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.

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.

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.

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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I’ve noticed that once the coolant temperature gets up towards 100C, it has a big impact on the A/C.    I’m assuming the condenser, already marginal, is just not able to pull enough heat out of the system in hot weather with a 100C radiator next to it.    My thinking is by modifying the auxiliary fan cut-in point, I can keep the temperatures a bit lower, and provide more air over the condenser.

My 300SE has gone beyond auxiliary fan cut-in modifications to hard wire the fan to run any time the compressor is engaged.  This does help with A/C performance, as you get constant air cover the condenser.  But it has some drawbacks I am hoping to avoid on the 560SEC.    Firstly, the auxiliary fan only runs when the A/C is on.   This isn’t the end of the world, as the A/C will generally be running on hot days.  Its not ideal though.   Secondly, as the fan is always running, it even runs at freeway speeds which is not a good idea.

Instead of hacking the system like that, with a lower cut in point, I should have the best of both worlds.  More air flow during stop and go traffic, protection even without the A/C and no unnecessary running of the fan.   My 450SLC already has this modification and it works quite well.   The 107 A/C system is even more marginal than the 126 with R134A so it needs all the help it can get.

To perform the auxiliary fan cut-in modification, a resistor is placed between the two wires that come from the temperature sensor unit on the thermostat housing.   From reading various forum posts, a 1100 ohm resistor should result in a cut in point of around 94C.   This seemed ideal, as with an 80C thermostat, it should be fully open around 92-93C.   Having the fan running before the thermostat is open is just going to have them running cross purposes.

The picture below shows the temperature sensor (the green sender unit).  The harness with the two wires is the one where the resistor must be placed.    Jaycar had the resistors I needed in stock.

Most people simply solder the resistor between the two posts and call it a day.   I tried this, but the modification didn’t work.   I’m terrible at soldering and avoid it where I can.   After that, I came up with a better solution anyway.  I could build something small and removable on the bench, so the car can go back to stock at any time.

Lukcily, I had a set of bullet connectors that fitted the factory connector.  I crimped them together and tested my work using a multi-meter.   My handiwork can be seen below.   A little messy, but no permanent modification to the car.  I may try and make a neater version and replace this one.

This was actually V2.   On V1, I used shrink wrap to make it look neater.   Even though I only used the heat gun for a couple of seconds, it warped my connectors to the point I could no longer push them onto the sender unit poles.   Back to the drawing board and using electrical tape this time.

After I installed my auxiliary fan cut-in modification, I took the car for a brisk test drive and then let it idle for a bit.   It worked, but not quite as I expected.   The cut in point seemed more like the high 90s rather than mid 90s.   I don’t know if this is just because my gauge is not particularly accurate.    I may swap the modification over to the SEC for a side-by-side test.   That will have to wait as the SEC needs new brake hoses.

The picture below shows the temperature not long after the fan kicked in.   Before that I will see how the car performs in the real world.

Obviously, the auxiliary fan needs to work properly before this modification is performed.   I have just replaced the fan on the car, and have tested it working fine.

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