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I've had my '86 RS Convertible for over 20 years. I finally figured out (2 years ago) why some of the fan control switches and sensors weren't working by way of no good ground to the thermostat housing. But due to other things not working I'm now on to a rather custom setup of aftermarket temperature gauge and a customized relay, and other upgraded sensors and connectors. I want to put things back more to stock now that I have managed to confirm that the digital dash temperature gauge seems to actually work.

I'd appreciate seeing pictures and also some sample ohm readings.

I have an official service manual and it seems to show that the switch for the idiot light and the input to the gauge use the same wire.

My primary mechanic's Snap-on diagnostic computer was able to confirm that the car's computer knows the engine temperature and it was within a few degrees of my aftermarket temperature gauge. I think the computer is measuring temperature from the upgraded sensor with a newfangled 2-wire connector in the lower part of the thermostat housing area that someone has already hardwired into the harness.

A specialty mechanic kindly replaced the heads on my engine a couple of years ago and not everything got put back correctly. After that is when I installed the aftermarket temperature gauge and had my way with a fan control system. And my aftermarket temperature gauge sensor is screwed into the back of the engine where a stock spade terminal stock part was. I don't know if it was a switch or variable resistance sensor.

Recently the car was in the configuration that a black plastic cap single green wire connector was loose, or that it was connected to a screw-in sensor that was just lying there, not even grounded. I don't know if this is superfluous given the updated computer's sensor, and I risk miscalibrating the computer if this is connected up. Or if this serves an independent purpose for the gauge.

And also that a black wire with a regular 1/4" quick connect was found stuck near the air intake tubes. If I ground the black wire, the idiot light comes on after a short while, and also the digital dash temperature gauge goes up. And the idiot light stays on for a while even after I release the wire from ground.

So I'd like to better understand, by pictures and any electrical measurements, what really should be where, and see what stock functions can be brought back.

While dreaming / musing, if only I could wire up an OBD2 Bluetooth dongle and monitor the computer's temperature readings and any other parameters... what I tried with an aftermarket $10 or so cable did not get me anywhere. What magic does Snap-on use with my mechanic's prior fancy tools and adapter cables?
 
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