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Old 01-26-2009, 04:51 PM
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Join Date: Aug 2008
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Diagnosing a faulty viscous cooling fan clutch

Anyone help determine if the cooling fan clutch is not engaging properly? When the 1995 SL500 is cold, and it is cold in Cleveland Ohio today, 20 degrees, the fan can be rotated with very little effort. I drove the car for its mid-winter excursion for about half an hour at speeds of 50/60 mph. Thermostat showed the temp in the 55/60 celsius range. Got heat from the heater but the dash gauge was in the very low range for this car. On warmer days, the temp is typically 80/90 celsius and I have never checked to see if the fan clutch activates. I thought that since today was especially cold outside and that the engine never really warmed up into its usual heat range, that I might not be able to determine if the viscous clutch is working or not.

Here is the question: At what point or temp should I expect the clutch and fan blade to "lock-up" and become harder to turn with the engine off? Its not doing it now but again that may be for the reasons mentioned above. Thanks, Steve
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Old 01-26-2009, 09:02 PM
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Join Date: Jul 2008
Posts: 1,236
Hi there... Yes, I'm skirting your fan clutch question because I think there is another issue here.

Your car should get hotter than 55/60c after 30 minutes of driving, which leads me to suspect that either your thermostat is stuck open or the temp sensor which goes to your temperature gauge is bad. If the thermostat isn't to blame, then your temp sensor is bad. Keep in mind that your car has more than one temp sensor. There's a dual element sensor that goes to the fuel system and computer, and a single element sensor that goes only to the temp gauge. Not too terribly expensive. I have a 300SE that only showed 60-65c and what it needed was a new sensor. It now reads 80-85c.

The fact that it DOES get hotter on warm days leads me to suspect your thermostat is stuck open. Your gas mileage will improve significantly when you fix this.

Good luck.

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Old 01-26-2009, 09:06 PM
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the fan blade will spin easliy with engine cold, with the engine warmed up fully the fan blade should be much more difficult to spin by hand....If it still spins easily with the engine fully warmed up the clutch is shot.....
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Old 01-26-2009, 09:12 PM
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and it really doesn't lock, per se, it's a gradual engage/disengage. that's why it's called a clutch....
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Old 01-28-2009, 04:50 PM
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Viscous cooling fan damper

Thank You Cliffmac and Tiny Panzer for your thoughts on my cooling fan and temperature issue with the '95 SL500. Two days ago was the first time in a year that the weather was cold outside (20 degree) and the roads and the roads were clear of snow and road salt that I could take the car out for a ride to get all of the fluids moving. The ride was about 20 minutes and about 10 miles and usually this is enough to get the car relatively hot. I was surprised by the temp reading not being "high" enough for this car because it would normally run at 80 to 90 celsius. My first thought was to check the viscous cooling fan clutch to see if was "spinning" or locked up and cooling the motor off too much. The fan spun around easily, so I am just attributing this reading of 40 to 50 celsius as an anomaly related to the cold and 50 mph speed I was driving. I plan on checking the thermostat in the spring and the temp sensor you mentioned. Thanks, Steve
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Old 01-29-2009, 12:23 PM
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'94 S500: only 793 sold!
Join Date: Mar 2001
Location: San Diego, CA
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An MB should warmup within 5mins or so (@ cold ambient temps) so the tstat is bad . . . probably stuck partially open . . as mine was. Had the same symptoms as yours.

It's not GOOD to run the car at low engine temps (50-60C). Shouldn't wait for spring to replace the tstat IMO.
Regards . . . . JimF
'94 S500 Cpe

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Old 01-29-2009, 12:57 PM
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Join Date: Apr 2006
Posts: 122
The B52's

What a coincidence. I just replaced the t'stat in my m119 engine for the same underheating problem. I suggest the same for your engine.
The best tests for the fan clutch are done when the engine has sat overnight and when fully hot. When started cold you will hear the fan force air at such velocity that is is noticable audibly for a few seconds only. At operating temp throw a fender cover or towel or something to restrict the airflow infront of the a/c condenser/radiator. Allow the engine to run without getting anything fouled in the aux. fans. When you see the engine temp elevated as indicated on the temp gauge (110-115 C) pull the restriction away and accellerate the engine to around 1500-3000 rpm. The sound of rushing air will increase. I think it sounds like an old WW2 aircraft.(sorry not a Messerschmidt). You should notice a momentary rush of air also.
Best to you

Last edited by dpkreuze; 01-30-2009 at 12:04 PM.
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Old 01-29-2009, 12:59 PM
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Waiting for his Model 3..
Join Date: May 2008
Location: Springfield VA
Posts: 5,172
When my clutch went bad you could spin the fan very easily when the engine was off, and stop it easily (using a newspaper or something) with the engine at idle.

1991 350SDL. 230,000 miles (new motor @ 150,000).

2013 Fiat 500E. 20,000 miles, 80 miles at a time.
2012 Passat TDI. 95,000 miles. More space, power, and fuel economy than the Benz
2004 Touareg V10 TDI. 150,000 miles. One of 450.
1999 Jetta TDI. 310,000 miles.
1992 Jetta ECOdiesel. 156,000 miles. 1 of 48. Sold.
1991 Jetta ECOdiesel. 430,000 miles. 1 of 700. Sold to VeeDubTDI, totaled in front of our house
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