300TE Temp Sensor Question

Hello to all,

I've got a 1991 300TE 4matic. I've verified the aux fan resistor is open and the coolant temp sensor on the receiver/dryer is not functioning. Both will be replaced shortly.

Here's my question:

Can someone confirm that the coolant temp sensor on the recvr/dryer does not breach the pressure boundary of the a/c system? I would imagine it does not, but I'd hate to blow R12 in my face when I try to replace the sensor if the sensor is part of the boundary.

Your help is appreciated!!

someguyfromMaryland

[Arthur Dalton]

I think you will find the aux fan sensor on the a/c drier is not temp sensed. It is a presure switch.
How have you diagnosed it as faulty?
The temp sensor for aux fan is for high fan and uis located at the thermostat housing . It is a negative resistance/temp thermistor.

Arthur,

AFAIK, on a 1991 300TE 4matic, the aux fan is operated on two speeds. The low speed is run through a resistor (which is open on my car) which gets its input signal from the temp switch on the receiver/dryer. There is also a pressure switch on the recvr/dryer to cut out at low head to prevent running the system without refrigerant. The high speed fan circuit receives input signal from the temp sensor on the cylinder head. If the low speed circuit is working properly, you will not see the car go above 100C unless you're in slow traffic.

I'm still looking for anyone who can confirm if the temp switch is exposed to a/c refrigerant.

Thanks,

someguyfromMaryland

[Arthur Dalton]

Originally posted by someguyfromMaryland
Arthur,

AFAIK, on a 1991 300TE 4matic, the aux fan is operated on two speeds. The low speed is run through a resistor (which is open on my car) which gets its input signal from the temp switch on the receiver/dryer. There is also a pressure switch on the recvr/dryer to cut out at low head to prevent running the system without refrigerant. The high speed fan circuit receives input signal from the temp sensor on the cylinder head. If the low speed circuit is working properly, you will not see the car go above 100C unless you're in slow traffic.

I'm still looking for anyone who can confirm if the temp switch is exposed to a/c refrigerant.

Thanks,

someguyfromMaryland

OK,
I guess the Wagon is different.
My schematics and CD data for a 124.230 chassis [91-300E-4matic] show low speed fan a/c drier sw part S-32 as
a pressure switch for high side pressures of cut-in/cut out of 20/16 bar.
The other pressure sw. is , as you say, to prevent compressor running with low freon. This one has nothing to do w/fan operation.

The CD procedure calls for evacuation/recharge when changing
presure switches.

As an added note on the high side low fan pressure sw. systems:
If this switch is jumper and now has low fan, the switch should not be condemned until freon level has been checked. [ a slightly low freon level will not allow the system high side to get to the cut-in pressure required for the low fan and often mis-diagnosed.

Right is right, and wrong is wrong.......

Arthur has his notes in order, contrary to what I've read on several archive posts, the red sensor on the receiver/dryer is a second pressure switch. The first p/s (S31/1) is for the compressor (on at 2.0, off at 2.6) and the red one (S32) is for Speed 1 on the aux fans.

Thanks go to Arthur for his patience in explaining my error and taking the time to verify his facts with the manual. Now I have to find somewhere to verify the charge on the system and then probably evacuate it to replace the switch. I have cold a/c and I can get about 22F cooling at idle, so I think I've probably got a good charge, and the symptoms are consistent with the switch not functioning. Basically, the car ran well, just warm in traffic and the fast speed fans were all I had to control temp. Hopefully I can get the low speed fans back and let the car run a little cooler in traffic.

Thanks again, Arthur!!

someguyfromMaryland

[Arthur Dalton]

Another note on the subject:

Be aware that the low fan is set to engage when there is a "High Thermal Load."
What this means is that if you have high ambient temp/humidity,
the high side pressures get to the point of cut-in pressure and the fans added air across the condensor helps drop this high pressure. The thing to know here is that if the system is not being Taxed/Burdened, than the a/c will run fine WITHOUT the low fan being called for.... so don't expect low aux fan if the ambient temp/humidity [ read LOAD here] are low..
this circuit is a safty valve for extreme conditions.
Some newer models use fan regardless of high side pressure,
but yours is not one of this design. [ I like this better]

The correct way to test the switch is with gauges on the high side for pressure readings and a continuity test for cut-in/cut-out specs. at the sw. connector.
This will eliminate the false diagnoses I mentioned earlier.

I might also mention that the confusion of calling this switch a temp one may come from the part sometimes being refered to
as a temp a/c switch as to not confuse it with the freon /compressor high pressure cut-out sw. when ordering.
But in reality, it is a pressure switch with bar ci/co, not
Centigrade temp...