I have done a search and have swapped out the high speed fan relay and replaced the thermo switch to the fan. Fan is still always on high speed.....as soon as I hit the key switch its on. What else could this be? THANKS GUYS!!

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'89 300SE
'02 E320 Wagon

I don't think an 89 should have a thermo switch, it has a temp sensor. If your switch is connecting the two pins together normally it will make the ACC control unit think the engine is overheating and run the fans on high. It will also turn off the AC compressor clutch.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Ok, it may not be a switch....it could be a sensor. It screws into the head up by the injectors. It has a blue head with 2 connectors. Thought for sure this was it, but no.

So what switch would cause this? Where is it located? My AC compressor wont turn on so thats why I'm starting with the fan.

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'89 300SE
'02 E320 Wagon

What's the part number on it. IIRC the one on my car (which uses the same system) is blue. The reference photos in the factory service manual are kind of hard to make out.

Do you have a multi meter?

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

The original Mercedes part # looks like 008 5424517

replacement I got from Pelican Parts. It is a Behr with the same part #

I do have a multi meter

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'89 300SE
'02 E320 Wagon

OK that sensor looks like the right one.

If you unplug the sensor plug and turn on the ignition does the fan come on high speed? I'm wondering if there's a short in the wiring someplace between the sensor and the climate control head.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

yes sir, the fan stays on with sensor unplugged

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'89 300SE
'02 E320 Wagon