OP we need the year of your 124.

They changed the aux fan control between 86-87 and all others.

86-87 uses a binary temperature switch in the water neck to directly trigger the fan relay on high speed.

88-up the water neck sensor is a thermistor. This goes to the CCU, and the CCU has an output pin that drives the high speed fan relay.

This relay applies direct voltage to the motor (no resistor).

Both versions use the dryer pressure switch to trigger the low speed fan (through the resistor).

For this reason you also have to be careful with the CCU controller, the 88-up has the extra feature for the aux fan control. You can put one of these in an 86-87 since the pins aren't wired in these model wiring harness and your fans will still work directly the original way. Of course you can also swap the water neck sensor and rewire the system to use the new style controller.

You can't use an 86-87 CCU in a later model as it does not support the CCU based fan control and your fans will not respond to temperature.

__________________
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2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

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