I recently purchased an old 300 SDL. Drove it 1000 miles with no problems. I have started to fix some window regulator problems and with turning it on and off to get the regulator out I noticed that the aux fan is always on. I checked the diy posting and did the normal checks switched out the relays, disconnected the thermostat and ac switches and it still will not turn off. I've disconnected it from the plug at the fan but would like to know what else controls the fan.

Sure you switched the correct relay?

You have checked the right places, ac high pressure switch and the thermostat. Check if it is High speed or Low speed always on. You may have a wire grounded near the switches.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Maybe we need a clarification of "always on." A stuck relay is about the only possible cause of a legitimate case of "always on," given a reasonable interpretation of "always."
If "always" means something else, then disregard.

Stuck on high speed or low speed? There are separate relays for high and low speed. Does removing one of those relays kill the fan? Did you wet the 105/125*C switch by the upper radiator hose fitting.

Sixto
87 300D