1994 E320 wagon has recently started to overheat. Usually runs at about 100 now creeps up toward red line on easy highway driving in cool weather in about 10 minutes or less.
Both aux electric fans operate when temp exceeds 105. Turning on defrost will drop temp back to normal then starts to climb again. Troubleshooting to date: Read all forums on topic. Fan spins freely even after bringing up to operation temps but I have reason to suspect something besides the clutch. I read that the clutch engages when heat off the radiator activates the bimetal strip on the front of the clutch. Before ordering and changing the clutch, I felt the front of the clutch and the radiator after it comes up to high temps and both the radiator fins and bimetal I can feel are cool to the touch. All radiator hoses are hot but the coolant in the overflow resevoir is cool. I removed the thermostat and the condition remains the same. How can I determine if there is enough coolant flow through the radiator to produce the heat needed to operate the fan clutch? Any Help or Speculation appreciated.

If there is no resistance to turning your viscous fan when engine is up to operating temperature I would say the clutch is certainly dead and due for replacement.
Geo.

First, test the opening temperature of your thermostat on the stove in a pan of water. It should begin to open at 87 deg. C and be almost fully open by boiling. If it fails this, replace. If not, then the likely culprit is the radiator. When was your radiator replaced? It sounds like the insides of the tubes are crusted with deposits since you say the fan clutch and radiator core don't get hot. Also, with the A/C off and the aux fans running is the air that exits the radiator hot? Again, if the thermostat is good and the air exiting the radiator is not hot suspect a dirty radiator. You could try to flush and clean the system yourself, send the radiator out to a shop to be professionally cleaned, or if the radiator is original, replace it. You will still need to flush the system but IMO even an 8 yr. old plastic MB radiator is living on borrowed time. Mark

Also, with the A/C off and the aux fans running is the air that exits the radiator hot? Again, if the thermostat is good and the air exiting the radiator is not hot suspect a dirty radiator. You could try to flush and clean the system yourself, send the radiator out to a shop to be professionally cleaned, or if the radiator is original, replace it. You will still need to flush the system but IMO even an 8 yr. old plastic MB radiator is living on borrowed time. Mark[/QUOTE]

I will check the thermostat today and also the exit air temp. I was afraid someone would suggest a plugged radiator might be the problem.

Ray

Thanks very much for the help and response.
I've cracked open a beer at 9am to celebrate the cheap fix with a $12 thermostat. ( I going to look around at how much a radiator replacement would run to determine how many beers this calls for).

Here are my thoughts on trouble shooting though I have close to no mechanical ability.

I thought to remove the thermostat (though I did not check it's operation on the stove till today). I found that my car overheated just as readily with no thermostat as with a non working one. I did check the operation of the new thermostat in the same pan on the stove with the old one before installing it (new one works, old one didn't).

The car now runs about 9 degrees cooler (about 96 instead of bouncing around 105 at what i used to consider "normal") It had recently been heading toward redline at 125.

However the fan still "freewheels" at 96degrees. Should I be concerned if the fan clutch engages at this low temp? I don't know what temp will activate the clutch but it does not concern me as long as things are running this cool.

I just offered this as an observation since so many forum replies suggest clutch replacement if it free wheels at normal operation temp. I wonder if the fan is even called upon for cooling under mild driving conditions and is working just fine by not working?

Your car should normally run at 87 degrees C not 96. You may want to replace the coolant temperature switch which could be giving you a bad reading. My 95 C280 seemed to run hot also (fluctuating between 95 and 105 degrees C even to 110 on occassion). The new switch made the temp guage virtually rock solid at 87 degrees C. Now only when it's in stop and go traffic in 100 degree Phoenix heat does it move to maybe 92 degrees C. Then it moves quickly back down to normal when moving. Your viscous fan is probably okay. It should rotate very freely even when hot.

Even given your lack of DIY experience, radiator replacement on the 124's is really very very straight forward. Simple actually. New radiator is $250 ish.

Steve