Ok, so I have an 87 300e 3.0 liter. The problem Ive been having is with the heating/cooling system of the car. Its very weird Im not sure if the two problems go together or if they are two different things causing these problems but they just started happening at the same time. I will be running my climate control in cold and the second i move the temperature wheel to another position other then the cold MAX it will start blowing extremely hot air, it will not blow to the temperature the wheel is set at, only really hot air. Aswell, when i do change the temperature on the wheel it takes about 2-5 minutes for the heat to accually start comming through. Because of this problem the auto feature doesnt do its job like it should on the fan control. The second half to that problem is, the other night I was sitting in traffic for about an hour only idling at 600 rpm and the engine temperature guage was slowly, slowly climbing higher and higher, it was comming near the 120 deg. mark when i decided i had to turn the climate control on extremely hot to try to get some heat out of the system, when i turned on the heat, it took about 4 minutes for the heat to accually come through and then the engine temp. started to drop, but started to drop extremely slowly. The only way the engine cooled was with getting on the highway and having the air blown into the engine through the grill, then it went down to normal temperature. Also to note, when hitting the (o) button to shut off the heat it takes 2-5 minutes for it to shut off and once the heat starts flowing through the vents and i try to switch the wheel back to cold, it will refuse no matter how long i leave it, i have to shut the whole thing off (takes 2-5 minutes) then turn the wheel to cold, and start from there. Is the engine overheating problem and this heat problem caused by possibly the same thing. PLEASE help, its getting so frustrating. Thank you for reading all this .

-Justin

 

For the first question -- Sounds like the unit that hides behind the wood. It is pretty common whenever anybody mentions something about hot air. The solder points inside get cracked/tarnished and it can cause this. I have fixed two units with "similar" problems.

Just pull it apart, open up the unit and look for any bad looking points then just touch it up using a soldering iron and some fresh solder if it looks low. See if it makes any difference. Took me a couple tries, but I knew after the first it made a difference.

That, or head to the junkyard and pull one to test....shouldn't charge you more than $20 for it.

 

thanks alot for the advice. when ever i get some free time i will take it apart and have a look at it. thank you.

-Justin

 

The unit is gonna be the only thing that would be causing those type of problems. Only other thing I can think of would be the little climate fan up by the reading light malfunctioning, but I highly doubt it.

Let us know if that fixes anything. Its always good to know so we can help others in the future.

 

I had the same symptoms on a 92 300E. The AC would function normally and suddenly it would blow hot air. Pressing ac function buttons and turning temp wheels can't stop the hot air from blowing. Turning off and starting the car seem to reset something and makes the AC blow cold air again but not for long.

It turned out that the mono valve was defective. It is a valve that controls the hot engine coolant to flow in and out of the of the AC system to regulate the cabin temperature depending on the AC temp setting. When the cabin gets too cold the system tries to regulate the temperature by letting hot coolant flow into the AC system and it would get stuck open. Once it was stuck open, hot coolant continue to flow in the system that resulted in hot air flowing in the cabin (However, I'm not certain if an 87 has the same set up as a 92.)