I'm having some cooling issues and would like to know at what temp and or RPM should the engine cooling fan engage? If I let the engine temp get over 100C and turn off the motor the fan spins 2-3 seconds, if I increase the idle the fan does not seem to increase in speed and or pitch, at idle and over 100C temp I can stop the fan with my hand. Is the clutch bad?

Thank you.

ezrider,

I believe the fan clutch is activated by one of the temperature sensors and it is set to do its thing at 105 Celsius. The electric fans come on with the airconditioning at slow speed and the engine temperature in fast speed when the temps get over 115 degrees Celsius.

If the car is running hotter than it used to, the chances are the thermostat is the problem. The thermostat on these cars is a little more complex than most, and has two moving parts, one that controls the flow direction and rate of water from the engine, and the other cooler water from the radiator. It does not take much for these to get out of whack in ten years of operation.

Since they are relatively complicated, they can be difficult to buy new in proper working order. There have been instances cited on the board here of multiple bad thermostats being supplied by the dealer and aftermarket parts places. Leads to confusion about what is going on.

If the engine is not suffering from a serious ill, like a cracked head or leaking head gasket, and you have done the routine coolant changes over time, the coolant system should be in working order. The thermostat is the most likely culprit, and I would focus on that item. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Jim, thermostat has been replaced with correct one.

Seeking answers to fan question?

Thank you.

ezrider,

I did a little research, and while I no longer own a 350SD, and have no technical data on hand from that car in particular, I did look at my gas cars and the W124 CD that includes features for the 350SD series engine.

I guessed at the temperature settings for the devices in my last post and was off a bit.

The fanbelt driven fan has an electrically activated clutch that gets the signal to engage and drive the fan at the pulley ratio speed of the crank pulley diameter and the fan pulley diameter. The sensor for the clutch is in or near the thermostat housing, designated "S25/1" and is set to have the clutch engage at 100 degrees Celsius.

The electric fans run at two speeds, which are controlled by sensors in the airconditioning system refrigerant and the engine coolant thermostat housing. The airconditioning system sensor monitors pressure, and at a set pressure the low speed fan comes on to help reduce system pressure by removing more heat from the "condensor" heat exchanger. The fast speed fan is controlled by the engine coolant temperature sensor set at 105 degrees Celsius.

If the engine should reach a temperature of 125 degrees Celsius, the airconditioning compressor is shut off by another engine coolant temperature sensor.

Hope this helps, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

JimSmith,

I just replaced the viscous clutch on my 350 SDL ... don't think it is electric. I have had abnormal increases in temp since the replacement so may have gotten a bad one or something could have occurred in the tstat at the same time (unlikely, but they do fail) ... I don't know if my fan is locking up at the correct temp (looks like there is a bimetalic tstat built into the clutch ...

What do you think? Comments from others?

I am always very suspicious when something changes radically (hi load temp on mine went from 82-85C to 100C) when I changed out the clutch ... will replace tstat next and see what happens?

__________________
George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg

ezrider and stephenson,

Once again I gave you guys some bum dope. I am struggling learning to use this CD manual. Unlike a printed version, this thing requires that you kind of play it like a video game, searching for something that may or may not be in it, and you won't find until you unlock the sequence of finding it. Enough excuses.

The 601 series engines have the magnetic clutch that is activated by the temperature sensor I noted earlier. The 603 series engines, like the 350SD/SDL have, use a viscous clutch as stephenson noted. Data for the 601, 602 and 603 series are all included in the manual, if you can figure out how to navigate the thing. Persistence has paid off though as I found stuff pertaining to the 603 series engines.

The viscous clutch is activated by a bimetallic spring on the forward face of the hub, that is set to make the clutch grab when the ambient air from the radiator that blows over it reaches 71 degrees Celsius, which corresponds to water temps of 90 to 95 degrees Celsius, depending on ambient temperature conditions, and if the airconditioning is on, and so on. These data may not be exact for the 350SD/SDL as air flow around the front of the car is likely slightly different , as is the radiator geometry, the shroud and so on, but I think it is likely consistent for those vehicles too.

The fan runs at about 1000 rpm max until the clutch is engaged, when it will run at up to 3,300 rpm max, with a generally linear relationship to engine speed until the max rpm value is reached.

The clutch is supposed to be "on" when the car starts, and then once the fluid inside is asked to do work without the control valve in the activate fan position, it runs back to the storage reservoir inside the hub, (out of the area where it is active) releasing the mechanism. Apparently enough fluid stays in the active chamber to take the torque load of the fan up to near 1000 rpm. It is not clear in the manual if the control valve, operated by the bimetallic spring in the hub, leaks the fluid from the storage area to the active area when the engine is at rest, or if the fluid stays in there when the car is shut off and the fan has been running because the operating temperature of the engine activated it.

So, if you start up and the fan runs faster initially, for a short time period, before the engine heats up and then slows down, it would seem the clutch is doing what it is supposed to do. Verifying this would require running the car over 1000 rpm in this time period and watching the fan slow down as the viscous clutch returns to the inactive mode. The manual says if you run the engine until the temp gets to 95 degrees Celsius, with the engine at 3,000 to 4,000 rpm, the fan will come on and be very apparent. I would expect the transition from 1000 rpm to 3300 rpm on the fan to be noticeable visually and audibly, even with a Diesel running at that speed.

It seems to me if the car, as I posted in the Tech section on this subject, is running at 80 to 85 degrees Celsius, the viscous clutch like the magnetic ones and the two auxilliar electric fans in front of the radiator are not going to be in the picture. Something is happening to get the car out of the stable 80 to 85 degrees Celsius operating mode. While it is suggested this is normal in hot weather and stop and go traffic to see the operating temperature rise to over 105 degrees Celsius, I never experienced it (the high operating temps not the traffic situation) in a Diesel until the engine no longer held the 80 to 85 degree Celsius temperature regularly. I have seen it in my 190E 2.3-16 from day one, but my 300E is rock solid in all conditions at 83 or so Celsius.

This is an issue that is constantly confounding owners on this site, and there are tales of trouble shooting episodes that systematically change everything but the block, with no benefit. Then there are tales of the same people changing the thermostat three or four times, and getting one that brings back the original steady 80 to 85 degrees Celsius operating temperatures. The changes to water pumps, radiators, head gaskets, heads and so on have universally failed to affect the operating temperature rise except possibly in a case with a grossly cracked head. Changing thermostats has been effective, but not on the first try for most, and not even on the second or third try for some.

It seems to me solving this one issue will ease the stress of MB owners that frequent this site who read the manual and hear that it is ok for the car to go over 110 degrees Celsius, but are anxious because it used to be stuck at 80 degrees Celsius or so for years. It makes you sure something unhealthy is happening.

Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)