It's incorrect for any car to have 20-30 temperature swings in normal driving. Maybe sitting in traffic on a very hot day or climbing a mountain. But in normal driving, no. These are the only cars I know where such swings are routine, due to a very wacky design. The fans are intended to engage progressively, but even in perfect condition, don't keep temps anywhere near level. And with three separate mechanisms for fan control, the odds of component failure are high. I've heard a lot of excuses for why this should be, not one makes sense.

As a design goal, a cooling system engineer wants to hold temperatures as steady as possible to limit pressure fluctuation. For a cooling system, varying pressure is like flexing a piece of metal until it fatigues. Highly variable pressure will eventually promote component failure.

I use the temp switch to engage the aux fans just prior to the clutch fan engaging. They run quite a bit more than they normally would. But my temperatures stay in a tight range, rarely over 90, and never hit 100. This effectively eliminates the clutch fan, because air temps almost never get hot enough for it to engage. And the added benefit is that I have working A/C, even in heavy traffic.

What year/model Mercedes diesel are you referring to that has a 20 to 30 deg swing? I have currently an 85 and had an 83 300D, and neither one have the swing you mentioned. They stay within 80 to 85 C on the temp gauge both summer and winter with the viscous fan.

I think I will be taking a different approach in controlling the AC aux fan. Rather than adding a temp switch to the T stat housing or the cyl head, I will build a circuit that interface with the coolant temp gauge sender (iirc it is located between glow plugs #2 and 3) to control the AC aux fan. I will make it adjustable so I can have the fan come on earlier or later. It will be less work in the end and provide better temperature control.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Every Mercedes I've ever owned has a "normal" range of at least 30 degrees. Normal driving, they'll hold to 80-85 pretty well. But a hot city traffic or a haul up a mountain road will send the needle soaring. I used to commute to NYC, and it was impossible to use the A/C on a hot day when I was stuck in a jam. A properly controlled electric fan keeps it under control.

Variable fan controls are available cheap, just buy one. But why not try something a little more ambitious...a proportional fan control, which automatically adjusts based on temperature. Some modern cars have this feature built into their ECU's, but I've never seen an aftermarket one that worked reliably.

Viscus fan clutch = "proportional fan control, which automatically adjusts based on temperature". Why re-invent the wheel here?

It is very easy to put a thermostat to control an electric fan and do away with the mechanical fan. Buy a 95C thermo switch ( about $0.50 each or less ) and glue it to the thermostat housing. You can select any temperature you like but use the Normally Open (NO) type. They are used in oven, toaster and are dirt cheap. I use JB weld. The thin layer of glue should be OK as it will not affect the thermoswitch to reach temperature. Connect one end to chassis and the other end to the relay that trigger the Aux fan ( one of the pins in the wiring harness on the 3 pin thermostat ). Bingo - now you have a temperature controlled electric fan. You can also wire it in parallel with the existing circuit, so whichever thermostat reaches temperature will trigger the fan.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Gotta be kidding....

A viscous fan clutch is an on/off device, controlled by ambient air temperature. When it's engaged, it's proportional to engine RPM, which makes it a poor proxy for a coolant thermoswitch. When it's not engaged, it's not proportional to anything. A proprotionally controlled electric fan will turn at a speed that's mapped to coolant temperature.

As for gluing appliance switches to the thermostat housing...doesn't rate a response.

You do not need to take it if you do not like it. This is a free world.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

I have not seen anything close to a 30 degrees swing in city traffic in mine, the most I'd say 5 degrees. AC didn't work in the 83. The 85 has AC with R134 but has low charge and bad cooling but neither one suffers the wild swing you experienced.

Found this variable speed fan controller. A bit pricy. Have you seen cheaper?
http://www.summitracing.com/parts/flx-33054

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Be nice, he was just giving his opinion, which I agree with BTW.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

This one http://www.summitracing.com/parts/flx-31163 has everything needed including a 1/4-18 NPT probe. My T-stat housing has a port with M14 threads. 1/4-18 NPT will go in the M14 but fit loose. I should be able to make it seal with a washer + o-ring +thread sealant.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

McMaster has a 1/4-M14 adapter for $7.02

McMaster-Carr

1/4 NPT is very close in dia to M14. It will be on a stand-off and will not be in contact with the coolant.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

that's a clever idea.

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

Where are you putting this that it wouldn't contact the coolant?

Tstat housing

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I don't see any problem with using an adapter at that point. OTH, a forced-in sensor sealed with goop may give you an unpleasant surprise one day.

I think that you're going to play with the adjustable sensor for a day or so. Once you find the sweet spot, you'll never touch it again. If you have 22mm plugs on the 617 as we have on the 60x engines, it's so much better to use a fixed temperature switch there. You'll probably find a 92 or 95C switch is about perfect: most thermal switches have 8-12 degrees hysteresis, which means your temps will be in the 80-85 range. Even if you have to try two or three switches until you find one you're happy with, it will cost 1/3 the price of the adjustable.

Try something like a Whaler 6010-92, from a VW.