I think I've read all the threads on overheating m103s on this and other forums.

I've just been driving around Southern Europe. 1990 Mb 300E. Temperature is stable between 80-90 most of the time. Had a few occasions of outside temperature of between 95-100 F. Idling in that temperature brings the engine all the way up towards the red line. I had to stop at the side of the road on the highway once to let it cool down. Next time I put the heating on max and I just about made it through a 2 hrs jam on the German autobahn. Again in 95F.

My viscous fan is pretty good I think. At least I can hear it engaging and roaring. But it doesn't cool enough when idling I guess.

I have automatic climate (button type) but there is no gas in the system. Two aux cooling fans in front of the radiator.

So heres the funny thing about my aux fan - low speed fan comes on with the ignition if outside temperature is more than 68F even with cold engine. it stops after a while but comes on again like it should if the engine gets very hot. But after an hour or two maybe, of driving - they are completely dead. Even when the engine was close to boiling they didn't start. Tried to tap the center with rubber hammer, tried to disengage the various sensor but could not trigger them in any way. Then when the engine cools all the way down, they start with the ignition again.

I have replaced the coolant temp sensor (4 pin) the one all the way in the back of the engine block. Replaced (aux fan high speed?) relay behind the fuse box, replaced ac pressure/temp sensor(?) the one down below the left head light. All fuses are good. Reservoire cap seems to be good. I guess my thermostat is good as it is only in extreme conditions overheating occurs.

I have a hard time diagnosing this in back here in Norway as it never gets that hot. Any clues guys?

The aux fan is not required to keep coolant temperature if A/C is not on.

The engine can keep temps well below 100C in traffic with a functioning system with just the fan clutch, water pump and radiator. If it can't keep below 100C then something is weak in the system.

I dont even get to the redline in 120F /w A/C on in traffic. About 108C and thats sitting in traffic excessively, but my climate is extreme. In 100F ambient, everything stays less than 100C with A/C and traffic.

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x2 It's possible your thermostat is tired and not opening fully. Also remove the radiator and check for dirt in the radiator and condenser fins (clean gently!) and dirt between the radiator and condenser.

Sixto
98 E320s sedan and wagon

Do as Sixto says and make sure both the radiator AND the A/C condenser are CLEAN CLEAN CLEAN!!! You may be surprised how much crap can be built up in there over the years.

When the engine is as hot as you say it is, the engine driven fan should be roaring. It will be loud, and it'll be obvious!

Move onto the basics, replace the radiator cap (when they get weak they can let the system boil and reduce capacity) and consider replacing the thermostat.

If after all the above are completed you still have overheating issues, it may be time to consider flushing the cooling system and having your radiator flow tested.

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Thanks! Will replace my thermostat and flush the cooling system thoroughly. But still - -even though i shouldn't need them, there is something wrong with how my aux fan operates.

The viscous clutch fan on these cars is marginal in this type of weather. You absolutely need to the aux fans on in order to keep the engine cool while sitting in traffic at 100f. Do the "cool harness" modification to keep the aux fans running in this weather.
http://www.benzworld.org/forums/w124-e-ce-d-td-class/1629476-auxillary-fan-turn-set-point-resistor.html

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Might be something wrong with his aux fan wiring or sensors. If they aren't engaging at near redline, then there is a definite problem.

To the OP, if you are in Norway and you are having trouble being able to diagnose your aux fans/sensors because temps don't get that high, try removing the coolant temp sensor and using a heat gun on the probe with the KOEO and see if the fans kick on. At least this way you can get somewhat of an idea where the problem is. I think you should have a 2-prong sensor and a 4-prong sensor. The 2-prong sensor would be the one that activates the high speed fans.

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1990 190E 3.0L

The low speed fan on this car is triggered by the refrigerant pressure switch only. If you have no refrigerant, it's not going to operate. High speed is a bit more complicated. On this car, there is a thermistor engine temp sensor (not a fan switch) that feeds the climate control unit. The climate control system then decides when the fan should come on at high speed. Generally, the setpoint is quite high, about 105. But yours seems to come on at random. That suggests either a bad sensor or a bad CC head unit.

But I also tend to agree that without A/C, you should have enough flow with the belt driven fan to suffice. There are probably multiple problems here.

Attached Files

1990 300E Cooling Fan.pdf (49.1 KB, 243 views)