Car in question is an '87 300E with 190k miles on it. It's been running warm for the past year or two - last summer we replaced the thermostat, which helped a little bit, but the changing of the seasons helped a lot more.

Fast forward to this summer: with ambient temperatures over 95F, engine temperature goes up to, and stabilizes at, 110C in traffic. Temps return to around 90C once I get moving again. A/C cuts out until after the engine temps go back down.

It looks like the fan clutch is probably bad, and I'm going to have to have someone else fix it, but I'm hoping to have more DIY luck troubleshooting the aux fan. This doesn't ever come on, not when the temperatures climb or when the A/C comes on. I bridged the leads off the green pressure switch, and the fan works. The fuses in the fuse box and the black box behind it look okay, so my questions are:

What does the (pre)resistor do? I've got a yellow one, without a metal bracket, and with two wires attached to only one side. Any way I can test it?

Which of the plugs on the engine is the coolant temperature sensor? What's the procedure for testing this one?

What else can I look for?

Thanks in advance,
anthony