M117 fan controller prototype

A couple of months ago I tried a modification on my 560SEL to trigger the auxiliary cooling fan earlier.   The primary reason for this was to provide adequate air flow over the A/C condenser in stop/go traffic.   The W126 A/C was designed for R12 refrigerant and is marginal at best with R134A.    I used a resistor to trick the fan into running at a lower temperature.   It worked, but the temperature kick in was a bit higher than I wanted and the wiring was messy.    I thought a better solution would be to build a proper M117 fan controller.

The heart of the controller would be a potentiometer.    I had previously used a 1.1k ohm resistor which wasn’t quite enough.   A 0-2k potentiometer should give me all the range I needed to fine tune the cut in point.   My plan was then to build the potentiometer into a housing and leverage Mercedes-Benz connectors so it snaps right into the factory wiring harness.    That way, removing my M117 fan controller would be a simple matter of unplugging it.

M117 fan controller prototype

The only downside of this plan for me is that it requires soldering. I hate soldering and use crimped connections where possible. The wiring above shows the rough layout of the controller. This then sites inside a metal hobby box with the plastic Mercedes connectors.

M117 fan controller

Once completed, I tried my M117 fan controller prototype in my 560SEC. I had checked the resistance with a multi-meter so I was surprised when I couldn’t get the fan to kick in even with the resistance at zero. Turns out the relay had died. Swapping the high speed relay with the low speed relay had the fan whirring into life.

Overall I am very happy with the M117 fan controller prototype. I have already identified two changes I would make to it. First one would be fitting the wiring to the housing before I soldered the Mercedes terminals so I only needed one hole at each end of the housing. Secondly would be to make the wires a bit longer to allow the box to fit into a better location. I have noticed that the knob is quite sensitive so minute adjustments are required. This isn’t really a problem as I will probably mark where the normal adjustment point I like is.

M117 fan controller

I’ll probably run the prototype for a few weeks and then build two more units as the permanent solution for both 560s.

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