K6JRF at the mic K6JRF's Page
formerly W6FZC

My Mercedes Benz
S500 Coupe

K6JRF's MB S500 Cpe
(Updated: May 28, 2004)

Analyze and Troubleshoot "Check Engine" MIL and Electronic Control Units (ECU)!

Modification of the Viscous Fan Clutch for M119.xxx Engine

Description of Problem
The Merecedes-Benz automobile is a work of art but some of the functions are not as 'usable' as they could be! Take for instance the viscous fan clutch (VFC) in most cars. It is supposed to cool the engine by drawing outside air into and through the radiator. Simple enough!

Viscous Fan Clutch for M119 Engine But the kicker is that the VFC doesn't engage until the temperature is into the RED region on the temperature gauge! Much too late! So this modification changes the clutch to lock up at all times. It's very simple to do and adds greatly to the cooling of the engine.

So much so, with the ambient temperature of 85 degress, it maintains less than 90C with A/C on full and running in stop/go traffic when used with my Cool Harness.

Although this specifics detailed here are for my M119 engine, this modification will work on ANY Mercedes with a VFC. Most MBs have one!

The drawback is increased 'noise' from the fan. It's not readily perceptible at idle (650prm) but at 1500 - 2000rpm the air noise is noticeable when outside the car. But the increased safety margin for engine temperature is well worth the minor drawbacks.

The picture shows a stock VFC made by Sachs with the protective plastic cover over the key element, the bimetallic strip (bms). Click in the above link to find a VFC for your year, model car. Remember to select "ENGLISH" in the bottom toolbar. The following sections show how the bms and VFC operate.

BMS Theory of Operation
Bimetallic Strip bends toward the greater thermal coeficient of expansion When two metals with dissimilar thermal expansion coefficients are bonded together, they produce useful devices for detecting temperature changes. A typical pair of brass and steel with expansion coefficients of 19 and 13 ppm per degree C respectively are commonly used. As heat is applied, the bms bends away from the metal with the greater expansion. In this case, brass.

So as the radiator/engine temperature increases, the bms bends releasing pressure on the pin (clutch) allowing the internal fluid to engage causing the fans to change from freewheeling to a lockup position and turn at the engine rpm. Now much more cool air is drawn though the radiator, lowering the coolant temperature and thus, the engine.

Sachs VFC Part Numbers
The table shows the current VFC model numbers for all Mercedes cars. The W140 and C140 use the 2100 013 031 part. But it appears that it can be replaced with the 2100 027 131 model.

Current Sachs VFC Part Numbers
Sachs: 2100013031
Model No: MNT9K-C1
MB Cross Ref: 119 200 00 22

E-class (W210)
E 50 AMG (210.072) (255 kW ; 02/1996 - 08/1997)

S-class (W140)
400 SE,SEL/S420 (140.042, 140.043) (210 kW ; 02/1991 - 10/1998)
500 SE,SEL (140.050, 140.051) (240 kW ; 02/1991 - 10/1998)
S 420 (140.042) (205 kW ; 01/1993 - 10/1998)
S 500 (140.050, 140.051) (235 kW ; 01/1993 - 10/1998)

S-class Coupe (C140)
SEC/CL 420 (140.063) (205 kW ; 10/1994 - 02/1999)
SEC/CL 500 (140.070) (235 kW ; 10/1992 - 02/1999)
SL (R129)
500 SL (129.066) (240 kW ; 09/1989 - 08/1992)

Sachs: 2100013032
Model No: MNT9K-L
MB Cross Ref: 119 200 01 22

E-class (W124)
E 420 (124.034) (205 kW ; 06/1993 - 06/1995)
E 500 (124.036) (235 kW ; 06/1993 - 06/1995)

Saloon (W124)
400 E 4.2 (124.034) (205 kW ; 10/1992 - 06/1993)
500 E (124.036) (240 kW ; 01/1991 - 06/1993)

Sachs: 2100027131
Model No: VL120L
MB Cross Ref: 113 200 02 22, 119 200 02 22

E-class (W210)
E 420 (210.072) (205 kW ; 01/1996 - 06/1997)
E 430 (210.070) (205 kW ; 06/1997)
E 430 4-matic (210.083) (205 kW ; 05/1999)
E 55 AMG (210.074) (260 kW ; 08/1997)

E-class Estate (S210)
E 420 T (210.272) (205 kW ; 06/1996 - 06/1997)
E 430 T (210.270) (205 kW ; 06/1997)
E 430 T 4-matic (210.283) (205 kW ; 05/1999)
E 55 T AMG (210.274) (260 kW ; 08/1997)

SL (R129)
500 (129.068) (225 kW ; 05/1998 - 10/2001)

Removing the VFC
View of S500 front area showing key screws to remove to gain access to VFC The hardest part of this modification is to get the VFC off the car!

For my car, had to remove the protective metal cowling in front that holds the Aux fans so that the fan shroud could be removed.

This photo shows how to remove the fan shroud bracket for my S500 cpe. This procedure is valid for any W140 chassis car. All screws are 10mm including the recessed one. The bracket is released by pulling it up and over the hard rubber hood bumpers.
View of fan shroud clips and center holding pin for fan shroud extender After removing the bracket, remove the fan shroud clips and release the center pin that holds the fan shroud extender. Then rotate the extender clockwise to release. Place extender back out of the way over the fan blades.

Gently pull up and 'wriggle' the fan shroud around the upper radiator hose and place it to the side. Now you can see straight down to the side of the VFC.
VFC Modification
This picture shows the modified VFC. The actual modification is quite easy to accomplish. Tools needed include a 3/8" socket with 8mm drive key for the bolt that holds it to the engine. There's not a lot of room to get large tools in front of the radiator, so this part is left up to you.

VFC with cover removed and modification installed This modification is courtesy of Stu Ritter, current technical editor of the MBCCA's "The STAR" periodical, and a former MB shop-owner with 35+ years experience. The following is in his words with some editing for clarity.

Take the VFC off the engine. Carefully remove the bms by pushing down on the strip on the open end. The bms will slide out.

Drill a small hole on the side that is closest to the clutch (pin) [shown in the inset picture], say 4mm with some overage so as to get a little 4mm bolt with two nuts. There isn't enough thread on that thin strip to trust the threading. Screw the bolt in and double nut it, one on each side of the strip.

Bow the bimetallic strip around 1mm to 2mm by adjusting the length of the screw to raise the bms. This will give you full tilt fan engagement right up to the high speed disconnect around 2,500 rpm. You won't believe the amount of fan noise but the engine runs cool as a cucumber.

For winter, I reach down with a 4mm wrench, loosen the nuts, turn the bolt 1mm or 2mm to have a normal viscous coupling during the winter when I don't need the added fan noise.

jrf notes:It turns out that raising the screw carefully a few mm, causes the vfc to engage when the engine (and radiator) is hot but not when cold. Also it disconnects (if it was engaged) at 2000rpm or so. A very nice feature.

VFC closeup with BMS removed showing the actuator pin The screw raises the small pin (shown to the right) to fully engage the VFC. Note that complete pin displacement is about 0.070", a little over 1/16". Check the bms picture below to see where to drill: The 'lift' screw hole should be drilled 13/16" in from the end. The large hole on the left is NOT needed. That was done while doing some experimentation.

If you wish to 'remove' the mod, remember to NOT put the black plastic cover back. Otherwise you will not be able to turn the screw unless you line it up perfectly with the holes in the cover. This can be done but requires some forethought. But even then space is at a premium so a very small screwdriver is needed.

jrf notes:It turns out that raising the screw carefully a few mm, causes the vfc to engage when the engine (and radiator) is hot but not when cold. Also it disconnects (if it was engaged) at 2000rpm or so. A very nice feature.

The picture shows a close up of the bms used in the VFC unit. It is the most extensively used general purpose bimetal, TB 1577 A-GE in Europe. These types are used for linearity ranges up to 200 degrees centigrade and useful deflection temperature range up to 350 degrees centigrade. Of course, this range is beyond where it would be useful to keep engine temps within a safe, reliable range.

Bimetallic Switch in the VFC

Future Plans
I'm looking at ways to replace the bms with a lower temperature version (90C for full lockup) that would allow the fans to freewheel until about 85C. At that temperature, the bms would start to deflect and at 90C, it will have deflected 1/16" so as to cause full clutch engagement. This, of course, would be the 'ideal' solution since the air noise and added drag would be completely eliminated until such time as the engine needs the added cooling.

Update: 6/22/03
There has been no continued interest, nor can I generate any re-design activity for the bms. The one company that showed some interest has fallen to the wayside. So it appears that the 'mechanical' modification will be it.
So enjoy this modification for a really cooler engine and colder A/C!

Send me Email Icon with your comments