here looking up how to test it without a scope.. but figured I'd create a new thread in case I've missed something stupid easy, which is a pretty common occurrence for me.

Thanks for the extra eyes..

[ChevyTech]

From what I understand the ignition system will provide spark even without the PCM, the PCM just sends a signal to alter timing is necessary.

This is correct.

Tested the pick-up coil in the distributor.
500 to 1500 ohms between terminals.
Infinite from one terminal to ground.

When testing the pick-up coil resistance, the key should be off, and the pickup coil should be unplugged from the distributor module.

Inspect the magnet on the pickup coil to see if it is cranking into pieces. It should be one part.

Make sure the distributor turns when you crank the engine.

[Zeke's Garage]

Thanks for the input, I checked the VR, and it tested 762 Ohms across the leads and infinity to ground. I decided to try swapping out the whole distributor with one I got for another project, and still no spark. I put the fuel pump relay fuse back in for my last test, and it is squirting fuel at the injectors.. I'm taking that to mean that the PCM is getting a cranking pulse, which means the dist, and ignition module must be working? I'm guessing now, that I have 3 bad coils in my inventory? Granted, I haven't tried all of them recently, so I'll try swapping them in to see what I get.

I've looked all over and can't find the procedure to test the TBI HEI coils, all I see are tests for the in-the-cap HEI coils. Does someone have that procedure? I'd like to bench test them before I start swapping again.

Thanks!

[ChevyTech]

Have you swapped the coil wire?
It is common for these trucks to burn the core open.

[Zeke's Garage]

Yes, I've tried 3 different coil wires, even though I've ohm'ed them out and know that they're good. Plus I'm just working off the coil terminal at this point attempting to coax some spark out of it.

So, let's recap..
1. New distributor which contains new VR, reluctor, ignition module, cap and rotor.
2. New coil, which tests good as per the Haynes manual (Yeah, that's sketchy at best).
3. Verified 12V to the pink wire at coil, and through to the pigtail at the distributor.
4. Injectors are squirting during cranking, indicating the PCM is getting cranking pulses from the ignition module.
5. All tests at this point are being done at the coil terminal, removing any issues with the cap, rotor, or wires.
6. Verified and bolstered the ground on the coil in case the secondary wasn't getting a good path.

I'm almost to the point of having it towed to a shop.. which if you knew me you'd shudder in horror.

But as a sanity check, I'm going to go through the steps on this site.. Looks pretty thorough, and I see a couple of things I can't say I've tried.

[ChevyTech]

The article is to long and wordy for the amount of time I have.

You are right, if the injectors spray fuel, the computer is seeing a reference pulse from the distributor.

My guess is, there is a bad connection, that is making and breaking contact, so it passes tests, but is not staying connected.

Be careful of all the connectors. They can get stretched out from testing with probes. Then they will pass the test, but won't make connection when you plug tings back together.

[Zeke's Garage]

After a couple more days of testing, replacing and head-scratching.. I finally found the issue. It seems that the white wire that runs into the cab to the tach, was fully grounded. This kept the primary side of the coil energized, and the ignition module couldn't break the connection on it's end and fire the coil. As I was trying to verify the make/break action of the IM, I realized the full-time ground.. so I just clipped the wire from the firewall harness and it fired right up. I'll have to pull the cluster out and see what caused the short to ground.

I really appreciate the help on this guys.. I had sworn that I would only put in another 100 hours or so before I took it to a shop

[ChevyTech]

Let us know what you find.