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Discussion Starter · #1 ·
I have a 90 Trooper LS that I have done a 3.4 swap on. Recently and randomly it will refuse to start. That engine cranks over just fine, but there is no spark and no fuel. My friend and I tried to chase down grounds and power leads to see if that was the problem but can't find anything out of place. Worked on it for 2 hours last night with no results, and then this morning went out and it started right up. This is very frustrating, any help or suggestions would be greatly appreciated.
 

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I was also poking at this truck yesterday. We tested voltage at the ignition coil and injectors and have the constant 12V. Tried a different ECU, both ECUs seem to function, CEL comes on with ignition on and it will post diagnostic code 12 with the jumper in the ALDL connector.

Everything SEEMS fine, but the magic that fires the injectors and ignition is not present. I was not able to hear the fuel pump prime either.
 

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We have not. Would that prevent the injectors from firing? Used a noid light, there was no pulse.

I was suspecting a failing fuel pump. But I haven't looked any further in that direction since I saw there was no pulse. I do know the ECU is fed 12v from the relay for the fuel pump status. Will it not run anything else if knows the fuel pump isn't running?

I was going to try testing or bypassing some relays, but I didn't have a pinout or diagram for the style of relay in the fuse box.
 

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Discussion Starter · #5 ·
So after fooling with it some more at lunch I think I might have a weak connection to my ignition control module on the distributor. After checking that connection it has started twice now. Does any one know if that controls the injectors?
 

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I'm thinking it sends a signal to the ecm so the ecm knows the engine is either running (or at least cranking) as a safety feature so the fuel pump doesn't continue to run on a dead engine, as in a crash or roll over.
Ahhh, here it is straight from the manual.
"The "reference" signal also triggers the fuel injection system. Without the "reference" signal, the engine cannot run.
 

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More than 20 years ago I was told that with electronic parts, either they work, or they don't, there's no in-between.

And relays were included in what was insisted worked that way.

I didn't believe it then, and not many people still believe it now.

A relay can be bad in such a way as to work sometimes, and not work sometimes.

Get a comparable, known good relay, and swap it in for the one in this truck.

If the problem goes away, simple enough, buy another relay.
 
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