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  #1  
Old 05-21-2009, 08:02 PM
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1989 420SEL Air Conditioning Temp. Switch

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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Last edited by Cigar Havana; 05-21-2009 at 08:10 PM.
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Old 05-21-2009, 08:11 PM
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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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Last edited by Arthur Dalton; 05-21-2009 at 08:17 PM.
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Old 05-21-2009, 10:29 PM
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Next Steps

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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Old 05-21-2009, 10:42 PM
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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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Last edited by Arthur Dalton; 05-21-2009 at 11:05 PM.
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Old 05-21-2009, 10:53 PM
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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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Last edited by Arthur Dalton; 05-21-2009 at 11:08 PM.
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Old 05-23-2009, 06:05 PM
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No Go

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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Old 05-23-2009, 06:19 PM
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Is you chassis early or late ?
Single fan or dual will tell.
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Old 05-23-2009, 06:21 PM
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Single Fan

Only 1 fan.
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Old 05-23-2009, 06:29 PM
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OK

Early

That F22 is most like the problem b/c you have pimary sensor circuit integrity hearing the relay click. But if you also have NO FAN with High speed sensor Default, then the suspect shifts to broken wire on R15..[ which could be why you only see one wire on each side [ The third wire would be Common feed for both speeds ]
I am not sure on R15 location for early chassis, but if you look at the part below, you can see what it looka lik and compare to what you see ,.
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Last edited by Arthur Dalton; 05-23-2009 at 06:42 PM.
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Old 05-23-2009, 06:38 PM
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http://catalog.peachparts.com/ShopByVehicle.epc?q=1989-Mercedes--Benz-4--0sel-Cooling--System&yearid=1989%40%401989&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6265%3AMBC%7C1533%3AED%7C10000055%40%40420SEL&catid=240909%40%40Cooling+System&subcatid=241010@@Aux%2E+Fan+Resistor&mode=PA

Click on the pic and it will zoom .....you can then even read the part # for your comparison.............................
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Last edited by Arthur Dalton; 05-23-2009 at 06:56 PM.
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Old 05-23-2009, 07:08 PM
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The R15 may still be located under the brake booster (covered with leaves) and the F22 is probably located on the booster side of the firewall (1 1/2" by 1/2" black plastic fuse block bolted to the back side of the aluminum firewall partition). The fuse box looks like this.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1985-Mercedes--Benz-500sec-Body--Electrical&yearid=1985%40%401985&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6291%3AMBC%7C1540%3AED%7C10000058%40%40500SEC&catid=241959%40%40Body+Electrical&subcatid=241968@@Fusebox&mode=PA

Mark
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Old 05-23-2009, 07:49 PM
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Try fuse 'D" for Single fan model [ load] instead of F22
Fuse 7 [ coil side]

They also list R15 as lower/left/front -engine compartment...

And plug the blu sensor back in to reset the ACC and then umplu git after the engine is running..........does that get default high fan?
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Last edited by Arthur Dalton; 05-23-2009 at 07:55 PM.
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Old 05-23-2009, 08:11 PM
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Yes for resistor

Hookedon210s - yes, your reference to the resistor is exactly the part I found, and referred to as the white ceramic under the brake fluid reservoir. I'm going back out to work on the beast, and see if I can find that other fuse box.
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Old 05-23-2009, 10:19 PM
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Found Fuse

I found the fuse link. It is on the rear side of the forward firewall almost directly in front of the brake fluide reservoir. It tested good with an ohm meter, no break.

I took out the resistor with an 8mm socket. There was melted plastic on it, I guess it gets very hot. Between the 2 terminals on the resistor, the circuit tested open, zero resistance. So it looks like I need a new auxiliary fan resistor. The metal is stamped 0.2

One thing I did not mention before, first step for me, whenever working on the electricals, I disconnect the negative side of the battery.

I'll try connecting the positive side of the battery to the single wire that came off the resistor. That single wire, when tested with an ohm meter, does not connect to ground. The double wire when tested with an ohm meter, does connect to ground. Is this correct, or is there a short circuit somewhere? Note that the negative side of the battery is not connected to the car, when these tests were undertaken.
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Old 05-23-2009, 11:05 PM
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Your other post stated R15 only had a single wire on each terminal..now you say it has a single and a double wire as I stated it should have.
Which is it ????????

< I'll try connecting the positive side of the battery to the single wire that came off the resistor.<<

That is not what the test is.
Read the test again..[ Post #4..R15 test] it is a fan motor and R15 test and that test requires all wires [ a single on on..e side and a double on the other] to be hooked up to the R15 terminals.
The test also requires the Battery ground to be connected b/c this is an In -Situ test.


The test again:

bring 12 v pos oover to the R15 resistor. Touch the 2 wire terminal and you should have HIGH Fan
Touch the single wire terninal for Low Fan.
If Yes to both, fan motor is good, as well as R15..if NO to single wire test, , but yes to double, R15 is bad.
This test does not need any key On or controls On , or engine running , etc. It is a direct 12v to fan in-situ test so we know which direction to take the next test. If this test is PASS. your problem is upstream


But in this case , we already know you have an open R15..which effects ONLY low speed.....so the relay is clicking and the R15 is downstream from the relay load contacts, so a Bad R15 fits the condition.
The true test in that case is to take a V reading at R15 single wire side while at the same time jumpering the pressure sw at the reciever/drier to energize the relay. if YES all low fan circuits are verified as good except r15.

You can also simply jumper R15 right at the terminals [ across them ] and then jumper drier sw to verify a bad R15 ..the results will be a high fan in place of the normal low fan b/c you have NO dropping resistane due to R15 jumper. Either way.... I do all those right off the bat at R15 cuz I can perform all those test variables in less than 2 min. I then work off those telling results.

End of test.

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Last edited by Arthur Dalton; 05-23-2009 at 11:54 PM.
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