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Old 07-26-2013, 04:53 PM
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MBeige MBeige is offline
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Join Date: Apr 2005
Location: SoCal
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Originally Posted by Mike D View Post
The cooling fan relays on most Benz are activated by the completion of the ground/earth portion of the relay so adding an additional 12V power leg won't work.

To test the cooling fan relay you ground the 100C plug and the fan should engage.

For the A/C portion you jumper the pressure switch which will either be in-line or mounted on the receiver/dryer with the A/C engaged.
Actually it's the temperature switch, and not the pressure switch, you need to ground to activate the fan. You can engage position 2 of the key (or have the car running). Disconnect the leads of the temperature switch (look for the pigtail harness on the receiver/drier) and bridge the two leads coming from the harness (not the pigtails). With the key in position 2 this should activate the auxiliary fan.

The fan should also have a plug located in front of the driver front wheel.
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