How can i test the EST circuit??

Hello everyone!!! I'm new on this forum and hoping to find some help for my '94 Chevy Silverado (350 TBI)!! After replacing my MAP sensor (code 33) wich made it run extremely rough, i got a code 42 for Ignition Control. I found out that my coil was shorted so i replaced it, but now he DOESN'T START anymore... Now the next thing i gotta check is my EST circuit, as everything else is testing positively!!! Anyone has some input on EST circuit?? HUGE THANKS!!

Hey Silverado350! Can I correctly assume that you're asking about the EST circuit because you have no spark? Just wanna make sure we're going down the right path.

For our info, is this a manual or automatic transmission? It matters because, according to the diagram, manual transmissions use a separate knock sensor module. Automatics have the knock sensor going directly to the PCM.

Before digging into the EST or distributor, let's back up and go to the coil. This thing ran before the coil was installed, so it makes sense to me that we should start there. At the coil, both pink wires should have B+ during cranking. Both white wires should show a pulsed ground during cranking. The easiest way to verify this is with an incandescent test light. This ScannerDanner video is an excellent example:

I own the same truck lol

Having the timing wire by the blower motor disconnected will throw that code!

Just in case that was overlooked!



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No kidding? :silly: RFI from the motor, I'd guess?

FWIW, I don't think you can get a flat out no start from an EST circuit problem. A start/stall, or a start and poor idle, maybe. If there's any doubt, you can always cut or disconnect the tan/black wire. That'll eliminate the PCM's control of spark timing.

Hey Tyler!!! Well now that the whole ignition system is brand new (just the ICM that is a bit older, replaced in last june…), he starts and runs but idles extremely rough like a cammed 454 since the E.S.T circuit doesn't seem to work....Oh and btw, it's an automatic transmission (4L60E if i'm not mistaken...Big pan and non-slip open yoke)!

what is the coolant temp sensor reading?? does it seem like its running rich when its running rough?? also have you checked for a stuck open egr valve??

Silverado350 wrote: Hello everyone!!! I'm new on this forum and hoping to find some help for my '94 Chevy Silverado (350 TBI)!! After replacing my MAP sensor (code 33) wich made it run extremely rough, i got a code 42 for Ignition Control. I found out that my coil was shorted so i replaced it, but now he DOESN'T START anymore... Now the next thing i gotta check is my EST circuit, as everything else is testing positively!!! Anyone has some input on EST circuit?? HUGE THANKS!!

Okay, this is a no start? A code 42 on these early bypass ignition systems will set a code 42 from extended cranking conditions. I would ignore the code 42 and find out what you are missing. I am assuming you have no spark? Do you have injector pulse?
Watch the video posted by another member here in this thread, where I am showing how to address a no spark condition on this type of system.