I have a perplexing concern, I have a 1986 240DL in great shape. I recently acquired the one owner car with about 140K miles. I have started doing the service on the car, drained and replaced the engine coolant and thermostat, changed the oil, new cap, rotor, plugs and air filter.
The car runs great but has this interesting issue which seems to happen only at night while the weather is somewhat cooler. When climbing a long grade the temp gauge will climb to the 10:30 position, never reaching the "red" zone. However climbing the same hill in warm weather will not produce the same results.
Could this be the result of a failing fan clutch? are the "thermal" type?


Any thoughts would be appreciated.

 

the temp gauge on the 240 (and 740 and 940) has a silly compensator circuit thats supposed to make it stay dead center as long as the vehicle is anywhere within normal operating range, this effectively reduces the needle to an idiot light. one possibility is this temperature compensator is flakey. it CAN be disabled by removing a small board from the back of the instrument panel and replacing it with a jumper, then the needle behaves more linearly.

 

Thank you, I have seen this modification and had intended to do it when I change the odometer gear.

 

Fan clutch won't cause this. What's the "10:30 position", how far from HOT? If it's overheating, it's more likely to happen when warm outside. Look at the gauge sending unit/wiring, make sure they are OK. It 's the one in the front of the head.

 

Thanks, Yes I agree it should be when it is hot. I live in Southern Ca and yesterday it was in the 80's. I climbed the same hill in the daytime and you would think that if it was going to run hot that's when it would. But it was steady just below the 9:00 position. It seems to only happen at night, and it never overheats, just gets to appox. to just above 3/4. I have tried to simulate the conditions; turning on the heat, the lights, etc. The whole thing is perplexing. I spoke to a mechanic friend and he was stymied as well, seems to be operating in reverse. Since the fan clutch is inexpensive I am going to change it out. I am also going to do he temp gauge mod.

 

Get an accurate reading of the ACTUAL temperature when this is happening. Buy/borrow an infrared temp gauge, get an instant reading. This way you'll be sure. Harbor Freight sells these things pretty reasonably priced.

 

Sounds like a good Idea

 

amazon has a good selection IR thermometers. I have this one, which is quite accurate and consistent. There are, of course, much cheaper ones.