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 […]

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 […]