I have a 1975 450 SL and the engine temp. gauge does not register. All other gauges seem to work ok. I replaced the sending unit and nothing changed. I pulled the instrument panel out and behind it I found a piece that was fitted into the back of the panel that is about 3 inches long and cylindrical fitted into one of the openings the size of the bulb inserts. The end had something that looked like it should plug into something inside the panel. The end sticking out of the panel had what looked like it should connet to something but there was no wire loose. I also found another piece that was not connected to anything hanging behind the panel. It has a small (approx. 1 1/2 inch) piece connected to the end of two wires. The end can be snapped off and there are two metal pieces, one to each wire, that when pressed makes a buzzer sound. It also has a small hole in the side of the plastic piece. Not sure if this has anything to do with the temp. gauge. Can't find anything in the Chiltons I have that mentions this. Please help, need a temp. gauge. Thanks! Leont

 

Wasn't the gauge. I had replaced the temp. sensor near the thermostat housing but I found another next to the firewall on the passenger side of the engine. It had a wire broken off. I reconnected it (riged it) and now the temp gauge works, runs between the 175 and next mark. The 3 inch piece sticking in the back was the light for the instrument panel, (the back was missing and a bulb went in the end it), on the side for the fuel and temp gauge. That other piece with the hole in it is apparently an alarm to let you know something about the key in the lock when the drivers door is open I think. All works now and I can pull the instrument panel in about 5 min. now! Leont