K6JRF at the mic K6JRF's Page
formerly W6FZC


My Mercedes Benz
S500 Coupe

K6JRF's MB S500 Cpe
(Updated: Apr 2, 2013)

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

Auxiliary Fans Turn-on Point Modification

Description of Problem
The 'S' class V8 5.0L Mercedes-Benz vehicles generate a lot of engine heat during summer driving in stop-go traffic. Especially if the antifreeze/water mixture is 50/50%. (To change the mixture ratio, see Menu #17) As a result, engine temperatures in excess of 110 deg C, as indicated on the instrument cluster temperature gauge, are not unusual! Mercedes-Benz maintains the official position that this is ok! While I might agree if it was just a transient, repeated exposure to this is not a good thing. If fact they even say that going into the RED area occasionally, which is above 120 deg C, is not to be of concern to the vehicle owner.

But high temperature is not good for any part of a car and it especially lowers the car's available power. For every 12 deg C rise, the car's timing is retarded by 5 degs so as not to ping. Thus a lot of power is wasted because of the high heat. This is not to mention the additional stress to electrical connectors, vacuum lines, rubber hoses and electronic parts.

This modification (as described) is specifically for 1992-1995 'S' class cars such as the S500 or S600 (W140), coupes (C140) and 500E/E500. However, this modification also works for most 1984 thru 1995 Mercedes models such as; 190E, 300E, 400E, 500E, 320SE, 420SE, 500SE, E320, E420, E500, S320, S420, S500 and S600 models.

The modification is very straightforward to accomplish and it dramatically reduces the operating temperature of the engine. The best part is that this only takes one part to accomplish, a 1/4 watt resistor and a few minutes to solder it into the connector!

Please note that this modification will not make the Aux Fan system operational. On the contrary, the Aux Fan system must be operational for this modification to work. Some have purchased a Cool Harness or made the mod thinking that by installing it, the Aux Fan system will now be operational. Many times the control relays are at fault; click here for a pictorial for locating the A/F relays.

Theory of Operation
The S500 series car has two (2) fans can draw up to 25 amps (MB claims 35 amps but there's only a 30amp fuse) at full tilt and are switched on in either two or three stages. Most of the newer Mercedes use just a two (2) stage system consisting of LOW and HIGH.

The two drawings below show operation of the aux fan system for LOW and HIGH speed. MEDIUM speed current draw is approximately halfway between LOW (12amps) and HIGH (23amps).

Functional schematic diagram showing Aux Fan operation on LOW speed Functional schematic diagram showing Aux Fan operation on HIGH speed

Mopunting positions for R15 and R15/1 resistors
The ckt values of R15 (MB PN: 000 158 37 45) is 0.9 ohms and R15/1 (MB PN: 000 158 39 45) is 0.6 ohms. The in-ckt values are smaller and are shown in the schematic.

The mounting points for R15 and R15/1 are shown in the drawing below. Both resistors are stacked vertically in the newer models but for my car, they are horizontally mounted end-to-end.

The fans can be activated by either temperature or refrigerant pressure. According to MB data, here's the as-delivered Temperature and Refrigerant pressure vs stage activation:

Auxiliary Fans Activation
Switching Points - fan speed 1st - low 2nd - med 3rd - hi
Coolant Temp (ON in deg C) 100 107 115
Coolant Temp (OFF in deg C) 95 100 107
Refrig Pressure ON (bar)[psi] 14 [206] 17 [250] 20 [294]
Refrig Pressure OFF (bar)[psi] 11 [161] 14 [206] 17 [250]
Eq resistance of CTS (ohms) 310 250 200
Actuation of Fans (Volts) 7 9 12
Three Stage Fan Activation for S-Class; Other Models have 1 or 2 stages

The fans can be switched on by either high temperature (over 100 deg C) or by high refrigerant pressure (over 14 bar or 206 lbs). This modification concentrates on the 'temperature' side so that the fans may be activated when a certain temperature has occurred and does NOT require that the A/C be in use.

The fans use the Coolant Temperature Sensor [CTS] (NOT the ECT sensor (B11/2)) and turn on when the resistance of the CTS [PN 008 542 45 17] is as shown above. Note that to TEST the fans cut-in, 310 and 250 ohms are substituted for the actual CTS (per MB Maintenance Manual). This means that at 100 deg C, the resistance of the CTS is 310 ohms; at 107 deg C, it's 250 ohms.

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, with 92 deg C being my choice. That's simple enough. But of course, you need a sturdy mechanical package to 'house' the resistor. After trying a few things, I made a choice. If you came here front my home page, you already know how I packaged it!. If not read on.

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 an in-stock part, so don't try to get one unless you special order it. There are numerous other sources for this type/value resistor, typically any electronic's parts house. A CARBON type should be avoided since they will drift badly with heat.

So what is the value to use?
98 deg C ====> 1780 ohms
95 deg C ====> 1200 ohms
92 deg C ====> 1100 ohms ==> preferred value!

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.

Doing the Modification
My picture shows how I did it. Locate the A/C CT sensor (B10/8) and disconnect it. Using a small blade screwdriver, take the connector apart and solder the resistor (1.1K ohm) into the pin leads where the wires go as shown below. Then snap the top back on, and plug it in. That's all there is to it!

Closeup of connector pins showing mounting method Testing shows that it turns on at 92-95 degs C instead of 105 deg C as before. Used the instrument panel gauge for the measurement. After coming on, it takes the temperature down to about 88 degs and then shuts off. With A/C on full, the temperature stays below 98 deg C, as measured on a 82 deg F day, doing city stop-go driving.

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 and auxiliary fans use the CTS sensor (ie a separate sensor). If NOT, this change will not work because the ECT sensor is used for fuel and ignition mapping. Changing the temperature will also change the mapping by 'some' amount.

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

If you do not feel as though you can not do this yourself, a complete package is available for you to simply plug-in. It's called Cool Harness(c). See more details on the main page. CLICK HERE for pricing.

Don't forget Purple Ice (PI): You can now (for summer driving) add PI to a 60/40% mixture of water/antifreeze. 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.

Aux Fan Relays Location
There are many things that can cause the A/F to not work. Chief among these is the wiring becoming brittle and failing. Also the dropping resistors burn out causing an "open" to the system. Some time the A/F "high" and "low" speed relays fail or become "ohmic" and cause intermittent operation. Both relays are tucked away behind the car's fuse panel and are somewhat difficult to find. The following pictoral by Benzworld's "slowhands" makes it very easy to replace and/or swap the relays to t/s the problem.

Aux Fan Relay Pictorial Aux Fan Relay Pictorial


Aux Fan Relay Pictorial Aux Fan Relay Pictorial


Aux Fan Relay Pictorial  Aux Fan Relay Pictorial

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