I just inspected my 1989 420SEL W126 air conditioning system and there seems to problem at the dryer with the refrigerant temp. switch. With the inside heating/cooling climate control unit (CCU) requesting 15c and the ambient temperature being 29c, the AC turns on, and blows cold air but when I check for power at the red temperature switch located on the dryer, there is no power on either wire. I would think that switch needs power whenever the AC is turned on and the compressor is engaged. When I unplug the engine coolant temp. sensor from the engine, the electric auxiliary fan does come on. The system is charged with 2.9 lbs of 134a, and I believe it has the correct pressure.

What relay or device controls power to the refrigerant red temperature switch on the air conditioning dryer? How can I find the faulty part?

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1989 420 SEL

They call it a temp sw, but it is actually a pressure sw.

It has a 6" pigtail wire set ..slide the sleeves back on the connectors at the pigtail end and jumper them right there w/key ON ..that will get you LOW aux fan...if NO , relay /fuse / or dropping resitor are bad.

Post what you have with that test.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1989-Mercedes--Benz-4--0sel-Climate--Control&yearid=1989%40%401989&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6265%3AMBC%7C1533%3AED%7C10000055%40%40420SEL&catid=242213%40%40Climate+Control&subcatid=242240@@A%2FC+Temp%2E+Switch&mode=PA

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A Dalton

Yes, that is the switch, as shown in your link. I always thought it was temp. switch, since it is labeled as such in the parts catalog, and the black switch on the dryer was the pressure switch.

All fuses in the fuse box look good, including #1 which is labeled auxiliary fan. Note that the auxiliary fan does come on if the engine coolant reaches about 107c and if the wire is disconnected from the sensor, which is on the pipe above the water pump.

Anyways, ignition on and engine running, CCU at max. cooling, ambient temp. 28c, compressor is on, cool air blowing in, disconnected both leads from the red switch, shorted them together, auxiliary fan did not turn on.

Engine off. Replaced relay 002 545 18 05, with another used relay, thought to be good. Bought it from an independent mechanic, who indicated it was good. Again, ignition on and engine running, CCU at max. cooling, ambient temp. 28c, compressor is on, cool air blowing in, disconnected both leads from the red switch, shorted them together, auxiliary fan did not turn on.

Now the resistor. I guess it is somewhere under the brake booster? How is it mostly easily removed for testing?

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1989 420 SEL

You do not have to have engine or ac on to test the low fan .
The high fan and low fan are the same , it is just which side of the resistor they are using that changes the speed.
So, the jumper test is just to verify that circuit.
Now comes the circuit.
The low fan has a relay. That relay has TWO fuses ..one fuse is for the coil side of the relay and the other is for the contactors/load side.
Here is a trick...w/key ON, jumper the switch as suggested and listen real close to the relay. If the relay CLICKS , but NO fan, you have verified power to the relay coil. You then know the problem is on the load side of the relay..meaning the LOAD fuse [ aux fuse F22 in your case ..outside fuse box], or a bad R15 resistor.
Here is R15 test. This test doe not require any key On or engine running...just a jumper from battery.
Locate the R15 and bring a jumper from bat + over to the R15. On the side that has 2 wires, touch that terminal w jumper and you should have HIGH fan...thouch the terminal with single wire and you should have Low fan..if you have high [ which we already know you will] but no low from single wire side , you have a bad R15. If you pass both test , the problem is upstream [ relay/fuse/wiring.

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A Dalton

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Yes, that is the switch, as shown in your link. I always thought it was temp. switch, since it is labeled as such in the parts catalog, and the black switch on the dryer was the pressure switch. >

They are both pressure switches ...the blk is pressure for comp activation..the other is high side pressure for aux low fan cut-in. The low fan only comes on when the high-side reaches 20 bar...it is then called for to alleviate the High Termal Load on the condenser ..that is why they call it a temp sw..even tho it is , by design, a pressure sw..you can see that in the pic by looking at the port in the threaded end and by looking up the cut-in specs ..they are in Pressure , NOT temp. [ I hate that they did that b/c I am always incorrectly getting called on that one ...]

On your chassis , there was a location change and I believe you will find R15 behind the driver headlamp area..........

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A Dalton

As recommended, more testing. Engine off, key on, shorted the wires at the red switch together, and I could hear a click inside the fuse box. The small silver relay inside the fuse box, was clicking. It was the one on the driver’s side closest to the windshield. The fan did not turn on.

This suggests that the problem is on the load side of the relay. Now I am trying to locate fuse F22. There is small module attached to the fuse box and there are 2 small blue fuses inside, both are good. Is this fuse block F22?

Could someone tell me where to find R15 and confirm the F22? I'll take another look behind the driver’s headlamp for the R15. Based on the description it seems it has 2 wires going into at one end and one wire at the other end. It is not behind the headlight.

I do see a white ceramic block attached to the fender, deep below the brake fluid reservoir. The white block seems to have only one wire at each end. Beside the fuse box, there is a small square black module that has a resistor inside, part number 000 540 24 81 with 744k ohms resistance between the terminals.

Next, I am trying to repeat the original test. Engine on, CCU to 15c, refrigerant compressor is on, the blue 2 wire temp. sensor above the water pump is unplugged, and the auxiliary fan does not turn on. Checked fuse #1, it is good.

It seems things are getting worse than better. Help

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1989 420 SEL

That's what I thought.

Go to post 15..that is what I was getting at back at post 4.

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A Dalton