I said that I'd look into changing the temperature point at which the Auxillary Fans (AF) cut-in. Well, I now know WHAT to do but I would appreciate some help in HOW to make it a "pro" mod!

Here's the theory. These comments are for my W140 S500 w/119.970 engine. They may NOT be applicable to a different class of car (E series) and/or different years (95 and on).

S500 has two (2) fans that draw 35 amps(!) at full tilt. Assume this means 17.5 amps/fan.
The S500 has a three stage fan, some models only have two stages.

According to MB data, here's the as-delivered temp vs stage activation:
Aux Fans:
Switching Points------------------ 1st ------------ 2nd ------------- 3rd
Coolant temperature (C)--------100 on--------- 107 on -----------115 on
------------------------------------------- 95 off----------100 off------------ 107 off
Refrig pressure (bar) ------------- 14 ------------- 17 ---------------- 20 on
Actuation of AF (V)---------------- 7 ---------------- 9 ---------------- 12
Eq resistance of CTS----------- 310 ------------ 250 ------------- 200 ohms

The fans use the [CTS] Coolant Temperature Sensor [CTS] (NOT the ECT sensor (B11/2)) and turn on when the resistance of the CTS is as shown above. Note that to TEST the fans cut-in, 310 and 250 ohms are substituted for the actual CTS. This means that at 100 deg C, the resistance of the CTS is 310 ohms; at 107 deg C, it's 250 ohms (per MB documentation).

Thus to 'fool' the system, you need to add a resistor in parallel to the CTS to make it look as though the temperature is higher than it really is! Resistors in parallel LOWER the resistance value.

What temperature to use? I think it should be between 90 and 95 deg C. That's simple enough but HOW to do this mechanically (mount the resistor) is not clear to me yet. You'll see later how I did it.

Resistor Value:
Use a 1%, 1/4 watt FILM type resistor to ensure stability. For example, Radio Shack PN RSU 11345741 (1.1K). This is NOT a stocked value, so don't try to get one unless you special order it. There are numerous other sources for this type/value resistor, typically any electronics house. A CARBON type should be avoided since they will drift badly with heat.

So what is the value to use?
95 deg C ====> 1200 ohms
90 deg c ====> 1100 ohms

Note that the higher you choose to have the AF kick-in, the larger the value of resistor that's needed. That's because the sensor value changes exponentially near 100 deg C. The resistor should be "electrically" across the two (2) leads to the CTS sensor (B10/8). A value that is in between the stated values will move the point accordingly although it will be hardly noticeable.

Now how to mount this permanently? Waiting for ideas. Would like to see a mating connector with the resistor mounted so it can be plugged in!

My pic shows how I did it. Take the connector apart and solder the resistor into the pin leads where the wires go. Then snap the top back on, and plug it in. That's all there is to it! Link:

Testing shows that it turns on at 91-92 degs C, as best as I can read the temp gauge in the instrument panel. And takes the temp down to about 87 degs and then shuts off. With A/C on full, it's less than 95 deg C.

So far haven't seen any downside to this mod, ie 'fooling' the A/C system by 5 degs or so.

Remember that this change involves a system with TWO (2) coolant temperature sensors where the power train management is done via the ECT sensor and the A/C is done via the CTS sensor (ie a separate sensor). If NOT, this mod will not work. For vehicles with only ONE temperature sensor, this mod will also not work!

This modification is also FAIL SAFE. If the resistor were to fail, which is OPEN, then the fan ckt will revert to 'stock' operation.

Don't forget WATER WETTER (WW): You can now (for summer driving) add WW to a 80/20 % mixture of water/antifreeze plus a can of WW. This with the CTS mod will keep your car's temperature gauge well below where it used to be! Hope that you find it useful.

[Edited by JimF on 05-12-2001 at 02:52 PM]

__________________
Regards . . . . JimF
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'94 S500 Cpe

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