HI! Need some advice on my 1990 240. The Temp Gauge isn't working (Gas gauge finicky as well). The air blows VERY hot so I drove it around for a little while and pulled over and checked the coolant. The reservoir was full and has me thinking that the broken gauge isn't telling the car it needs to be cooled down. Not sure if that is factual tho. What should my next step be? Im at a stand still. Thank you!

 

The gauge doesn't tell the car that it needs to cool down, it's just a thermometer measuring the coolant temp. Get an infrared thermometer to check the actual temp. of the car when warm. Then investigate the cause of the inop gauge, the sender or the gauge itself.

 

The temp gauge system has a thermistor sending unit in the engine - that's a resistor than changes in value with temperature. Then the gauge which is really just an ohm meter, measures that resistance. To keep the voltage stable so the gauge won't fluctuate with the different voltages from the alternator - there is a voltage stabilizer, it's a transistor like device mounted on the back of the instrument cluster. Those (in your year) don't go bad. On your car there is also a temp compensation board - a small rectangular circuit board inside the instrument cluster that makes the gauge lie when the ac is on - those boards are known to get poor connections and make the gauge not work correctly.

The nuts that hold both of those gauges to the circuit board get loose sometimes, and create erratic gauges - so a few things to check.

The gauge does not control the temp of the coolant it just reports what the temp is. - The thermostat in the thermostat housing controls the engine temperature, by allowing coolant to flow to the radiator to be cooled. The upper radiator hose is connected to the thermostat housing. Thermostats do break and get stuck closed, or stuck open - thermostats make the engine run at 180-200 degrees depending on what temp thermostat was installed -

 

Our aim is to not confuse the beginners but rather speak plainly!!!!
check your wiring to the gauge sensor!! If its broken/ disconnected you will get no reading on cluster.
it is just under thermostat area and infront of knock sensor.
its a two wire connector... Start there my friend.
Regarding your fuel gauge, try a can of berrymans from walmart to break up sulfur deposits on sending unit.
Good luck
Goldenblock

 

also to be clear, there's TWO temp sensors in the block, one for the display gauge, and another one that goes to the ICU and ECU and is used to adapt the fuel injection and ignition timing to the cold vs hot motor. this is in addition to the mechanical thermostat that controls the coolant flow through the radiator.

 

Got it fixed! The issue was the circuit board behind the dash. Replacement was fairly easy once I figured out what was going on. Thank you!