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1990 Chevy K2500 Silverado CODE 42
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Kden
New User
Feb 14, 2015, 3:42 PM
Post #1 of 2
(2629 views)
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1990 Chevy K2500 Silverado CODE 42
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Hello Everyone,
I was on the web searching "how to fixe code 42 on 1990 Chevy Silverado" and this forum came up.
I know there was a thread back in 2011 or so, but my question/fix was not addressed.
My truck has a 350ci in it and here's what I've done...mileage is N/A as my odometer was changed due to it not working...last known was 189K
Last week I changed out:
Distributor
Coil
Pick up Coil
Ignition Module
Rotor
Cap
Fuel pump
I was having other no spark issues which turned out to be my pick up coil.
I've changed out the module twice thinking a new one might have been defective...the one that is in there now is know to be working.
I did not touch the ignition bypass wire (tan/blk) a anytime, so I don't suspect that, but I did check it anyway just in case it got loose...looks good.
I notice in my code book, 42 could also be "Fuel cutoff relay circuit - open or shorted to ground". Where is this relay?
Any advice/suggestions is greatly appreciated.
Thank you very much!
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Discretesignals
Ultimate Carjunky
/ Moderator
Feb 14, 2015, 5:35 PM
Post #2 of 2
(2615 views)
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Re: 1990 Chevy K2500 Silverado CODE 42
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Code 42 is an electronic spark timing problem. I could fill a whole thread with information about how GM's bypass ignition system functions, but basically in a nut shell when the engine is cranking, the rpm is low, the circuitry inside the ignition module is controlling spark timing. When the engine starts and the engine speed gets above 400 rpm, the engine computer takes over controlling the spark timing. That is unless you have the bypass connector unplugged or code 42 is set.
For the computer to control spark timing it has to send 5 volts down the bypass circuit that causes a transistor inside the ignition module to close a circuit called the EST circuit. This circuit is used by the engine computer to control the spark timing. In EST mode the computer has taken over from the circuity in the ignition module to control spark timing. When the computer isn't sending 5 volts down the bypass the EST circuit is grounded by the ignition module and the ignition timing is base timing.
The way the code sets is the engine computer is not sending 5 volts on the bypass circuit, but the computer sees that the EST circuit is not grounded. The engine computer not only sends a timing control signal down the EST circuit, but also monitors the voltage on that circuit because that is the way it determines if there is a problem on that circuit.
One more time:
Computer sends 5 volts down the bypass circuit to tell the ignition module it wants to control spark timing. The computer sends the timing signal down the EST circuit and also sees that there is voltage on it. All is good.
The Computer doesn't send 5 volts down the bypass circuit, but notices the EST circuit isn't grounded (puter is seeing voltage). The computer assumes the EST circuit is open or shorted to power. Code 42 sets.
Read that as much as you can so you are somewhat familiar with how the EST system operates. Once you understand how it operates, you'll be able to troubleshoot it.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Feb 14, 2015, 5:43 PM)
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