Hi Guys
Just getting around to fixing a high heat problem on my 560sel. When the a/c is turned on the aux fan is supposed to come on high according to Mercedes dealership. Some one, before I got my car had disconnected the green sensor on the a/c drier and took the two wires that went to the drier sensor and hooked them up to the two heavy wires that go to the aux fan. This caused the fan to run on high speed all the time. I disconnected the two wires that were mickey moused to the aux fan wires and wired them back to the drier high pressure sensor the way it should be. The fan does not come when the air conditioner is turned on. Since my a/c was a little low - I was told that a low charge would not trigger the sensor on the drier. Well, I now have had the a/c topped up so it blows nice and cold - but the high speed aux fan still won't come on when the a/c is on. I checked with Mercedes dealership and they suggested that I disconnect the two wires from the sensor and short them together and the fan should come on high. It doesn't. Is there a relay or something else I can check to try and find it? The fuses are all good - no corrosion on the tips. I have a funeral to go to in Michigan next week and I would like to fix it before I leave.
Thanks ahead of time for any help.
Morris

Your system may be working correctly -- I have been looking into this the last week on my 560sl. If you do a search you will find much information.

The system in the 560 did not turn the fan on when the ac was first turned on -- it only turns on if pressure is high in the ac system or the water temp is too high.

If the ac is a little low on gas the pressures never get high enough to trip the switch. This is what they feel is my problem -- I had it converted a few years ago -- I have made an appointment next week. I do not really think that this is the best layout.

The tech was saying if everything is right the fan will rarely go on -- the engine driven fan on this model is very large and powerfull and does a good job of cooling the system. He was saying that the high water temp cut in is very rare and could indicate something else up -- unless you were in very hot weather

If you bypass the pressure switch the fan will run all the time - even with the ac off! So this simple change will not solve the problem.

Thanks for your reply
However" The tech was saying if everything is right the fan will rarely go on -- the engine driven fan on this model is very large and powerfull and does a good job of cooling the system. He was saying that the high water temp cut in is very rare and could indicate something else up -- unless you were in very hot weather

If you bypass the pressure switch the fan will run all the time - even with the ac off! So this simple change will not solve the problem."
On my car - when I travel when it is above 90F outside on the highway - my engine temp rides about 95C with the a/c off - which is OK. When I put the a/c on - the engine temp climbs to 110C after about 10 minutes and the aux fan does not come on high - from what I have read on the subject - the fan should kick in at about 105C. It doesn't. When I bypass the pressure switch - the fan does NOT come on. If I apply 12v to the fan - it works at high speed AOK. That is the problem. That is why I feel there is a relay problem - but where?? If you have any idea why - I would appreciate it. If I find it this week end - I will tell you Monday.
Thanks
Morris

The fan will not go on until you hit 115 -- it will then run until the temp drops down to 105. If it stays at 110 on the hightway in very hot weather MB says this is design intent.

To get the low speed they have a resistor - in my car it is right up front in the airstream -white - it looks like the resistor for the ignition. You can bypass this and then bypass the pressure switch (red on my car) this will check both of the components the fan will then run on high.

I did not have to look into the rest of the controls for the fans - I was surprised that the fans did not come on with the as on low as other cars do in fact as many other MBs do - But then I have never had any problems - and I may not now if it is just low gas charge. But I would think that a simple relay of the compressor to switch on the low side of the fan would increase the cooloing capacity

Problem solved for now - but I hate it when a repair has to happen this way. I pulled the 6 silver relays in the fuse box and checked them - they were all good. I pulled the #1 fuse for the aux fan and though it looked good with absolutely no corrosion - I cleaned the ends with 4oo grit sandpaper and re-installed it. That is all I did. Now the fan comes on properly when the two wires from the high pressure sensor are shorted together like it is supposed to and for the first time - if I remove the cap from the temp sensor on the top of the engine by the cruise control - the fan comes on high like it is supposed to. Temps up here only went to 77F this week end and when I took the car on the highway with the air on max - I froze but the engine temp never went over 95C - appears to cycle from 85C to 95C - which is normal. Since everything is now working good - I can only assume that there was a faulty connection where the relays plug in and by uninstalling and reinstalling the relays - it cured the problem. Before I did this there was no 12 volts to the wire to the high pressure sensor - now there is. Go figure - I will enjoy it while it works and wait to see if it lasts. Thanks for everyone's suggestions. I will keep you all posted.
Morris

Morris,

Can you tell me what wires you shorted together and how to test this system. I pulled off a plug between the air intake and the radiator and tried to short the pins (two on left, one on right) but the aux fan did not start. Is this the same thing you did and which pins did you connect. Also, what does the temp sensor by the cruise control look like and where is it? I just got my car recently and want to test that this was all hooked up properly (after new radiator installed in '98). I will try the relay trick, as well.

Thanks,

John

87 420SEL
00 C230K
00 A4 2.8Q

On the driver's side behind the headlight you will find the air conditioner drier - a black round object. On the top of it you will find a sight glass and a sensor that is green in color screwed into the top beside the sight glass. The two wires to this sensor should cause the fan to come on if they are shorted together as the green sensor activates when the a/c pressure goes high enough to engage it. Mine did not cause the fan to come on when they were shorted together. Now - miraculously - they do when shorted together. The temp sensor is on the top of the engine at the end of the upper rad hose. There are two sensors here - one with one lead (temp guage) and one with two leads - Aux fan controller. The system is designed so that if the sensor goes open - the fan starts on high. So to test it - simply pull the connector off the sensor and the aux fan should start right up - when the engine is at operating temperture. If it does not - change the sensor - This one is easy - only slight antifreeze loss. The green one on the drier screws right into the a/c system and the system has to be evacuated and recharged to change that sensor.
Hope this helps you. I am leaving for a funeral in Michigan this week for my sister who died of ALS - so I am hoping the repair holds - if not - I will patiently check things out when I get back and find it once and for all. By the way - when I had pulled my temp sensor on the rad hose - prior to the fuse and relay thing - my fan did not come on high speed - it does now.
\Morris

Thanks for the details, Morris. Sorry the trip in your ride has to be for such reasons...

John

Morris

The reason for your NO Fan on ETC sensor test is the same as your No fan a/c ...
The fuse feed for the coil side of both relays is common in the fuse box..
The load side for each fan circuit [ relay secondary circuit] is the Contactor side of the relays. These have seperate load fuses.
So, if you have a bad fuse / connection on the relay coil
side [ primary side], No fans will work [ low /ac fan OR ect/ high fan.].
This fits your remedy/cause diagnosis.....
Bad contact on relays primary circuit feed ..

Just remember ... the sensors trip the COIL side of the circuit .
This is the low amps/current side of the relays
The working. high amps [LOAD] side of the relays are the CONTACTS in the relay and they carry the high current to the fan motors... [ that is why we use relays] --low switching/sensor current used to pull-in contacts that are capable of carrying larger current [ in this case , fan motor...]
A tip off to a tech as to wether the problem is on the relay coil side is to listen for the relays to CLICK when testing the sensors ..if a relay is clicking and one still has NO fan, one knows that the problem is not on the relay primary side of the circuit..
so you go to the load side for further testing ..[ usually fuse on load side for high fan and good chance of bad dropping resistor/connection on low [a/c] fan......
As you already know, the most common mis-diagnosed NO low a/c fan is the fact that LOW freon level will not allow the high
side pressure to reach the switch cut-in spec...so , anyone getting a fan after jumping sw. test must first check freon level/pressures before condemning the sw......

Thanks Arthur
You hit it on the head. If the circuit acts up again - I will have an ear to the relays to hear if the primary is pulling them on. I get the feeling that the relays have never been touched since 1988 and by simply unplugging them and plugging them back in - better contact was made on the pins. So far all is well. If you could help me understand something - I would be appreciative. I have found out that the temp sensor on the upper rad hose will engage the fan at high speed when the internal resistance goes up because of high engine temp. I have found out that the high pressure sensor on the drier will kick the fan in on high speed when triggered. What sensors kick the fan in on LOW speed and where are they located? According to Mercedes dealer up here - the green sensor on the a/c high pressure will only cut in to turn on the fan if the a/c is on and the outside ambient temp. is above 90F (roughly) Apparently they say that lower outside ambient temps do not cause the a/c to build up enough high side pressure to kick the fan on unless it is working harder from outside higher ambient temps. So far with temps around 77F - a/c set on minimum - car gets cold but engine temp stays below 95C so the fan has no reason to kick on. I guess I will have to wait until I hit Chicago with their 90-95F muggy temps to find out if all is well.
thanks
Morris

You are getting correct info..
The aux fan for a/c is simply a saftey circuit to aide the High Side pressure from getting out of wack under High Thermal Load by cooling off the condensor with air-flow... [ this reduces the high side pressure]..
This circuit uses a dropping resistor ans is LOW fan... if you are seeing HIGH fan in this circuit , someone may have jumpered the resistor [ guys in the South do this to give a little more flow to condenser demand]
In Hot climates , it is common to see the fan come on fairly consistently, but in cooler climates, there is no DEMAND for condensor air-flow aid as there is no high side burden due to lower Ambient temp...

In cooler climates , one will usually see the fan cycle when at idle coming off a run [ ie. at a stop light ]. This is a good indicator of proper operation in mild climate[ I actually use a high idle , hood closed , 2-3 min test for this condition].
Remember , the high side pressures change at different thermal load conditions, and the aux fan is there to keep the high side in check when called for ... it's main puorpose is to keep the high side from getting so high that the sw will cut off the compressor
due to un-checked high side pressure ...in other words -it works in that window of pressure conditions....
On the ECT resistance thing, the ect sensor is actually a NEG
thermistor --ie- the resistance gets LOWER as the temp increases... and that lowering trips the fan module control...
Some fool with this sensor for a lower trip point, but others have had better luck with a lower thermostat... [ my preference]

I went out to test this. When the car is parked, operating temp, and the AC on full (ie both arrows filled) the fan kicks on. As soon as I turned the AC off, the fan stopped. I take it from this the AC pressure sensor is working. When, however, I then pulled the temp sensor plug (with two pins) the fan didn't come on. Can I take this to mean the temp sensor is probably not working?

John

OK Arthur - got you on the a/c circuit - my trip this week to warmer climes will tell if all is well. On the engine temp sensor - you have me confused - if it is a negative thermister - lowering in value as the temp goes up - why was the fan test from Mercedes dealership to pull the connector off the sensor and the fan goes on? With open circuit -resistance will be infinity not low. Are we talking about two different sensors???
Please advise
Morris

Cuz the ECU has a DEFAULT detect for open/failed sensor circuit

so if the fan comes on when engine temps are normal - chances are the sensor has failed (to be verified by checking ohmage of disconnected sensor) - right? So if I understand what you are saying. This is the only sensor (on rad hose) that will kick aux fan into high. The high pressure sensor on a/c drier is what kicks the fan into low speed (unless load resister has been jumpered). Are my assumptions correct?