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Old 04-29-2015, 07:52 PM
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jay_bob jay_bob is offline
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Join Date: Oct 2011
Location: Columbia, SC
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On the 87 124s the auxiliary fan is controlled by the two sensors directly. (88 and up they changed the control of the high speed to the PBU)

Low speed power path: fuse D -> relay K10 -> resistor -> fan motor
High speed power path: fuse D -> relay K9 -> fan motor

Low speed control path: fuse 7 -> relay K10 coil -> S32 (refrigerant pressure sw at dryer, directly behind the left headlight) -> ground

High speed control path: fuse 7 -> relay K9 coil -> S25/5 X1 pin 2 (this is the 3 pin sensor switch on the water neck on top of the head just to the left of the fuel filter - pin 2 of the 2 pin connection, ground is pin 1 of the 2 pin connection) -> ground

If you short the leads at the pressure switch, K10 should pick up and you should get low speed.

If you short the leads on the 2 pin connector at the temperature switch K9 should pick up and you should get high speed.
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2014 ML350 Bluetec (wife's DD)
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both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech
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