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Old 06-09-2003, 12:26 PM
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Join Date: Apr 2000
Location: Indianapolis
Posts: 109
Question on Aux Fan operation with AC on

Friends - 1992 300TE, quick question on the AC. The system charge is a bit low (bubbles showing on drier), but the compressor does come on. HOWEVER, the Aux Fans do not engage. The electrical dwgs show a pressure switch in the Aux Fan circuit. Should the fans come on whenever the compressor does, or is this a situation where there's enough pressure for compressor engagement but not the Aux Fans (meaning fan resistor might be suspect).



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Old 06-09-2003, 12:40 PM
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I would like to be more specific in my response to you but I couldn't log into ALLDATA for some reason. Any how the aux. fans are pressure and temp. actuated. Pressure from the A/C system will engage a low speed setting on the aux. fans and I think (not sure of the exact temp.) about 107'C engine temp. will activate the high speed setting on the aux fans. Other car brands are switch activated as soon as you turn on the A/C system but not the Benzs.

The air in the system could very well be keeping your friends aux. fans from coming on. An easy way to tell is to ground the pressure sensor and see if they come on, the sensor is behind the left headlight on the A/C line. Can't miss it!!
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Old 06-09-2003, 01:56 PM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
< >>

There are 2 switches , one for compressor and one for low fan [a/c].
The first sign of slightly low freon is no low fan..
If fan comes on with the leads to the fan pressure sw jumpered,
the resistor and electrical integrity of circuit is fine.
If that works, then you are prob. not getting to cut-in spec of the switch [20bar]
due to low freon.
See other post from today...

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