Testing Blue Temp Sensor

[German-Whips]

Following on from my rough/low idle issues, I have bought a multimeter so I could test the Blue Temperature Sensor to see if that's what is causing the problem.


The question I have is that how do I test it from the ECU and where would I find the ECU on a cabriolet? Remove the glovebox and find it there?

Once I've found it, is it correct that I simply pull off the connection plug, and test it using the relevant pins?

Does the car have to be switched off with the key out, or in the 1st position?

A step by step guide would be appreciated!

Thanks

[zaust]

From the WIKI ↓

Testing

While it's relatively simple to test the Blue Temp Sensor, it is also important to test that the information is reaching the ECU. This test needs to be done first, so that you don't disturb any wiring in the engine bay - especially the sensor plug itself.

Testing the actual Sensor is a relative simple affair, requiring a multimeter.
Locate the ECU and remove its loom connection
Set your multimeter to measure resistance (Ohms)
Putt your multimeter electrodes onto pins 24 and 45 of the ECU (for facelift engines. For others, such as the 318iS, check the ECU Pinouts)
From cold, expect a reading in the region of 3500-4000 Ohms. Any more or less than this is not a huge issue
Reconnect, warm up the engine, and retest
From hot, expect a reading in the region of 330 Ohms. If your Blue Temp Sensor has not changed its resistance significantly, then it is at fault and should be replaced.

Once that's done, you can test the Blue Temp Sensor itself.
Locate the Blue Temp Sensor and disconnect its plug.
Repeat the test, this time testing the pins directly.
Confirm that the readings you got at the ECU are the same or similar to what you read at the Blue Temp Sensor.

If the readings differ, then the most likely cause is corrosion of the C191 plug (facelift cars only). Once that is ruled out, the fault must lie with the sensor itself.

[daimlerman]

Test at the engine ECU.
This lives above the driver's legs on a RHD car,remove the panel that lives under the steery column and over your legs to find it.It's silver in colour and has a large black plastic connector on one side with a lot of wires going into it.All the wires are different colours,so you need to check at the blue sensor to establish just which two wires you need.
Remove the connector with all the wires from the ECU and test at the connector side.There should be a large difference in the meter reading between a cold engine and one at running temperature.
As the ECU has to be dis-connected to carry out the test,it would be very difficult to test with the engine running!
Meter needs to be set to read Ohmns,BTW.

[German-Whips]

Thanks for that!

So no keys in the ignition at all then, car must be totally switched off?

[Jona1971]

yes