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Date registered: Nov 2002
Vehicle: 2008 ML550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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Make sure your jumper is making good contact with both the battery and the terminals on the resistor because the resistor contact that hidden under the headlight in that attached photo is connected directly to the fans, so if the fans run at any time, they should also run when +12 is jumpered there.
Just to reiterate how I would test: With the car off (key out), use a wire jumper from the +12 terminal of the battery to the rear terminal of the resistor (see pic). If the fans run at low speed, the resistor is good. Next, jumper to the front terminal (that you can't see in the photos). The fans should run at high speed, even if the resistor is bad. You may have to push the jumper hard onto the terminals as they tend to oxidize and will not make a good "casual" connection, so I would suggest repeating the test.
If the fans run with the key on, but do not run in the above test, your doing the test wrong, or someone has played with the wiring and all bets are off.
BTW, you note that removing the connection to the temp sensor causes the fans to run with the key on. This should not happen because the temp sensor is an NTC thermistor - high resistance (or open) = clod engine; low resistance = hot engine. This is why everyone puts a fixed resistor in parallel with it to fool the controller (the climate controller, actually) into thinking the engine is hotter than it is so that it turns the fans on earlier.
Has the upper wiring harness been replaced in your car? I just posted the same thing on another thread, but if your car still has the infamous deteriorating eco junk wiring harnesses, that may be your problem. The wires from the engine temp sensor run in the same bundle with others so when the insulation starts to break down you get shorts to other circuits that can cause the relays to energize at the wrong time(s). Carefully check the condition of the insulation on harness, in particular, the wires leading to the temp sensor.
Floobydust: a contemporary term derived from the archaic Latin miscellaneous, whose disputed history probably springs from Greek origins (influenced, of course by Egyptian linguists) - meaning here "a mixed bag." - National Semiconductor Audio Handbook