1986 Ignition trouble

[Uncle Larry]

I bought a 1986 C20 with 50K miles. It had been sitting for a long time. The seller said the knock sensor was bad, causing a no spark condition. He had disconnected the wire harness from the ECM to distributor and jumped out two wires on the distributor side of the harness, effectively bypassing the knock sensor. The truck ran decent so I drove it 25 miles to my home. By the time I got home it would not run at idle. I replaced the knock sensor in the drivers side engine block close to the oil pan as well as the distributor assembly. I still have no spark with the harness from the distributor connected to the ECM. There are four wires in the harness, black, white, green & tan. If I disconnect the harness and jump the black & green wires on the distributor side, I get spark. If I connect the harness and jump the black and green I get no spark.
Any help will be greatly appreciated.
Larry

 

[jgasca]

I bought a 1986 C20 with 50K miles. It had been sitting for a long time. The seller said the knock sensor was bad, causing a no spark condition. He had disconnected the wire harness from the ECM to distributor and jumped out two wires on the distributor side of the harness, effectively bypassing the knock sensor. The truck ran decent so I drove it 25 miles to my home. By the time I got home it would not run at idle. I replaced the knock sensor in the drivers side engine block close to the oil pan as well as the distributor assembly. I still have no spark with the harness from the distributor connected to the ECM. There are four wires in the harness, black, white, green & tan. If I disconnect the harness and jump the black & green wires on the distributor side, I get spark. If I connect the harness and jump the black and green I get no spark.
Any help will be greatly appreciated.
Larry

Same thing was happening to me. Truck would only run if I jumped the those same wires. I ended up replacing the ECM and that fixed the problem.

 

[89Suburban]

 

[HotRodPC]

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