190E Fluctuating Temperature Gauge

[allentc]

Hi,

I have a UK 1993 Mercedes 190E 2.0 Auto. For the past few weeks my temp gauge has been wildly fluctuating. Up to about the 100C mark and it seems to behave itself. However, the needle has started to twitch/fluctuate and then it soars to 120C and once went off the scale(!).



If I tap the cluster the needle will often go back down to about 90C (where it should be). If I turn the car off and on again it has the same effect: drops to about 90C.



The car doesn't appear to be overheating and is no loosing any coolant. It drives well and the head gasket was done 7 years ago. There is no mayo in the coolant bottle. However, understandably I have a niggling worry in the back of my mind that I might be frying the engine...
I have bridged the connector for the fan sensor with a paperclip and the fan does come on.


The fan temp sensor has three prongs. Two of the prongs are to activate the fan and the third is a single prong which drives the temperature gauge. I would like to test the reading given out by the third prong. If it says something reasonable like 90C but the gauge is reading 100+ then I know the fault is likely to be the wiring to the gauge or the gauge itself. Problem is I don't know what to measure at that third pole (resistance)? Does anyone have a resistance/temperature chart for this sensor so I can compare the sensor output to the gauge?


I could replace it and I do have a spare new one:

Access is very tight (sensor is the one to the left):




I tried getting a 22mm spanner in there but can't get it on the nut part of the sensor. Is the recommended tool a deep socket? It seems like easiest way would be to attack it from the top but don't know if a deep drive socket will fit over the rounded top of the sensor. I don't have a deep socket hence why I thought I would ask before wasting money purchasing one.


Any ideas as to how I can test that third pole of the fan/gauge sensor or what tool to use to replace it?


I suppose a quick and dirty way of proving the engine temp vs gauge would be to take a reading of the one that goes to the ECU. That is located just to the right and has four prongs. I could read the temp off by reading the resistance of the diagonally opposite prongs which would give me some piece of mind.

[duxthe1]

The electrical connection becomes poor where the gauge connects to the circuit board. There are three 7mm nuts that retain the gauge to the board. Slightly loosen them, wiggle it a bit, and then tighten them back down.