1989 Silverado K2500 Code 42

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dizzyh

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Hi All,
I have been trying to help a friend troubleshoot his 1989 Chevy Silverado K2500 5.7L. The truck runs great when cold but when it warms up it hesitates. I confirmed that the truck has a code 42. I looked up the troubleshooting steps and found the EST circuit troubleshooting chart. Following the steps on the chart, I am getting less than 500 ohms when the 4 way plug to the ignition is connected and probing CKT 423 on the computer harness. When I probe the test light to CKT 424 I get lights off. With my multi meter still connected to CKT 423 and probing 424 I get no reading. The steps states that I should get over 5000 ohms. My multimeter just reads “1”. With the 4 way plug disconnected and probing CKT 423 and ground my meter still reads “1”. The final result on the chart says the white wire CKT 423 is shorted to ground.

I have tested for continuity on the white wire (CKT423) and the Tan/Black Wire (CKT424) and they both have continuity. I have taken apart the black wire loom all the way to where the bundled wire clips on the firewall and see no breaks in any wire. The plugs/connectors looks good both on the distributer side and the computer harness. What else could be grounded? What else can I try? I even swapped the computer and it made no difference.

The troubleshooting chart I was using is this: http://www.fieros.de/en/v6help/code42.html
 
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I'm having trouble following the instructions you supplied versus the steps you are taking. For instance you state you tested CKT 423 but the chart has no such listings, I assume you mean D4 in the written instructions which is called a white wire but the diagram of the 86 to 89 shows the white wire as C8.
Have you tried replacing the ignition module?
 
Thank you for your response, and pointing that out. I updated the link to the chart that shows the 423, 424 wire. Yes the white wire is called 423. I have replaced the ignition module. Even a new distributor. Nothing helps.

Seems like my original post was deleted here it is again with the updated chart:

Hi All,
I have been trying to help a friend troubleshoot his 1989 Chevy Silverado K2500 5.7L. The truck runs great when cold but when it warms up it hesitates. I confirmed that the truck has a code 42. I looked up the troubleshooting steps and found the EST circuit troubleshooting chart. Following the steps on the chart, I am getting less than 500 ohms when the 4 way plug to the ignition is connected and probing CKT 423 on the computer harness. When I probe the test light to CKT 424 I get lights off. With my multi meter still connected to CKT 423 and probing 424 I get no reading. The steps states that I should get over 5000 ohms. My multimeter just reads “1”. With the 4 way plug disconnected and probing CKT 423 and ground my meter still reads “1”. The final result on the chart says the white wire CKT 423 is shorted to ground.

I have tested for continuity on the white wire (CKT423) and the Tan/Black Wire (CKT424) and they both have continuity. I have taken apart the black wire loom all the way to where the bundled wire clips on the firewall and see no breaks in any wire. The plugs/connectors looks good both on the distributer side and the computer harness. What else could be grounded? What else can I try? I even swapped the computer and it made no difference.

The troubleshooting chart I was using is this: http://www.fieros.de/en/v6help/code42.html
 
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Thank you very much for the info. I will look through them, and try to troubleshoot further.
 
Zora, Thank you so much for the info. I was finally able to resolve the problem. I removed both the 4 pin and 2 pin connector on the ignition module and cleaned them with some CRC electronic cleaner. I then removed the connectors on the ignition coil and cleaned those. It seems like the 2 pin connector on the ignition coil, that goes in the ignition module 2 pin connector (The pink and white wire) is part of the EST system and also responsible for switching from ignition module control to ECM. I then probed the ecm harness 424 brown/black wire with a test light connected to 12volts while checking for resistance on the 423 white wire. I finally got resistance over 5000 ohms. Before cleaning I was getting nothing on the meter. Truck is finally running great even after warming up. No more code 42.
 
excellant, you may want to use some dielectric grease on those connections to prevent future corrosion.
 
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