Greetings all,

I picked up a 2008 MCS in *questionable* condition and have been giving her all the TLC possible since purchase. Most recently, I changed the thermostat housing and AUX water pump because the pump was leaking and I was already going to be working right there next to the thermostat. While swapping out the aforementioned parts, the temp sensor connector disintegrated in my hands. I ordered a new two pin connector and installed it on the two wires (Yellow=1, White=2). All was well in life until one night my car wouldn't turn over. I was dumbfounded because it has been starting up just fine up until that point. I disconnected the coolant temp sensor and it started up, a little rough but cleared out and began feeling normal at idle in about 5-10 seconds. I got it home and looked at the connector and everything looked good to me. I plugged the sensor in and the car started so I let it sit and re bled the coolant because that was the first thing I saw on the forums was some people having over heating problems likely due to trapped air. I really took my time and re-accomplished multiple times even though I think it was way good before I drove to work earlier. After shutting down and leaving it over night, the car wouldn't start again with the temp sensor plugged in and the ECU read ***CTMP MOM so there was certainly a problem with the sensor or wiring to it. I took apart the connector that I just replaced and adjusted the ends of the terminal to be tighter and re-verified I had yellow wire in slot 1 and white in slot 2. I re started the car and anything I do now reads 135C from the ECU. Even after letting the car sit for hours and getting as cold as it will get, always reading 135C. I ordered a new (OEM) temp sensor and new connectors and terminals which should be here before this upcoming weekend. I'm just curious if anyone has any idea what else I should do apart from the new temp sensor and connectors. I am at a loss here. The water hoses get warm while I let it idle in the driveway but not hot. I can feel coolant cycling through the system and my overfill res is not dropping at all. I would appreciate any feedback because this is super frustrating to me. I just want to enjoy the mini :\

 

Reads like the new sensor is mis-reporting temperature (voltage) or the wiring from the sensor to the ECU is bad. But there is a chance the input pin the sensor at the ECU the sensor ultimately connects to is bad. It can no longer receive an analog voltage signal so the underlying ADC can convert this from a voltage signal to a digital temperature value.

The high value is a clue at least based on my knowledge of at least how some temperature sensors work/fail.

Disconnect the sensor and check the voltage output at ambient temperature. Then connect the sensor and check voltage a close to the ECU as you can get. It should be very close to the same voltage reading. If not then there's a problem with the wiring between the sensor and where you probed the wiring. If it is about the same then this casts suspicion on the ECU.

No guarantees but that the temperature was ok for a while after the new sensor was installed but now is not ok points to a failure of the replacement sensor. I know with other engine sensors: O2 sensors, MAF, MAP; the advice is to buy factory. Cheap knock offs while tempting at least with their price are just not that good. (No sensor issues with my MINIs but with my other cars from a VW Golf TDi to a Porsche Turbo every time I had to replace a sensor: O2, MAF, even one coolant temperature sensor; I bought the necessary sensors from the dealer parts counter.)

 

Hey, thanks for all the info! I haven't shot too many wires yet for this issue but ill give it a try when I get home. I'm leaning hard towards sensor being bad and made sure to pick up OEM. Im learning that buying OEM with these cars is well worth it in the long run. Once again, I appreciate your response!

 

***UPDATE***

I checked the voltage as suggested above and the wiring is all good. My new coolant temperature sensor came in today so I threw it in. The car started right up and I am getting an accurate reading from pg 7 on the tach. Lesson learned, I will buy OEM from now on when it comes to sensors and the like.