Chevygirl93

Update to an older thread I had posted. 1996 K1500 5.7 TBI. Turns over great just no spark. I have replaced and tested everything except my ECM. DUI Distributor, Coil/Ignition Module, CrankSensor, and Ignition Switch. Have 12 volts to everything. Ohms test out on coil and ignition module. All fuses except my dash illumination have power. Compression is good as well as timming. No spark from anywhere. Did also do a test light on coil while cranking and it not flashing when cranking. But coil does test good even replaced good coil with another acdelco that tests out and still nothing. (Do not have fuel pump hooked up yet but still it should spark) Have no engine codes. Gauge cluster is the only thing that is weird as if I turn on my headlights gauges go crazy, turns on my blinker lights. All grounds on engine are good. I'm at a total loss. Anything is appreciated before I go broke.

Gumby22

I’m trying to double check the wiring diagram (access issues to pro demand), but try either disconnecting the cluster or cut the white wire coming off the coil (or, if connector/wiring has been replaced, cut the ground side). This wire feeds the tach and a shorted tach or tach circuit can prevent spark.

Gumby22

I was finally able to verify - it’s the white wire, pin B in the coil connector.

Chevygirl93

Thankyou I will try tomorrow.

Chevygirl93

Okay, I've tried everything. Also bought a $300 scan tool and still nothing. Lots of codes though that could point to my ECM being bad. It's my last thing I haven't replaced. I'm at a loss. 🤷

kevinkpk

If the ecm is original, I'd recommend replacing it. I have a 96 lumina, replaced the ecm twice, last time was 4-5 years ago. Had the same issue woudn't read injector pulse. If you are sure the no spark is the issue, ebay is where I got mine from. It was $60 shipped, no core, and I sent the guy the vin, preprogrammed, still running.

Chevygirl93

That is my last option, I'm gonna buy one that will do that but I get a lifetime warranty with it. My last option.

Gumby22

What codes are you getting?

No, it isn’t your last option - diagnosing the problem is. I get you’re frustrated, but how frustrated are you going to be if you install a PCM and there’s still no spark? What’s your next move?

To be clear, I’m not saying the PCM can’t be the issue - I’m saying only replace it if you KNOW it’s the issue as opposed to a “last option” in the hopes that MIGHT fix it.

If you’re getting codes, follow diagnosis for those codes - you may be surprised how codes that are seemingly unrelated to your problem can actually lead to the solution. Trouble codes don’t tell you what’s wrong but they at least give you a place to start looking.

A functional PCM needs only 3 things to trigger the ICM to start providing spark to the coil:

1) ALL power feeds to be good - both from the battery and from the ignition switch.
2) ALL ground feeds to be good.
3) A good, consistent signal from the crank position sensor.

That’s it. If you’re wondering, the cam sensor is only there for the PCM to use as a reference in determining misfires - it isn’t used at all for spark or fuel injector control.

Now, the PCM may only need those 3 things, but there’s other things that are necessary for the PCM to receive power, ground, and crank signals.

Good wiring is the first and most important. You can ohm check wires all day long, but realistically, ohm checks aren’t very useful for diagnosis, especially if you’re not checking the terminals for good contact. If you’re gonna go to the trouble of checking resistance, perform a loaded voltage drop test instead - that will not only tell you if the wiring is intact, but also how functional it is. All you do is substitute an INCANDESCENT test light and jumper wire in place of your meter. Use the jumper wire to tie one end of the circuit to battery negative, then hook up the test light to battery positive and complete the circuit from the other end - once the test light is on, measure the voltage drop across the wiring. Less than 0.3v dropped and that wire is good.

I stress incandescent test light because the LED test lights only require 0.6 volts to turn on - that can be deceiving. An incandescent test light will light with only a few volts but its brightness is a clear indicator of whether or not there’s sufficient voltage and current moving through the wire.

I’ll post the PCM power and ground feeds tomorrow.

Chevygirl93

I'll have to send a PDF of my scan tool cause there's alot.

Gumby22

Ok - does your scan tool note the he difference between current (or active, depending on the scan tool), pending, and history codes? Because it’s only the current or pending codes that need attention - history codes are codes that have been set and then resolved, either by repair or by the PCM’s protocol for clearing them.