Sorry to bug everyone with this but I have done extensive research on this site and others and can't find the answer on this.

1990 300SL M104

I've replaced the ECT sensor and added JimF's resistor mod. The Aux fans turn on high at about 105c. They do not turn on low with an increase in temperature.

I've had the relay apart (its a double relay affair) and can trigger low and high speeds manually.

My refridgerant is low (another project) but I can trigger low speed fans by pulling the A/C pressure sensor leads and jumping them (the compressor also engages)

Should the aux fans engage at low speed based on temp change on this model? If so, what would be the next thing on the list to check?

Many thanks to JimF, Bobterry et al.

I can't speak to the effect of JimF's mod on low-speed fan operation, but as designed, low-speed aux fans come on due to increase in refrigerant pressure and have NO trigger related to engine temperature. Remember from your research that the main function of the aux fans is to protect the compressor.

I would suspect that your being low on refrigerant could be the culprit.

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Steve
'93 400E

Correct . The fan speeds are two completely different circuits/funtions.
Low fan is a/c high side pressure activated to keep freon head pressure in check at the condensor and high fan is engine temp sensed by the coolant CTS at 105......the only part shared for both circuits is the fan motor/s themselves. The low fan circuit is accomplished by simply putting a voltage droppin resistor is series with the fan 12v feed from the low fan relay.

I don't know about your 1990 model, but my 96 will only turn on the fans if the outside temp is 86 degrees or higher, unless of course the engine temp or Freon pressure gets up there, which it usually doesn't unless it is hot outside. This was a learning experience for me as well because I thought something was wrong with the car as I would figure anytime the AC was on the aux fans should come on, but I guess with the lower temp and the engine fan, it is enough to keep the AC cool. I did replace my fan clutch because while in traffic the engine temp would get kind of high and I don't want my little motor to get too hot! ha ha.. Good luck!
Brian

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Brian W. Heitman

www.MBCA.org/northtexas

Join MBCA!!!

Thanks everyone. That answers it for me. Without this community I'd be several dollars lighter in the pockets.

Cheers,
John