I've done all the tests mentioned in this thread. Put 12v from battery to one end of the resistor ballast behind left headlight, the fans fire at high speed. This resister has been bypassed with a wire by the previous owner. I then put 12v on the other end, fans DON'T come on. This confirmed that my low-side circuit is fine.

Hence, I'm only interested in fixing my high-side circuit. That's what I was testing when I took the connector of the blue aux fan thermistor off. Been reading that this should turn the Fans ON at high speed if the circuit is fine. But they don't on mine. I can hear the relay click though.

I went further and tested resistance. On the blue aux fan thermistor at 80 Celcius, it reports 607 Ohms. No idea if that's within spec.

Tmrw I'll dissemble some things and inspect the wiring.

If you put 12v on one end of ballast resistor and it works, on high speed, but not the low speed then did you test the resistance of the ballast? I think it's 5 ohms. By the way, I think you are confused. You said you put 12v to the ballast resistor and the fans turned on in high speed which confirms your low side is fine??? That doesn't make any sense. If you put 12v to the ballast resistor and you got high speed fans, then one of 2 things is going on: 1) your resistor is registering at 0 ohms or 2) you put 12v on the wrong end of the resistor. You stated you performed all the tests, if that's true then when you bridged the 2 wires on the receiver drier what was the result? You should've gotten low speed fans. You did mention the previous owner bypassed the resistor....How was it bypassed? Also, did you undo the "bypass" before you started performing your tests?

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1990 190E 3.0L