Auxillary Fan Turn-on Set Point Resistor Modification
10-18-2011, 11:33 PM
#1
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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
06-10-2015, 09:29 AM
#3
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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
06-10-2015, 12:52 PM
#4
Out Of Control!!
Although I can appreciate the OP's aux fan prowess, what he hasn't mentioned is the aux fan comes on when two actual values are recognized--that is refrig pressure exceeds 20bar(this varies by MY) and engine coolant temp of 105C(this varies my MY)
Why would anyone want to lower those values and endanger the AC system and or the engine---do the correct thing test the coolant temp sensor and the refrig pressure switch--forget the HS resistor---what you are doing by changing the resistor is hide a real problem of pressure and coolant---choose wisely!!
Why would anyone want to lower those values and endanger the AC system and or the engine---do the correct thing test the coolant temp sensor and the refrig pressure switch--forget the HS resistor---what you are doing by changing the resistor is hide a real problem of pressure and coolant---choose wisely!!
Last edited by Plutoe; 06-10-2015 at 01:08 PM.
06-12-2015, 01:09 PM
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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, ....
06-12-2015, 01:32 PM
#6
Out Of Control!!
You are reading way to much into what I have said--let me repeat myself--the aux fan adds extra cooling when the engine coolant and or refrigerant temp exceeds their design set points---in the practical sense if your coolant and AC systems are working to design the aux fan would never come on---there would never be a need.
When the aux fan does come on you know you have an issue--what the OP has done is lower the coolant temp when the aux fan comes on---that just creates more problems
Additionally the so called coolant resistor fix does nothing to reduce the temp that the aux fan will come on for a high re frig temp---high temps lower cooling(simplified)
PS; It has been eight years since my aux fan has come on(every two years I clean the corroded connector)
When the aux fan does come on you know you have an issue--what the OP has done is lower the coolant temp when the aux fan comes on---that just creates more problems
Additionally the so called coolant resistor fix does nothing to reduce the temp that the aux fan will come on for a high re frig temp---high temps lower cooling(simplified)
PS; It has been eight years since my aux fan has come on(every two years I clean the corroded connector)
Last edited by Plutoe; 06-12-2015 at 01:35 PM.
