Bingo! that's the correct operation.

Also at higher revs the fan clutch disengages.

At over 30kms per hour the air movement through the radiator renders the fans redundant. Even at extreme ambient air temperatures the system should keep the engine temperature at or less than 110 degrees - not up in the red. This is why I suggest a coolant circulation problem.

It is my H.O. that the M103 engines should have idiot lights rather than a temp gauge - which seem to worry many people by the temp oscillations.

His car was only barely touching the red after sitting, idling in traffic for a long time. No mention of overheating while driving. The aux fan also did not kick on at 105 as it should have. A very worn fan clutch will definitely affect the small amount of air the engine fan is able to produce at idle.

I know that just sitting in the drive through food lane, my car will shoot up to 100C within 5 minutes of idling. If it's anywhere above 80F ambient, it will hit 105C. Then the aux fans will kick in full force. But if I had a weak fan clutch, and no aux fans, I could see it hitting 115 easily.

Anybody have a pic of the blue sensor on the head? I don't see one on my 87. Maybe different color?

Test the fan clutch by simply turning the fan with the engine off and see if it coasts. If it does, replace the fan clutch.

The electric fan is actuated by high side a/c pressure and will not come on unless the a/c is on and the high side pressure is dangerously high. It's only purpose is to bring down that dangerous high side pressure.

The main fan can be adjusted to come on earlier at around 95 'C.
as can the aux fans.

True, but not the complete answer. It is also activated by the blue base temp sensor on the head, back about cyl 4 (farthest right in the photo). That's what activates the high fan at about 105C, while the pressure switch on the drier responds to condenser pressure (somewhere between 16 and 20 bar, depending on red or green switch).

On my 87 it is not blue....It is a big green sensor with two 'plugs' that go into it...
Anybody done it with this model?

Cal Learners image might be from the later model M103's.

Here's an image from the earlier M103. The ECU temp sensor is a two pole green sensor in this variation. Your aux fan arrangement maybe a single fan and not the dual as per later update W124's.

http://farm3.static.flickr.com/2162/...413ca8f5_o.jpg

So it is the water temp sensor on that image?

If so, that is the one I "attempted" to do. I unplugged with Key ON. Then started the car and the aux fan did not turn on...I thought maybe it was the wrong sensor, but maybe not? I checked the fuse and it looks fine.

Like I said though, when the A/C was functional, the aux fan was working fine.

That sensor is the water temp gauge sensor for the gauge in the instrument cluster.

http://www.ps2cho.net/downloads/phot.../Headimage.jpg

Is it the green one then? If so, which plug do I remove? Is it the same process so I can just do a trial-error to figure it out?

How many sensors do you have there? Three or four?

The green sensor is normally in the very rear of the head. That "should" be the sensor for engine control.

I don't want to second guess here - U.S. Specced cars are different from Euro's and cars specced for the R.O.W.

In the later models with dual fans there are two relays
one actuates the fans at low speed - AC derived operation
one actuates the fans to high speed - sensor derived operation.

I'm not sure it's the same as the diesel, I only have diesel and M104 powered 124s right now.

If it is the same, the green sensor has a single-pin connector, which is for the temp gauge, and a 2-pin connector, which turns on the fans, high-speed, direct / no relay. Low speed goes through a ceramic resistor behind the left headlamp and the high-pressure switch on the receiver/dryer next to it.

Babymog is wright.Two pole sensor is the one you looking for. With unpluged sensor,ignition on engine not runing, connect those two wires and your fan should run if wiring is ok.Wiring goes over resistor (lego look) at driver side near by head light. Position of the sensor is different with year and market.

Gotcha. Then yes my very rear one is for the KE-Jet coolant temp sensor. Do I need to bridge them or just unplug? Should the fans kick in right away or when the car is started if it is functioning correctly?