I have a 92 300SD and I have some tempeture issues. I'm driving on the fwy 65-70 mph and the gage at dash board some times reads allaway 120 and I tap the plastic on front of the gage and goes down to 80-85 then slow start going to 100-110 sometimes to 120 I tap the plastic glass on the gage and does no move down I stop the car and look at the water reservor and water is not boiling (good thing) but I don't know, if is the temp sensor, the controls the gage at the dash board is going bad or thermostat is acting up, how to test for this issue.

If tapping the gauge changes the reading there is something going on in the Gauge.

Heating the Temp Sensor changes the resistance of the current going through the sensor and the Gauge points due to the different current flow.

A loose wire or electrical connection in the Gauge loose will also change the resistance to the flow of current; loose it would produce an erattic resistance causing an erattic reading.

To answer your question:
The FSM would need to give you some sort of resistance in Ohms compared to a specific temperature.
You run the car up to that temp and check the resistance at the Sensor Terminal/s.

An alternate way would be to remove the sensor and heat up a pot of water with a thermometer in it (the have inexpensive thermometers for making candy and jelly) and submerge the probe of the sensor while giving it some time to heat up and check the resistance.

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84 300D, 82 Volvo 244Gl Diesel

Thank you Diesel for your advise, I will check the sensor. By the way Is the sensor between injectors 2 & 3 just to the bottom? Also when I check do I need to have ignition ON ? Thank you again.

I do not know where the sensor is located.

This is a Russian parts site similar to the Mercedes EPC site. You may be able to look up where the part is if they have your Car listed.

http://www.detali.ru/cat/oem_mb0.asp?TP=1

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84 300D, 82 Volvo 244Gl Diesel