1991 350 knock sensor question

[tsr2185]

While changing out my starter, i broke the blue wire going into the knock sensor. Was in a rush and was told it would still run like this so i put the new starter in.

Did some online research and it looks like all i need is a new pigtail? Its seems like this one was pretty jerry rigged with some electrical tape. Was about to order a new today, and mess with it when I next get the chance.

Thoughts?
pigtail

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[Erik the Awful]

Yup. I'd use a heat-shrink butt connector and throw a new terminal end on it and call it good.

 

[tsr2185]

Yup. I'd use a heat-shrink butt connector and throw a new terminal end on it and call it good.

Took the old pigtail off. Seemed it was just the pin held on with silicone. Could this have never been a good connection, and still not throw a code? Also, can a bad knock sensor/connection cause rough idle when hot?

 

[Erik the Awful]

If the connection is open, it will not throw a code. The knock sensor is a piezoelectric crystal that produces an electric current when it's subjected to an impact. If it's unplugged, it just never sends a voltage to the computer. The engine can knock and the computer won't pull the timing back.

Rough idle when hot? Not likely. Pinging under a load? Yes, especially when hot.

 

[evilunclegrimace]

You can still buy a factory style pig tail for the Knock sensor.

 

[evilunclegrimace]

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[tsr2185]

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Yessir. I got one coming in tomorrow. I confirmed the sensor is still intact and pinhole is clean.

 

[Schurkey]

If the connection is open, it will not throw a code. The knock sensor is a piezoelectric crystal that produces an electric current when it's subjected to an impact. If it's unplugged, it just never sends a voltage to the computer. The engine can knock and the computer won't pull the timing back.

Rough idle when hot? Not likely. Pinging under a load? Yes, especially when hot.

Are we sure? I thought the computer would test the knock sensor now and then. Doesn't the computer advance the timing looking for knock indications from the sensor? If it doesn't get any, it triggers the knock sensor code.


Look closely at the knock sensor wire. The wire probably had a sleeve or jacket on it as a heat shield. It would be best to recreate that heat shield.

Verify that the remainder of the wire is in usable condition. Those wires to the knock sensor and starter solenoid are infamous for getting road-splash. The wires corrode green or even black. You won't get a good connection until you cut the wire back far enough to get to copper-colored copper.



I went through this with my starter solenoid "S" terminal. The previous owner had cut the original wire back about eight inches and spliced-in a new section. Butt-connectors, no heat-shrink. Of course it rotted again.

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When I got to it, it was rotted considerably farther up the harness. I cut the harness back as far as I could reach behind the engine, but it was still corroded. Had to splice in fresh wire above the engine and drop it down to get to the solenoid--about three feet altogether; but I wouldn't have needed that much if I had access to the harness behind the engine.

This is an eight-inch+ section of the wire, the PO had spliced to the black end. Even cutting eight+ more inches off, I still had corroded wire. It looks "ok" in this photo compared to the black end, but in real life it was ugly.

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[tsr2185]

The wire probably had a metallic coating or jacket on it as a heat shield. It would be best to recreate that heat shield.

Verify that the remainder of the wire is in usable condition. Those wires to the knock sensor and starter solenoid are infamous for getting road-splash. The wires corrode green or even black. You won't get a good connection until you cut the wire back far enough to get to copper-colored copper.



I went through this with my starter solenoid "S" terminal. The previous owner had cut the original wire back about eight inches and spliced-in a new section. Butt-connectors, no heat-shrink. Of course it rotted again.

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When I got to it, it was rotted considerably farther up the harness. I cut the harness back as far as I could reach behind the engine, but it was still corroded. Had to splice in fresh wire above the engine and drop it down to get to the solenoid--about three feet altogether; but I wouldn't have needed that much if I had access to the harness behind the engine.

This is an eight-inch+ section of the wire, the PO had spliced to the black end. Even cutting six more inches off, I still had corroded wire. It looks "ok" in this photo compared to the black end, but in real life it was ugly.

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Yep same here. The S terminal wire had so much insulation missing about 3-4 inches up the wire i cut about 6 inches off. I see what you mean. The blue wire going to the knock sensore has about 6-8" of slack i can cut back if i need to. My new pigtail comes in tomorrow, hopefully i have no issues then.

 

[Erik the Awful]

Are we sure? I thought the computer would test the knock sensor now and then. Doesn't the computer advance the timing looking for knock indications from the sensor? If it doesn't get any, it triggers the knock sensor code.

Good point. I don't know.