1990 Toyota Pickup knock sensor

This is what I found on the subject
and I figured I would share it hope it help

A head gasket replacement on V-6 Toyota trucks may be all it takes to kill the knock sensor. The CHECK ENGINE light may come on and a knock sensor code may be set when the engine is started for the first time after the repairs.
Be very careful during disassembly and repair, as the knock sensor wire can become very brittle internally. Moving the harness around during the engine disassembly/reassembly may cause an internal break that will not be visible on the outside of the wire insulation.
Separate wire harnesses are now available from Toyota dealers. We have also had some success in the past by making a new knock sensor wire from microphone cable. This cable is available at your local Radio Shack or similar electronics store.