W124 -Diagnosing Why High Speed AUX Fan Is Non OPT (1)

Found out why High speed auxiliary cooling fans are not working to cool car when overheating.
Links for what I use!
Power Probe, must have to diagnose electrical problems!
amzn.to/3r9nw4s
Soldering iron comes in handy!
amzn.to/46iEZpI

[@shaazy]

You're highlighting some extremely important things that NO ONE knows about on the m103 Mercs of this era! Thank you for making this! Would be absolutely worth it to add Timestamps especially when you talk about Freon pressure, and its effect on the low speed circuit of the aux fan! So crucial

[@Nolebad]

a true legend, thank you

[@orln]

thanks for documenting this, very detailed

[@marcusfinanger1116]

I'm kind of confused. I thought the four pin thermostat sensor at the back of engine controlled the aux fans. But it's 2 pressure switches? My aux fans are always on and I'm trying to figure out what actually controls on and off on these fans. Is it the pressure switches? Thermostat sensor? Ac or freon level? Do they turn on when AC turn on? My ac doesn't work so do I have to get the AC compressor to work before I get the fans to work. It's like you said everything has to work perfect or nothing works.. I'm going insane.

[@mygermancars7264]

The 4 pin is for fuel management , the blue 2 pin is for AUX fans, wires for it go to climate control unit in dash, the signal then goes to relay that powers AUX fans , the pressure switch at receiver dryer turns on low speed fans when AC is on, I think there's also a high speed fan pressure switch there that senses really hot freon temps, it activates to protect AC system.
Its a strange system for sure, if your low on freon the low speed AUX function won't work., if your fans are on from the get go, the 2 pin could be bad.