Hey guys I'm having over heating problems with my 1990 300e the driver side fan only comes on when I first start the car but then turns off (which is normal) but it won't always kick back on only sometimes when it feels like it regardless of the temp of the motor and also my passenger side fan won't turn on at all is this a common thing with these cars? I would like to hear some feed back before going out replacing the 2 fans. I'm thinking both fans are working there just might be something wrong that won't signal them on when needed. Thanks in advance

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The two aux fans in the front are for the ac system and switch on and off somewhat independent of engine temp.
87 300D aux cooling fan diagnosis

If your engine overheats my 2 cents would be on a faulty fan clutch on the cooling fan behind the radiator. It shall engage around 90C and you should really hear it blow air when from 2000 rpm.

So your saying the clutch fan might be causing the over heat? The fan behind the rad that's on the motor does turn on and stay on

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ehm. I think you have either a electrical cooling fan or a mechanical one controlled by a viscous clutch. If you have the latter the fan is loose and just slightly spins along with the engine until the air temperature reaches a certain level. Then the clutch engages and the fan follows the engine rpm. That's how it is on my 1990 300E (euro model). Again - if indeed you do have the mechanical visco fan, it is possible to do the following check - JUST BE CAREFUL WITH FINGERS!!! - when the engine is hot (>90C) and running, take e.g a piece of cardboard (something that will not break fan blades, but sturdy enough to give some resistance) and see if you can stop the fan with that. If you can, your clutch is dead and needs replacing.

Your passenger side fan is toast, they should both turn on and off at the same time. Take the fan off and bench test it. The electric fans have two triggers:

- When a/c pressure is above 300psi, the fans are turned on at low speed. This is accomplished by a switch on the receiver dryer. There are two switches on the dryer, the fan switch is the one with pigtail leads. Remove the harness and short the harness leads together. That should start the fan. If not, the most common problem is the fan resistor, on the inner fender near the dryer.

- When engine temperature is above 105. This can be triggered one of two ways, I'm not sure which applies to your model. (1) There may be a three way temperature switch on the engine water outlet. If so, then you can test by shorting the top two contacts together. (2) Later models used a temperature sensor which feeds the HVAC head unit. The head unit then manages the fans. I'm not sure of the test procedures for this version, but it should be in the archives somewhere.

As mentioned, the primary cooling fan may also be a problem, and you may need a new fan clutch.

daphatch, the thermostat may be faulty and not opening completely. I have experienced sudden, consistent overheating for no apparent reason. Replacing the thermostat fixed the problem. You can remove the thermostat and place it in a pan of boiling water to check that it opens completely.

You are likely aware when troubleshooting it's best to start with the simple, less-expensive, probable causes and go from there.

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Fred Hoelzle

I figured the same and replaced the thermostat but unfortunately that did not solve my problem

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daphatch, recommend checking the fan clutch next. Any signs that the water pump is leaking and not functioning properly?

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Fred Hoelzle

"Overheating" is a vague term. You have a temperature gage and should report what coolant temperatures you are seeing under various operating conditions.

I believe most models of the era were set up like my '88 190E 2.6.

The viscous fan clutch keys off radiator exit air temperature, and it will typically be fully tightened when coolant temperature reaches about 100C. You can tell by the "fan roar" you hear at about 2000 revs, and if you shut down the engine at about 100C and turn the mechanical fan by hand it should be much stiffer compared to when the engine is cold.

Even with a fully tightened fan clutch, idling in hot weather may not provide sufficient air flow at idle speed, and coolant temperature will continue to climb. At about 105C the auxiliary fan(s) should engage at high speed, and they can usually be heard. This should result in rapid cooling until they cut off at about 100C, and the cycle will repeat if the vehicle is stationary and the engine continues idlling. The above behavior is common in a hot weather traffic jam.

105C is about 221F and a 50/50 blend of ethylene glycol and water with a 15 psi cap boils at about 265F, so you still have plenty of boilover margin at 105C.

Independent of the above the auxiliary fan(s) will engage at low speed based on closing of the high pressure AC switch to provide adequate freon condensing. Often coolant temperature in hot weather, low speed driving will be lower with the AC on because of the high low speed duty cycle of the auxiliary fan(s) based on the freon condensing requirement. Coolant temperature many not reach the 105C requirement to engage the auxiliary fan(s) at high speed.

Search for a thread started by me a couple of years ago - auxiliary fan failure analysis.

Duke

Yeh the aux fans only turn on when starting the car (driverside Only) and sometimes it will kick on but only after reaching 100c+ the passenger side aux fan won't turn on at all. Car usually runs just above the 80c Mark on the dash while driving regardless of outside temp and if a.c. is on. Will rise to about half way of the 80 and 100 Mark but only when climbing a hill but that's normal.

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Unplug the passenger fan and put 12v to it. Saying there is a problem with the passenger side fan and not doing anything to test it does not help. At least you will narrow the problem down to the fan or the wiring. As for the fan clutch, pop the hood and spin the fan by hand. Start at the 12o clock blade and give it a nice spin and see if it continues to spin. It should not even make one complete revolution. Next get the car up to operating temp and then some then have someone turn the engine off while you watch the fan. It should stop when the engine stops. If it continues to freewheel, replace the clutch or service it with silicone. View the links.
https://youtu.be/S2W5BiqFfoE

https://youtu.be/Pgvxl9rSpVE

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1990 190E 3.0L

Thanks !


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