I write this article for Mercedes owners, and others, but particularly Mercedes 603 Diesel engine owners, as those are the vehicles and engines I have experience with. But the information contained here can be pretty well applied to any modern engine, Diesel or Gas, Mercedes or otherwise.
I own a Mercedes 300D and 300TD, both w124 1987, 6 cylinder diesels, 603 eng.
I have observed going to the wrecking yards the number of Mercedes that have not met with any accident, but appear to be there due to engine malfunction. I would suspect the majority are there due to overheating, and damaged cylinder heads. On the Mercedes Diesel six cylinder 603 engine, this is a huge problem, as not only is the head Aluminum, but so is the block. Usually when you overheat these engines, you destroy the cylinder head, which apparently can’t be rebuilt, and sometimes the block as well. They have a bad reputation for head damage, and I think I have discovered why. It is actually nothing to do with mechanical design of the head, but the auxiliary cooling equipment and sensors that are attached to the engine.
It is an easy problem to avoid and fix. ….. Here’s how.-
All Mercedes cars, diesel and gas, all seem to employ a Thermo Mechanical Clutching cooling fan on the radiator. These are great, as they free wheel when the engine is sufficiently cool, (like at high speed on the freeway), but engage when the engine begins to heat up (like in traffic), to cool the radiator. However when the mechanical clutch on the fan fails, the fan does not engage, and your engine suddenly overheads. You notice that on the freeway when traveling at speed there is no problem, but when you get in traffic, suddenly things start getting hot. (If you have an older MBZ, I suggest you start carrying a spare Thermo Mechanica Fan Clutch around with you, as it is as important as spare radiator hoses and fan belt.) By the way, you will need a very short 7 mm allan key socket set to undo the Thermo Mechanical Clutching fan when you need to replace it.
As a safety back up, Mercedes installs a huge electric Auxiliary Fan, which is mounted in front of the radiator, and visible when you open the hood. (Most modern cars also have this). This fan is activated at 105 degrees by a heat sensor on top of the engine block, where the water flows to the top of the radiator. The heat sensor in turn activates a relay, which is hidden behind the fuse compartment, which sends power through a fuse and onto the auxiliary fan. If anything in this auxiliary fan electrical system of sensors and relays fails, your overheating back protection has gone, and you are well on your way to destroying your cylinder head and possibly engine block.
In the last 6 months I have experienced three thermo mechanical fans failures on my Mercedes engines. I figure the clutches were the originals, and thus 20 years old. On all my vehicles, when this happened, the back up Auxiliary fan also failed, as I figure they had probably never been activated in 20 years, or since the cars were new.
On one vehicle the auxiliary fan failed because the 16 amp fuse in the fuse holder had developed a layer of oil encrustation over 20 yrs, and when it activated, it arched a spark, and melted the fuse holder, and thus disconnected the electrical connection, The other vehicle, the fuse was clean and OK, but one of the controlling relays (there are two), had failed, and thus the auxiliary fan also failed to activate. The third car, the heat sensor was not working, so once again, the Auxiliary fan failed to activate.
In all three cases I caught the overheating immediately, and turned the cabin heater on full heat, which enabled the engine to cool down via the cabin heater.
ALWAYS do this if you notice your car has overheated, it could save your engine!
So there is a super simple solution to this problem, which will give you a good backup if your Thermo Mechanical fan clutch fails, and so does everything else including the electric auxiliary fan system.
The solution is to install on your dash a back up switch to manually activate the electric auxiliary fan, and secondly, run two wires from the 12 volt supply to the auxiliary fan to a light in your instrument panel in your dash, so you can monitor if the auxiliary fan is getting power or not. In the Mercedes dash, you can simply remove one of the other instrument indictor lights, such as washer low level, or Fog lamp indicator lights, and use it’s position to monitor your auxiliary fan.
So on my Mercedes 603 engine, I ran two wires from the auxiliary fan heat sensor on the engine block to the new switch I mounted in the dashboard below the light switch. When activated, this switch closes the circuit on the heat sensor, thus activating the relays and everything else that starts the Auxiliary fan (on some cars this is only one wire to the sensor, and the other end of the switch connects to the car chassis, or ground). Also from the two wires that feed 12 volts to the Auxiliary fan, run two more wires back to your instrument panel in the dash, and connect it to the dash indicator light that once displayed your fog lamps or wind shield water low indictor, but now becomes the indicator to tell you if the Auxiliary fan is activated or not.
Every now and again, you can activate this switch to test everything if your Auxiliary fan back up system is working. This will test everything from relays to fuses. The only things that could fail are the sensor not activating at 105 degrees, or the auxiliary fan motor burning out.
Also, if the engine ever begins to over heat, and the sensor activates at 105 degrees, as it should, and because you now have a monitor light on you instrument panel, you get a visual indication that your engine is overheating and the auxiliary fan has been activated. Mercedes does not provide this warning light feature, only the temperature gauge. So this is an added bonus which didn’t come installed on the car before.
Furthermore, by having his manual override switch installed, you always have an emergency back up plan for when your Thermo Mechanical fan suddenly stops working, and you have to still get home so you can get out your tools to install your stand by Termo Mechanical Clutching fan.
For sure it will save your motor one day.
Further 134a A/C advantage. A further advantage of having this manual override feature on your auxiliary fan is you can activate it to assist the cooling abilities of you’re A/C, if you have converted it to 134a, The AC cools better when there is air flowing through the cooling condenser which is where the auxiliary fan is mounted. Turning the fan on increases the airflow in slow traffic, and thus helps the cooling abilities of the AC..
I hope you find this information helpful