On a 1989 420 SEL, (248,000 KM) the auxiliary fan does not turn on when the temperature reads 110. Normally it would turn on at about 105 and stay on until the engine water temp. dropped to 100. I did replace the blue dual pin thermostat on the pipe above the water pump. The coolant is at the correct level. The water pump and thermostat on the pump were replaced about 3 months ago.

Another odd event is occurring, but I'm not sure it is related. When first starting the car, the AC compressor keeps cycling on and off when the inside temp is well above the requested temp. I would have thought the compressor would stay on steady until the desired temperature is reached. A hose from the dryer to the evaporator was replaced about 2 months ago and the system was filled to proper capacity. The system has been converted to 134a.

Any ideas on how to diagnose the problem?

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1989 420 SEL

I'd examine the fan first. Old fans can seize up. It should freely spin by hand. There's usually a resistor on those fans too and it may have died. The other thing you can try is shorting the wires from the fan thermostat. Take off the electrical connector to the fan's cooling system thermostat and short the 12v wire to the ground wire and it should run with the ignition on. If you have a meter, check the voltage to the thermostat (temp sensor ) for this fan with ignition switch on. Post your results so we can help you. A paper clip is a good shorting device.
Steve

The fan spins freely.

I have a multimeter so here are the readings:

1. Ignition switch off, engine cold, resistance across the two pins on blue thermostat is 4.77 K ohms.

2. Ignition switch off, engine cold, resistance from either pin on blue thermostat to ground is open, infinite ohms.

3. Ignition switch on, engine not running and cold, voltage measured in the plug to the 2 pin blue thermostat is 5V.

4. Ignition switch on, engine running and hot (110 degress), voltage measured in the plug to the 2 pin blue thermostat is 5V.

5. Ignition switch on, engine hot (110 degrees), resistance across the two pins on blue thermostat is 270 ohms.

I did not understand the statement about shorting the plug. Did you mean that I should stick one end of the paper clip into one pin socket of the plug and the other end of the paper clip into the other pin socket in the plug?

Next Steps?

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1989 420 SEL

Anyone have some advice?

Which relay in the fuse box controls the fan?

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1989 420 SEL

In my 124, its the black cube is for high speed and the silver cube is for low speed. Look for the one with the higer rating - 30amp I think...

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1992 124 E200 (Euro)

I disconnected the battery and applied 12v directly to the fan. The fan spins, so the fan is good. What else is in the circuit that I should check to find out why the auxiliary fan does not turn on above 100c?

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1989 420 SEL

Check Hi / Low Speed Relays --- The 124 uses seperate hi / low speed relays to feed the fans. They are located in the back of the fuse box.
I believe they have a fuse built in them as well.

They kick in with temp > 105 Also circuit includes the low speed resistor ---mine literally burned up!

The factory wiring diagrams for your car are helpful, not sure if they are available on the CD or not. I believe mine was in the chassis manual.

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Bryan Harter
1992 500E Smoke Silver
2002 CLK55 AMG Cab Silver/Black
2001 Harley Fatboy - Pearl White
2004 F150 FX4 Silver/Grey
1969 GTO Liberty Blue
1956 F100 Black

If anyone else has the same problems, check #1 fuse, which is red. Also, when you unplug the blue thermostat on the water pump, the fan should turn on.

The fan should also turn on when you short the wires together that go to the red thermo switch on the AC dryer. Does anyone know how to diagnose why the fan does not turn on when the these wires are shorted together?

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1989 420 SEL

The cause of your problem was possibly suggested twice above, and is likely the pre-resistor for the low-speed fan (white ceramic rectangular-shaped part with a heavy coil in the middle attached at each end to an 8mm nut, often covered with a metal cover in the middle to protect the relay from fire damage in case any debris falls into it). It is located on the 126 chassis in the most awkward place on that vehicle, underneath the brake booster, and is one of only 3-4 jobs on that vehicle that is truly cuss-worthy. You could also have a short in the wiring between fan and fuse box, and that would still be easier IMO than replacing the preresistor (which gets corroded and does not come out like it does when new).

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Don't ask me, I'm a shop-owner by default