Hey All. So, after chasing down the rabbit hole, I think I have eliminated all but an issue with why my injectors aren't firing. I had a crank/no run condition that was solved with a new PCM-- the main pump didn't time out (and I had a few other issues). I had good spark, but it seemed weak and the truck is 35 years old. It needed the usual tune-up parts so it got a new cap, rotor, plugs; checked the SP wires and coil wire (all good) and for insurance replaced the ICM and coil (NGK). Ignition system is solid. The thing starts right up with starting fluid.

On the FUEL side...it's wonky. 2 tank model, both lift pumps OK, pressures spec out. Main pump makes 40 psi at the rail with key on, motor off. About 45psi directly after the pump, before the (new) fuel filter. Fuel pressure regulator is good--diaphragm is intact, no vacuum leaks. The old FP relay looked tired (inspected interior, definitely water intrusion) so replaced it also.

I knew it had bad PCM (main pump did NOT time out) so opened it but saw no leaking caps or burned traces. Replaced it (reman, did NOT want to spend $700+). Installed it, motor fired up...but ran a bit rough. Adjusted timing, it got better, but still as a smooth as it was. It idled for 15 minutes, decided to drive it around the complex, all good. But after going over a speed hump, it shut off. Just quit. Would not retire. Friend got me back into my parking space and I tried it again. Fired, ran rough for a bit, quit again. After more diagnostics I determined the injectors are NOT firing. 12V on BOTH wires going to the injectors at all times. Is it correct they ground through the motor, red wire is 12V continuous and the second wire (yellow or orange, by bank) is 12V that is de-energized by the PCM telling the inj to fire? Is it correct that that wire should cause a test light to blink rapidly as the inj works? If so, the secondary wire is now hot continuously. And the new PCM looks to be toast.

I *think* I have a bad wire somewhere on the injection circuit, causing a failure. It ran with a new PCM but don't want to replace it (again) before I sus out potential wiring issues. I'm looking for a detailed diagram of the FI wiring circuit. I'm thinking a broken/grounding wire is killing the PCM and injection circuit. Today I'm going to check continuity/resistance from each inj to the corresponding pin at the PCM. Does that make sense? Anyone have a similar experience? Insights? Majikal Incantations?

91K original miles, was a farm truck in NorCal, drove it cross country to relocate, 2600 miles and not a hiccup. 7 years not one problem. Well maintained, wiring harnesses are all intact, properly tied up, not a roach.

Any direction/thoughts/advice greatly appreciated.

 

Going over a bump and stalling out, would make me suspect the column mounted ignition switch or a connection issue at the battery negative terminal and the ground wire that goes from the battery negative, to the engine block and frame.
How are the connections at those locations?
The fuel injectors are powered from the ECM relay, and should have voltage on both sides, until the ECM grounds them.

The fuel injectors are grounded through pins 58 and 59, which causes them to open.





The fuel injectors are powered the ECM relay (blue dot).
The ECM relay is powered from the ignition switch (green dots).
Did you save the old Ignition Control Module and the old ECM?
I would look at the grounds where they connect to the battery negative terminal and engine block and frame, and clean those connections.
Then I would visually inspect the ECM relay and it's socket, for corrosion.
Does the key cylinder feel loose at all? The column mounted ignition switches that are operated via a linkage rod from the key cylinder, are known to fail.

 

Before you guys ripping into it, it's worth a look at the inertia switch. It is possible the bump tripped it. Would be an easy fix.

 

Some of these ECMs can be repaired by replacing the capacitors on the circuit board. You might be able to repair your old one.
Is your Ignition Control Module correct for your year of truck? There is a known issue with parts suppliers listing incorrect ICMs. Is yours distributor mounted? Did you use thermal paste when you installed the new one? Yours should be gray as opposed to black.
While you're checking grounds, see if you have these two ground wires coming out of the engine harness, near the upper passenger's side corner of the radiator. They need to be connected to the battery negative terminal. They're the power grounds for the ECM.

 

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Grounds, all solid. Checked every ground, all good. IGM is correct and, yes it is the grey one. Distributor mounted, sans the 3 vertical spade pins that disappear into the dist body. Slathered with dielectric grease, really tight to the dist for good heat distribution. SPOUT is within specs. All the connections I've checked have been thoroughly "greased". I've learned "the devil is in the details". Been spinning wrenches for decades, love machines, have a strong diagnostic mind, mentored by really competent people. But this is a Majik White Smoke issue, where I'm NOT as strong. I'm leaning to look again at the under column wiring. But this is an intermittent failure. And, again, it started immediately with the new PCM. Until it didn't.

 

Lower the column and check the rod actuated ignition switch. See if it looks like it's separating.
While the column is lowered, move the key through the different positions and see if the rod moves with it correctly. If it's an automatic, be careful with the shift indicator cable.
Check the ECM relay for corrosion.
The blue capacitors on the ECM circuit board are replaceable. It's not as hard to do as it seems.
When you said the spark seemed weak, were you watching it with a spark plug tester?
Weak spark could be a coil issue, a ground issue, or the PIP sensor in the distributor.