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STOP YOUR MERCEDES FROM OVERHEATING and other useful information

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davidknaus's picture
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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

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pch2021's picture
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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

its an 8 mm.

this was a very, very useful thread. it also applies suitable to cars with a 102 and 103 engine....and many of you know ALL of the 2.6 litre 103 motors like tog et hot in traffic!

excellent observation and a very thorough description.

just to add to it, please observe the following directions:

make sure you are using the correct MB coolant.

make sure you change your thermostat every 2 years or so.

make sure you have a clean cooling system [every 30,000 miles, clean it out!]

make sure your system is purged of all air.

make sure your freon levels are up! low freon causes the AC temp switch on the drier to turn on later, as the freon temperature rises slower when less is in the syetm!

and make sure your coolant/water concentration is somewhere between 40/60 or 60/40 (:

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1972 280SE 4.5 252K
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Re: STOP OUR MERCEDES FROM OVERHEATING and other useful informa

Very usefull. Another overheating issue on the 602 comes from plugged up trap oxidizer before the recall, and those unfamiliar with the MB replacement system.

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

Excellent post! On my 89 535i BMW I had a switch mounted on the dash to do exactly as you desribe here. The Big BMW sixes also run quite warm in the summer here in NE Louisiana.

Onto my 86 300sdl which also has the 603 diesel in it: My alternator seized the other morning and it threw my serpentine belt in traffic. I immediately turned on my heat to high and fan to high and used the heater core as the radiator. I jumped out and went to the trunk and retrieved a jumper wire from my tool kit and jumpered my high speed fan switch so I had the electric fan and the heater cooling my motor as I limped the remaining three miles home. This saved my car, and the coolant gauge never went above 80 doing this.

I will run a switch this weekend, just like in my BMW, so that I can manually control the high speed electric fan.

John

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

Hello, and thanks for this great information!

I own an '92 300TDT that is getting quite hot. When driving at the highway and normal slow driving, the temp is usually 85-90, but when idling, or driving in the city with AC working, it is often at 100C or above. It once got up to 115C before i panicced and turned on the heater. I don't know what "hot weather" is like where you live, but here, it is pretty hot at 25C.

I have heard the el. fan kicking in when idling at under 105C engine temp. I reckon this is because of AC. The fan shuld kick in at 80% when preassure in AC exceed 18bar, and stop when preassure going under 15bar. The strange thing is that i was almost unable to hear the fan.

I suspect the main fan being out of order and will deal with that later.

Today, after reading this post, I took off the connector at the temperatur sensor and short-circuited it. After turning on the ignition, the fan did start, but it hardly got any rpm. I was almost able to see each blade as it was spinning. The RPM went up and down like a 18-year old with his brand new '85 honda civic at a local gas station trying to impress someone :-)

I tapped the grille in front of the fan, and the RPM increased for a very short while. I put my sho on the middle of the fan giving it a little resistance, and the fan revved up like cracy. Now it was looking more like it. And smelling like burned rubber from mye shoe.

So. I suspect my fan is not working like it is supposed to be. I don't know if the sensor is working either, but if i can first solve one problem, I'll soon discover if there are any more of them. Can this fan be repaired, or do I have to buy a new one? Anyone tried it before?
This fan, living at the very front of the car has a hard life up here in the north. Half the year, our roads are covered in salt to keep the ice away. It destroys almost everything except rust, that loves salt. :-(

There are several fans used, but mine is no. 230 in this link : http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=124193%5F15U&M=603%2E960&GA=722%2E357&CT=F&cat=15U&SID=50&SGR=015&SGN=01

Part no : 0005007993

Expencive as *#*#*#*# :-(

Accually, the viscose cuppling is cheaper. (part no : 6062000022)

I heard/read some time that the fans are controlled like this :

Viscose : approx 102C (74c air-temp throu the radiator)
El : 50% at engine temp 105C
80% at AC preassure 18-15bar
100% at engine temp 108C

Is this right?

Is there any way I can alter the fans to kick in earlier? E.G at 90C? I really dislike the temp going up and down between 82C (thermostat temp) and approx 100C when going downhill / uphill.
The temp-sensor, is it just a short-circuit-switch set to 105C, or does it give different values based on the temp?

This was a lot of questions. I hope someone can answer some of them. I've attached an image of the sensor. It is the upper cable.

tempsensor.jpg
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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

I need help. I call on your expertise for the 124 cooling system. My car a 1992 300ce is located in Phoenix AZ. June thru september the temp is over 100 every day. A few questions about the cooling system. What is the capacity of the cooling system. How much should be coolant, how much water? Does it come premixed? What would happen if no coolant was used, only water. If there was no/little coolant and mostly water could this cause a radiator hose or the hose to the car heater to rupture?

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

cooling capacity is about 10.6 quarts.

I use one gallon zerex g-05 , water and a bottle of Wetter Water

I think you have to see if your Aux fan is working.

100 degrees in Phoenix in city driving would be about right...but you should drop to 90 degrees on the highway

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

You could also proactively just replace the 20-year-old parts.

Adding auxiliary switches, etc. seems a bit silly.

Lots of people think Mercedes last forever. They're wrong. The biggest majority of 1980s Mercedes are in fact ready for retirement.

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

lkchris;119640 wrote:
You could also proactively just replace the 20-year-old parts.

Adding auxiliary switches, etc. seems a bit silly.

Lots of people think Mercedes last forever. They're wrong. The biggest majority of 1980s Mercedes are in fact ready for retirement.

You migh be right regarding the Gas versions, but the Diesels have got another 20years left in them. So although it is necessary to ensure the entire system is working, it is super handy to have an electrical switch right there on the dash when that heat sensor you checked last week gave up this week.

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

Not sure I understand this.

20 year old diesels have another 20 years left in them but gas models don't.

Since a W126 chassis is the same regardless of the engine, how does the one with the diesel not need suspension rubber and other parts for another 20 years.

My own 300SDL chassis parts have worn out at the same rate as those on gas models. I know, I have replaced them.

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

After $17,000 in overhauls and restorations, my 1987 Mercedes Benz is ready for another 20+ yrs. What do you think?

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

1987260e;120407 wrote:
After $17,000 in overhauls and restorations, my 1987 Mercedes Benz is ready for another 20+ yrs. What do you think?

I thinks it's environmentaly responsible to maintain rather than replace. Though I'm sure there are those that would argue newer technology is more efficient, therefore better to upgrade.

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

Depends on how the $17,000 was spent. My local dealers could charge that for an engine and transmission job, throw in a few light bulbs and poof, your money is gone.

Under these circumstances, the engine and transmission would likely go another 20 years but the light bulbs and suspension will never make it.

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

Sounds like you have done your homework,I too have a Benz with the 603 engine have you ever encountered con. rod(s) being bent? I had cyl. 5 and 6 rods that were bent so bad that you could tell just by "eyeballing"I have heard of problems with these engines but never found what the cause or the fix,I replaced the two rods in question and hope I don't have any more problems,the rods bent at 130k miles,any input would be helpful--jim

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Re: STOP YOUR MERCEDES FROM OVERHEATING and other useful inform

OM603 engines in the U.S. were used in W124, W126 and W140 cars.

Two varieties were the almost bulletproof OM603.961 in the W124 (300D and 300TD) and W126.125 (300SDL). The other variety is the OM603.971, 3.5 liter engine.

Many, or most of the 3.5 liter engines failed due to connecting rods. Such engines later rebuilt by Mercedes went on to work well.

A 3.0 liter engine failure is an anomoly. My guess is that water entered the combustion chamber and since it is not comtessable, would have caused the tops of the pistons to break, the rods to bend or both.

There may be other possibilites as this is only a guess on my part.

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