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Old 10-12-2001, 10:49 PM
Posts: n/a
300TE A/C / Temp Control

I have a 300TE and I'm trying to understand the pressure switch functions on the Receiver/Dryer. I don't get aux fan operation until the coolant hits 105C and I think I'm supposed to get low speed aux fans with either receiver/dryer pressure or temp switch actuation (I think it's the press switch but I'm not sure).

Any help would be appreciated. Really!


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Old 10-13-2001, 10:58 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Yes, you should get low speed fan operation due to pressure at about 250psi (at least thats what the C280 had this week after I put a new fuse in the low speed relay).

You will need to view a wiring diagram to see whats involved (there are many changes by year). It is very likely that you have a fuse or resistor problem.

The switch that is involved is the one on the drier with the short pigtails. The one with no wire is the low/high pressure switch.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 10-13-2001, 11:56 AM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,517
The colored switch on the receiver/dryer is the one which activates the electric fan(s) based on pressure. It should be either green or red - probably red. I don't have the specs for the red switch in front of me, but it switches the fans on at about 225PSI and off at 175PSI. I wouldn't expect to see high enough pressure this time of year to activate the fan, unless you live in Florida or some such.

A quick test is to unplug the leads which connect to the switch and short them together. The fan should switch on. (Don't forget to have the ignition switched on!) If the fan runs, you've got either a defective switch or low refrigerant charge.

If the fan does not run, then it's electrical troubleshooting time. To do this right, you'll need the maintenance CD and an inexpensive volt/ohm meter. Probably the two most common problems are a dead fan or a burned out resistor. The fan can be tested by directly connecting it to the battery. Testing the resistor (it's right next to the receiver/dryer) requires an ohm meter. There's also the possiblity of a bad relay, faulty ground, or even defective wiring.
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Old 10-13-2001, 05:32 PM
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Thanks, Steve and JC (?),

I will verify the circuit is good up to the fan by shorting out the leads to the switch. This circuit not functioning would answer a longstanding mystery on why the car could run hot, only with the a/c on, when the cooling system has been fully reconditioned with new w/p, radiator, hoses, etc....

I'll let you know what I find out.

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