Tip & How-To about Mercedes-Benz 300-Class

Cooling system health and diagnosis for 90's Mercedes.

This is listed under the 300 series but fits most 90's Mercedes Benz cooling systems.
The first thing most people notice about there Mercedes benz is the the temperature needle climbs or sneaks towards the red in there car. This is not uncommon. Is it wrong or bad for the vehicle? The plain answer is no.
First lets talk about what is going on. The motor heats the water, the thermostat holds most of the water back to keep the block a more constant temperature. The water pump is always pumping while the motor is running so none of the hot water goes the wrong way backwards into the radiator or coolant overflow tank. The radiator is cooling off the coolant by either driving, viscous fan or the electric fan or fans your Mercedes has up front when stopped or slowed. When the thermostat reaches its set temp around 85-95 it fully open until the block fills with cooler water then shuts again, filling the radiator with hot water to be cooled. All in all pretty simple system to keep the block cool and evenly heated to keep stresses down.
Now some do not like the fact there system goes so close to red. Well one contributor is the Air conditioning system. When that compressor kicks on it basically is just a pump that transfers heat out of the car into the engine compartment making the cooling system work harder.
Another cause of climbing heat systems is a failed thermostat closed or open that keeps the system water hot in the block or hot throughout the system. Best fix is to of course replace it. Just follow the top hose unbolt it from the block and insert the new unit with proper gasket and direction and its good to go.
A bad or failing water pump can limit water circulation so if your car doesnt cool off after replacing the thermostat its the next to replace. Kind of not a simple Do It Yourself job with all the belt and pulley issues that may come up due to age. Not impossible just not easy.
Leaks in the system can cause boiling and rising temperatures so look around. The coolant tank small hose to the radiator on either end is normally the first, or even the winshield washer deicer or heater core lines. Cap can go bad but you will normally see a bad gasket on it if it fails.
Radiators can become clogged with scale after years and years of service with limited flushes and not adding distilled water. Sometimes the plastic cracks and just need replaced.
Ok now there are some external parts to the coolant system that directly effect it.
The viscous fan, it can lose the oil it has internally and not spin enough to cool. The rule is with the engine running if you can stop it easily with a rolled up newspaper replace it.
The engine temp sensor is normally 2 wire sensor near the front of the engine close to the intake normally. When it goes bad it will not let the electric fans come on when needed. If your having issues, cheap fix replace it.
The last piece of the system is the electric fan/fans. These have alot of integrated systems to look at if they do not work so just be careful. First there fuse is not in the fuse box. It is a aluminum strip fuse normally located on the firewall inboard of the brake booster in a tootsie roll sized container. When it goes out the fans do not come on. First without the A/C on operation is as follows 105 fans kick on low speed kick off at 95. So as you can see the needle will easily climb close to red in traffic again stay calm this can be normal. When the A/C is engaged (in a system with proper charge and working order) the high speed fan operation will happen independent of engine temp. This is because all the heat being transfered by the compressor as noted earlier.
I hope this sheds some light on some cooling system woes of anyone. This may apply to alot more vehicles but is specific to Mercedes. Operation of the coolant system is similar no matter what car you drive (unless its electric or air cooled of course).
Thank you.

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