87 Turbo knock sensor issue

[supraman7]

I have a 1987 Toyota Supra Turbo (MKIII), the check engine light dashboard lamp has been lighting up approx 15 to 60 seconds after the motor is started. The light will come on sooner rather than later, if I put a load on the motor.
The following are the checks I’ve done and their results:
I put a load on the motor while listening for sounds and found no detectable knock or ping coming from the motor. There is, however, an exhaust leak where the exhaust pipe meets the turbo, but I don’t believe that would cause the sensor to activate.
This model has an OBD 1 computer. When I did the diagnostic code check, the trouble code read out was #52, which indicates it’s a front side knock sensor. I removed the computer and both knock sensors from a friend’s 1987 Supra Turbo and tested them on my car. They had the same part numbers as the parts from my car. When in my car they functioned the same as my parts and presented the same check engine trouble light and knock sensor code number. This would indicate that my car’s parts are functioning well and that the sensor connectors are not bad.
I checked the communication lines by back probing from the sensor to the computer, they checked out OK. Also, I tested my computer and sensor with an ohm meter, I got the right input and received the right output values.
I’m perplexed. Is there anyone who has knowledge of this type of OBD 1 computer knock sensor problem or any other OBD 1 issue? If so, please let me know how it was resolved. Thanks.

[cre]

The MKIII doesn't have OBD-1, it's diagnostic system predates the California state specifications known as OBD-1 and is much more primitive... there is a serial out within the ECU, but it is not something available even to general service techs; it's pretty much just used for diagnosing the ECU itself.

Your problem is VERY common and is usually solved by replacing the knock sensor wiring. I'll find you a link to a writeup on the rewire and post back.

[cre]

http://www.supras.com/06/techcenter/display.php?QID=69

[supraman7]

Thanks for the heads up about the ECU differences.
Other people have also suggested that it’s probably a wiring problem. So I borrowed a friends special ohm meter that gives me exceptionally detailed readouts and does more than a regular meter. It has indicated there is a normal reading in my knock sensor wires. I also heard that some cars had a slightly lower than normal reading in the wires creating a problem. This is not true in my car.
I still may replace the knock sensor wires if I can’t find any other way to fix this. Thanks for the link it will help me.

[cre]

People are suggesting it from experience. Define "normal" readings... What are you measuring? Resistance? Voltage drop? Capacitance? The knock signal is a VERY fine signal which is VERY susceptible to interference unless you've put an oscilloscope to it testing isn't going to prove much.

Good luck with it; Let us know what else turns up.

[supraman7]

You were right, replacing the high frequency knock sensor wires solved the problem. I’m impressed with this forum. Thanks again for the advice.