I recently aquired this 1990 Blazer 5.7 ltr with a 700r4 4x4 with a 6 inch lift about a week ago and been working on it ever since. The service engine soon light came on and i got the light to blink out the codes. It gave me a code 12 and a code 42. I was told 12 was a normal code seperator. I replace the ignition control module. It made the truck run worse than it did. Idled rough and missed at high rpms. i stuck the old one back in and ran better but still a code 42. So i was told to replace the ECM and i did and still runs the same and still a code 42. Don't know what else to look for. Please let me know if yall have any ideas or suggestions.

 

check under dash on passenger side, in the main wiring harness bundle, there should be a tan color wire with a connector in it. if it is there, and not plugged in to anything, then find the other connector by it somewhere. this is the timing bypass wire, and if it is not connected, then it will cause motor to run rough. also, make sure everything is in good shape as far as cap, rotor, plug wires, plugs. make sure all connections at distributor, battery, and starter are good too, as well as all ground wires.

 

thanks for the quick reply!!! I changed the distributor and rotor. It ran better for a little while. Now it's doing it again. I checked all connections and nothing. i even checked for that timing bypass wire there isn't one that i could see. Not a tan wire behind the glove box.

 

Here's an update!!! I took the truck to a stealership today. Had them run the codes on the computer. They said it was the TPS and the EST. They also said if i replaced the TPS it might clear the EST. So i replaced the TPS and still have no daggum power and the service engine soon light on. They said might check the timing and see if it's out. I am not sure how to do that since i cann't see the marks or the harmonic balancer that well to adjust the timing. Oh and one more thing does anyone know how or why my parking brake light doesn't come on? No it's not the bulb i already replaced it. Please help!!!

 

I think i might of found it. While the truck was running i wiggled the wiring harness and the light went off. So there might be a short some where. Does anyone know where i might get a new wiring harness for the motor at?