I have a '94 SW1 that I just finished rebuilding the engine on. It runs great now, and I'm just trying to iron out the few remaining bugs... First thing is that the temp gauge doesn't work. I ran it enough to make sure the engine got up to temp, checking to make sure coolant was flowing by touching the upper radiator hose. Everything seemed to be fine, but the gauge wasn't moving at all. I had earlier replaced the ECTS with a new brass one (before the rebuild) and the gauge was working fine. I have the SOHC engine with both the temp sensor and the temp sender. If I have it right, the ECTS is the one closest to the upper radiator hose (2 wires, black & yellow), and the temp sender is about an inch lower and a few inches back on the head (1 dark green wire), right? I thought maybe I hooked them up backwards since they have the same type of plug, so I switched them, but it made no difference. So I switched them back. I was playing with them while the engine was running, and weirdly, when I pushed in a little harder on the ECTS connector, the engine started loping/idling rough... when I let it go, it ran smooth. I guess one question is what is the difference between the 2 sensors, if any, and to make sure which is which... Should I also replace the temp sender?

Also one other thing, I started it up and let it idle for a while to get it up to temp, and after a couple of minutes, the fan came on. I checked the engine and it still felt pretty cool, and nothing had started flowing through the radiator hose yet so it obviously hadn't gotten up to temp yet. So why did the fan come on??

 

On your car, the ECTS (2-wire) sensor goes directly to the Powertrain Control Module (PCM), and it's signal allows the PCM to adjust both timing and air/fuel mixture. On early models such as yours, it has no impact whatsoever on the temperature gauge. The ECTS signals also controls the cooling fan (via the PCM)

The CTS (1-wire) sensor goes directly to the temperature gauge, and has no impact on the driveability of your car, nor the operation of the cooling fan.