M117 Auxiliary fan cut-in modification

Under normal operation, the auxiliary fan on the M117 engine cuts in at 105C.  It can also be triggered by A/C system pressure.   These settings work fine to keep the engine at the correct temperature.   The challenge is that most of these cars have been converted to R134A refrigerant.  R134A is far less efficient than R12.   The A/C system in the W126 was designed for R12 so is now under specified.   Add in the hot Australian climate and stop/go Sydney traffic and you end up with a hot engine and poor A/C performance.

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.

auxiliary fan cut-in modification

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.

auxiliary fan cut-in modification

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.

auxiliary fan cut-in modificationAfter 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.

Fan runningObviously, 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.

2 comments to M117 Auxiliary fan cut-in modification

  • Jason P

    I’ve tried this modification but don’t like it for this reason: It makes the heat come on before the coolant is warm enough.

    Instead, I’ve rewired the fan circuit a bit: I moved the trigger from the high side pressure switch to the high speed fan relay, and with the help of another relay have the low speed fan triggered whenever the compressor is engaged. There’s no concern with having the fan run when moving down the road… on my 350SDL if the fan clutch is bad the car will overheat if it’s a hot day out, the ac is on, and you’re blasting down the highway at 130k.

    For whatever reason the fan clutches for the OM603 seem to be problematic. They either don’t work, or the spring flies off and the fan is on all the time. It moves a lot of air… and consumes a lot of horsepower in this state!! I removed the clutch fan and installed a BMW thermostat switch in the thermostat housing. This triggers the low and high speed fans appropriately. There’s very little space between the water pump and radiator on the 6 cylinder diesel, so I was only able to mount a small-ish fan on the engine side of the radiator, and have a large fan in front of the radiator and condenser pushing air back in. Being a 91 my car originally had the twin fans, but one motor went bad and spares are hard to come by at reasonable prices.

  • […] couple of months ago I tried a modification on my 560SEL to trigger the auxiliary cooling fan earlier.   The […]

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