Had a quick question on 3 pole coolant temp sensor located near the top radiator hose. I power washed the engine and looks like some water got on this sensor, every time I start the car even from cold the aux fan kicks in.
I unplug the sensor the fan goes off.
Question is that my sensor has a RED top and the only ones I see for sale have a blue or green top. Do these units superceed the red top sensor? What are the differences between them?
1987 300D

__________________
1987 300D
2005 E320 CDI

I deleted the previous post with colors and trip points from memory. Here's info on all but the red switch -



IIRC the red switch has trip points of 95*C and 105*C. The trip points are etched into the wrench flats of the switch.

Jeremy mentioned that he experienced the aux fan kicking in with the coolant temp gauge around 100*C with a red switch. Then he fitted a new fan clutch and the engine didn't run that hot.

Sixto
87 300D^2

Is this what youare talking about? Auxillary fan switch w133-1630507



Charlie

__________________
there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

If the first sentence in post #1 is any indication, probably not.

I missed this line "question on 3 pole coolant temp sensor ".

When I read his sensor has a red top, my mind jumped to the sensor I posted.

Charlie

__________________
there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

Ok, so the grey switch sounds pretty good. But looks like the red switch has trigger points at 95 wouldnt that make it a better choice? Also I am showing the green switch 0065454524 has superceeded the blue switch.

__________________
1987 300D
2005 E320 CDI

Sixto,

Do you know the thread size of this 006 545 6424 switch? I am doing a feasibility test on eliminating the viscous fan in my 85 300D and need a temp switch to use in the T stat housing to run the electric AC fan as a engine cooling fan.

__________________
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

Good question. I'm without MBs at the moment so I don't have one to measure. FWIW, it's a common temp switch format across OM601/2/3 and M102/3 engines. I'm sure there's a 300E in a scrap yard near you.

Sixto
MB-less

The red switch was never factory installed in a diesel by MB. It shows up in gas-engined cars. Do you need the part #? I have it somewhere.

[edit]

Mercedes-Benz S25/5
Auxiliary fan (high speed) and compressor cut-out switch part numbers are:
• 006 545 64 24 (used in OM603 of '87 300D/TD only) 105ºC/128ºC with blue top
• 006 545 61 24 (used in M102 and OM601 engines plus OM603 of '86/'87 300SDL) 105ºC/120ºC with gray top
• 006 545 45 24 (used in some M103s) 105ºC/115ºC with green top
• 006 545 42 24 (used in M102-3-4 and OM602 engines) 100ºC/110ºC with red top

Don't forget to install a new crush washer if you change switches.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The red switches were used on M102 190E's. I've never understood why Mercedes chose such incredibly high temps for the Diesel cars. The only thing that makes sense is that they were conserving horsepower so the DIN testing looked better.

Funola:

These are 14mm switches. You won't find much selection in that size, especially if you want the double throw functionality. My trick is to remove one of the water gallery plugs on the left side of the head, and replace it with a 22mm fan switch. 22mm switches are used in many VW, Porsche, Jaguar, and Fiat applications. You can find them in setpoints from 72C to 120C. I'm currently using a 95C Porsche switch, and I think the thermal control is as good as it can be. If you wanted to use the electric fans as primary, a set point of 92C, as used in VW's would be perfect. I have access to lots of thermal switch data if you need some help with this.

Also, that isn't the thermostat housing, it's just a coolant outlet.

Attached Thumbnails

 

Please explain what you mean by "Mercedes chose such incredibly high temps for the Diesel cars"? Are you referring to the T-stat temp or the viscous clutch engagement temp?

Did you eliminate the clutch fan and run only the AC electric fan via a 95C temp switch on the head? How does it perform? How often does the fan kick on?

As to whether it's called a thermostat housing or a coolant outlet, how about we call it half a thermostat housing? The thermostat sits halfway in it. The other half sits in the thermostat cover.

__________________
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

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.

__________________
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.

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

Thanks for the p/n's . The 006 545 42 24 100ºC/110ºC may be a good choice. Do you know if it's M14 threads?

__________________
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