82 300 SD Heat Problem

[richclay]

The heat on my 82 300 SD does not work consistently. I cannot determine a consistent pattern with it but upon starting the engine on a cold day the system acts properly by letting the water temp get to the proper point and then the heat system starting up. It will initially begin heating the car and shortly begin blowing cool air. Under idle conditions, it waits longer before blowing cold air, but if I hit the highway shortly after starting up, the heat does not last long. After coming to a stop, I have noticed that every so often the heat will start up again.

I have changed the thermostat and the mono valve and back flushed the system. No change.

I want to look at the sensor that senses the inside temperature of the car but I don't know where it is. I suppose such a sensor is part of the system since there must be something that tells what the temperature is so it will maintain the temperature that is set. DOES ANYONE KNOW WHERE THIS SENSOR IS ON THE 82 300 SD?

Richclay

[anhydrous7]

I own an 82 300SD. The temp sensor is above the rear view mirror in the center of the head liner. You should have a small square hole with a grill on it made of plastic. It is right before the sunroof (if you have one). Since you changed the monovalve, I am not sure what the problem may be. When I bought mine, I replaced the monovavle, but also had intermittent operation of the fan and such do to the ACC. I purchased a rebuilt unit and everything cleared up. If your ACC is the original, then you may be having trouble with the flex circuits. Over time they fail and cause intermittent operation. Hope this helps.

My guess is if you have to check that temperature sensor, you would have to remove the headliner to get to it. My best guess is that it is not the temp sensor. I did a search in the threads and could not find any one with problems with the temp sensor on a 300SD at the time.

Good luck.

Rob

[84300DT]

what happens if you put the heat to max on the temp dial? ie. past the stop. this is supposed to negate the function of the inside temp sensor.
does your electric aux coolant pump work?

[Pete Burton]

sensor is where anhydrous7 described. You can easily remove that little grille and pull the sensor out and unplug it. Check it's resistance with a multimeter at different temperatures. There are specs for these temps in the manual, I forget what they were....maybe near 2000 ohms? Anyway, I did this once on my 300SD and the values matched pretty close. At least try and see if it responds by leaving it outside to get cold, and then the resistance should change when you breathe on it.