Auxillary Fan Turn-on Set Point Resistor Modification
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Joined: Nov 2002
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From: Massachusetts
2018 GLE350, 2014 G550, 2000 SL500, 1980 TR8, 1995 E320 Cabriolet
Auxillary Fan Turn-on Set Point Resistor Modification
I have read a number of posts from people who have added a resistor to the coolant temperature sensor used by the climate controller to turn on the auxiliary electric fans when the engine temperature goes above 105 deg. C. Adding this resistor "fools" the system and causes the fans come on at a lower temperature which helps with issues like heat soak, etc. I have done this myself and it does work (except for one side effect noted below). But most people seem to have attached the resistor to the actual temperature sensor or they buy a modified sensor with the resistor already attached.
But when I was replacing my engine wiring harness, I noticed that the circuit for the coolant sensor passes through a small junction box on the left inside fender well. This makes sense because almost every other wire on the harness goes directly to the ECU so components that feed non-ECU functions must "exit" the harness somewhere else. I looked at the connector and noted that it can easily be disassembled so that the "cheater" resistor can be soldered directly to the pins here. This has the advantage of being a more reliable connection than directly on the coolant sensor and it does not need to be disturbed if the coolant sensor is replaced.
So I have attached a few "how to" pictures in case anyone wants to do it this way. The resistor value value I used was 2.7K ohms, although I have seen recommended values as low as 1.1K ohms. 2.7K provides auxiliary fan switch-on at 100 deg. C which is fine for me; lower values will lower the turn on temperature even more. A value too low will cause a fault in the climate control computer (12 flashes on pin 7, diagnostic connector). The resistor should be soldered to the Blue/Yellow wire (pin) and the Brown/Yellow wire (pin). The resistor leads should be insulated with some heat-shrink tubing or similar.
As for the side effect I mentioned (true no matter where you mount the resistor), in cold climates, the climate control system will always think the coolant is warm enough to run the heat, even if it is not. Not really a major problem, but it does blow cold air for a while whereas a stock system does not start the fan until there is actually heat available. To deal with this, I'm probably going to fit a "winter" switch in series with the resistor to disable the modification when it's cold and overheating is not likely to be a problem.
Anyway, I though it was a clean way to make this mod, so I posted it.
- FD
But when I was replacing my engine wiring harness, I noticed that the circuit for the coolant sensor passes through a small junction box on the left inside fender well. This makes sense because almost every other wire on the harness goes directly to the ECU so components that feed non-ECU functions must "exit" the harness somewhere else. I looked at the connector and noted that it can easily be disassembled so that the "cheater" resistor can be soldered directly to the pins here. This has the advantage of being a more reliable connection than directly on the coolant sensor and it does not need to be disturbed if the coolant sensor is replaced.
So I have attached a few "how to" pictures in case anyone wants to do it this way. The resistor value value I used was 2.7K ohms, although I have seen recommended values as low as 1.1K ohms. 2.7K provides auxiliary fan switch-on at 100 deg. C which is fine for me; lower values will lower the turn on temperature even more. A value too low will cause a fault in the climate control computer (12 flashes on pin 7, diagnostic connector). The resistor should be soldered to the Blue/Yellow wire (pin) and the Brown/Yellow wire (pin). The resistor leads should be insulated with some heat-shrink tubing or similar.
As for the side effect I mentioned (true no matter where you mount the resistor), in cold climates, the climate control system will always think the coolant is warm enough to run the heat, even if it is not. Not really a major problem, but it does blow cold air for a while whereas a stock system does not start the fan until there is actually heat available. To deal with this, I'm probably going to fit a "winter" switch in series with the resistor to disable the modification when it's cold and overheating is not likely to be a problem.
Anyway, I though it was a clean way to make this mod, so I posted it.
- FD
Junior Member
Joined: Jan 2009
Posts: 19
Likes: 0
From: Huntington Beach
E 320 coupe
resistor add for temp sensor.
I have read a number of posts from people who have added a resistor to the coolant temperature sensor used by the climate controller to turn on the auxiliary electric fans when the engine temperature goes above 105 deg. C. Adding this resistor "fools" the system and causes the fans come on at a lower temperature which helps with issues like heat soak, etc. I have done this myself and it does work (except for one side effect noted below). But most people seem to have attached the resistor to the actual temperature sensor or they buy a modified sensor with the resistor already attached.
But when I was replacing my engine wiring harness, I noticed that the circuit for the coolant sensor passes through a small junction box on the left inside fender well. This makes sense because almost every other wire on the harness goes directly to the ECU so components that feed non-ECU functions must "exit" the harness somewhere else. I looked at the connector and noted that it can easily be disassembled so that the "cheater" resistor can be soldered directly to the pins here. This has the advantage of being a more reliable connection than directly on the coolant sensor and it does not need to be disturbed if the coolant sensor is replaced.
So I have attached a few "how to" pictures in case anyone wants to do it this way. The resistor value value I used was 2.7K ohms, although I have seen recommended values as low as 1.1K ohms. 2.7K provides auxiliary fan switch-on at 100 deg. C which is fine for me; lower values will lower the turn on temperature even more. A value too low will cause a fault in the climate control computer (12 flashes on pin 7, diagnostic connector). The resistor should be soldered to the Blue/Yellow wire (pin) and the Brown/Yellow wire (pin). The resistor leads should be insulated with some heat-shrink tubing or similar.
As for the side effect I mentioned (true no matter where you mount the resistor), in cold climates, the climate control system will always think the coolant is warm enough to run the heat, even if it is not. Not really a major problem, but it does blow cold air for a while whereas a stock system does not start the fan until there is actually heat available. To deal with this, I'm probably going to fit a "winter" switch in series with the resistor to disable the modification when it's cold and overheating is not likely to be a problem.
Anyway, I though it was a clean way to make this mod, so I posted it.
- FD
But when I was replacing my engine wiring harness, I noticed that the circuit for the coolant sensor passes through a small junction box on the left inside fender well. This makes sense because almost every other wire on the harness goes directly to the ECU so components that feed non-ECU functions must "exit" the harness somewhere else. I looked at the connector and noted that it can easily be disassembled so that the "cheater" resistor can be soldered directly to the pins here. This has the advantage of being a more reliable connection than directly on the coolant sensor and it does not need to be disturbed if the coolant sensor is replaced.
So I have attached a few "how to" pictures in case anyone wants to do it this way. The resistor value value I used was 2.7K ohms, although I have seen recommended values as low as 1.1K ohms. 2.7K provides auxiliary fan switch-on at 100 deg. C which is fine for me; lower values will lower the turn on temperature even more. A value too low will cause a fault in the climate control computer (12 flashes on pin 7, diagnostic connector). The resistor should be soldered to the Blue/Yellow wire (pin) and the Brown/Yellow wire (pin). The resistor leads should be insulated with some heat-shrink tubing or similar.
As for the side effect I mentioned (true no matter where you mount the resistor), in cold climates, the climate control system will always think the coolant is warm enough to run the heat, even if it is not. Not really a major problem, but it does blow cold air for a while whereas a stock system does not start the fan until there is actually heat available. To deal with this, I'm probably going to fit a "winter" switch in series with the resistor to disable the modification when it's cold and overheating is not likely to be a problem.
Anyway, I though it was a clean way to make this mod, so I posted it.
- FD
Junior Member
Joined: Jan 2009
Posts: 19
Likes: 0
From: Huntington Beach
E 320 coupe
Plutoe, what kind of endanger the AC system you mean? Despite: lowering the set point provide more protection - in case earlier shoot off A/C ( at 110 C - 115 instead)
Not mentioned the A/C cooling improving, ....
Not mentioned the A/C cooling improving, ....
