w124 temperature sensor

[JumboBeef]

The temperature read out on my dash has suddenly started doing odd things. A few minutes after setting off, the temperature read out will drop by about 6C! Sometimes, it corrects itself, other times it will not.

After doing a search through these forums, it would seem my temperature sensor is now duff.......is this right? If so, does anyone have a spare one I could have please?

Thanks.

 

[crammy69]

I replaced mine (im a real noob when it comes to car diy) it was only about £20 from merc, and was a 5 minute job to do.

 

[andy_k]

Assuming that we are talking about the external temperature sensor

If you leave the car in the sun, the temperature will rise as the air around it gets hot. Once you start moving, the air is circulated and the temperature will drop

If you need any further confirmation, put a thermometer out in direct sunlight and then out it in the shade. The air temperature is still the same but without direct sunlight the emperature will invitably drop.

Stop looking for faults and just enjoy driving your car

 

[JumboBeef]

OK, thanks. It is the sensor which would be the duff bit, wouldn't it.........?

Assuming that we are talking about the external temperature sensor

If you leave the car in the sun, the temperature will rise as the air around it gets hot. Once you start moving, the air is circulated and the temperature will drop

Thanks, but today for example it dropped from +7C to around freezing and then back up to +7C, in a matter of seconds.

 

[crammy69]

sorry i thought you meant water temp sensor! ignore me im thick.

 

[JumboBeef]

Oh, right: sorry. Yes, external temperature sensor!

 

[andy_k]

first step is to take it out and clean it - see if anything is interfering with it

 

[JumboBeef]

first step is to take it out and clean it - see if anything is interfering with it

Thanks. I can see it on the front bumper: how do I get it out? It seems pretty solid to me.

 

[JumboBeef]

Anyone, please?

 

[andy_k]

I thought they just clipped in

 

[Ian B Walker]

remove the front number plate. Behind it is a phillips screw. Undo that and the sensor pulls out.

 

[JumboBeef]

OK, thanks! Do I just give it, and it's connectors (assuming I can get to them) a good clean? It was all working OK today, but not yesterday.

 

[JumboBeef]

To drag this topic back up, as I really must fix this................

The read out gives me random readings: sometimes (but not often) it is correct, then it will drop 10C, and back up. It might flash, or only display the numbers faintly. Today, it dropped to -45C then back up to +50C!

I took the unit off the front bumber, but couldn't see how to get inside it. I also couldn't see where the join is in the cable. I followed the cable back to the fuse box, where it goes inside, but I can't see how to disconnect it (to replace it, or clean it, or swear at it!).

What should I do next please? I would like to get this fault fixed!

Thanks!

 

[grober]

Its normally connected to the instrument wiring loom inside the fuse box. You have to slacken off and gently lift the fuse carrier assembly to get at the connections underneath. You may need a new sensor but it may be the display unit in the instrument cluster that's faulty= its expensive to replace compared to the sensor.

 

[Smiley]

Just buy a new sensor and connect it in to the existing wiring somewhere out of sight with some decent wiring connectors.

 

[grober]

Just buy a new sensor and connect it in to the existing wiring somewhere out of sight with some decent wiring connectors.

Personally I wouldn't do this since the temperature measurement depends on the electrical resistance of the sensor and the display unit will be calibrated against it. Changing the length of the connecting wires and adding extra connections may affect the overall system calibration?

 

[pos.k]

Personally I wouldn't do this since the temperature measurement depends on the electrical resistance of the sensor and the display unit will be calibrated against it. Changing the length of the connecting wires and adding extra connections may affect the overall system calibration?

The sensor will be thermistor whose resistance will change in line with temperature. I would imagine that as long as the cables are joined carefully and correctly (soldered would probably be best) - the difference in properties between that and a 6ft length of the original straight cable should be negligible / insignificant.

 

[JumboBeef]

Thanks for the advice. Since I bumped this thread up, it has been working correctly for the first time since I got the car! I wonder if it is a loose connection somewhere (or would that be too easy?)

 

[JumboBeef]

Nope, spoke too soon: it's telling me is it just above freezing again...........