Now that I am driving my daughter's fresh engined 300D I am much more aware of the climate control problem.

It acts like the cabin temp sensor is lagging in temp change. It will heat like crazy, then when it finally stops heating it blows cold for ages until you're freezing or you decide to shut it off. Is the temp sensor right there at the little square grill in the top middle of the dash, or is there a tube that goes down to the sensor in a remote location like an SD?

Does anyone know the resistance of a properly working sensor so I can test it once I find it?

Any suggestions or ideas will be greatly appreciated.

Have a great day,

My 1977 300D has the interior temp sensor in the grill work of the front passenger side speaker. You can see it. The tube just serves to suck air past the sensor.

Althought I can't help you with resistance, I did find that the tube did have a tendancy to fall out on occasion. I just supported it with those plastic ties and all was right with the world.

JCD

Thanks for the response.

The '77 to '80 is the servo system, the '81 to '85 is the electronic system, two different animals. Does anyone know if the sensor is in the same location on the late as it is on the early?

Thanks,

My climate control system was essentially useless until I took out the glove compartment and replaced the disintegrated connecting tube between the air temp sensor and some pipe that appears to have the purpose of pulling air through the sensor.

The glove box comes out by removing the center locking pin from each fastener, and then pulling out the fasteners. You have to manipulate the glove box courtesy light somewhat as you're pulling out the glove box in order to separate it from the glove box or disconnect its leads.

You will probably find a foam tube that looks something like pipe insulation. Or the remains of it. I cleaned all the remains away and replaced this section of foam tubing with 3/4-inch heavyweight "agricultural" rubber pipe (a lot like a car's heater hose in construction). Cost about $1.50 at a farm supply store. The 3/4" pipe provided a snug fit to the plastic pipes of the climate control system. I cut it a little longer than necessary, ran it up over one CC pipe way back, and then pulled it onto the other CC pipe. That way I have good overlapping connections that help the system stay relatively airtight. I didn't see any need for gasket shellac or hose clamps. It just has to pull a little air through the sensor.

Results are incredible now. I can set the temperature dial somewhere and the interior stays at that temperature. What a concept!!! Why MB used that cheap foam pipe I'll never know.

Ken300D
'82 300D

Ken,

Thanks for the feedback. This helps me expect what to find from someone's experience rather than doing exploratory surgery.

I really expect that this is the problem.

Thanks and have a great day,

You definitely need to check out and probably replace this section of foam pipe I'm describing. Made all the difference in the world on my '82 300D.

My original symptoms might be slightly different from yours. To get heat, I'd turn up the dial to a higher temp. Out comes lots of strong heat. Then it would cut off and I'd start getting cold air, and it seemed the heat would never come on again. So, I'd bump up the temp dial some more. Same thing over and over until the temp dial was way in the red zone, near maximum.

My theory is that since the heat sensor had no air flow, it was sensing the heat inside the dash which the ducts go through nearby. So the ducts were heating up the inside of the dash hotter and hotter.

With the pipe fixed and the sensor air flow moving again, it seems like the air coming from the vents is just about always comfortable, not really really hot nor cold. And the cabin temp is very stable.

Ken300D

Almost certainly that foam hose has turned to powder. For a quick check, see if the sensor will draw in cigarette smoke. I replaced the foam hose with a piece of vinyl tubing on mine. Another piece of that same crappy foam tubing is used as the evaporator drain. When that one goes, the back floor pans fill with water. I would vote for replacing that while you are under the dash.

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

I used round insulating foam from Home Depot when I replaced mine - don't remember if it was 3/4" or 1" diameter. Once you pull the glove box out look up toward the left and you should be able to either see what is left of the round foam tube or the places the tube attaches to.

__________________
Jim

I found some cloudy white plastic tubing of the right size in a scrap pile in my shop. It is some tubing that I used in fixing up a neighbors "high boy", which is a funny looking rig that is used to spray crops.

I put that on and it works much better. However, it took awhile this morning for the temp to stabilize. I theorize that this hard plastic line was cold and took awhile to stabilize at a temperature near the desired cabin temperature. I'm going to try again tonight with some pipe insulation like engatwork used. I believe the thermal properties will be more conducive to this application.

Thanks for the help,

Palangi:

How do you gain access to the evaporator drain pipe? Where is it? Can it be seen inside the dash from the glove box opening?

Last summer I never really saw all that much water dripping from the car during humid weather with the AC on. I thought that was kinda weird. Floorpans seem to be dry, but I'd like to follow up and probably replace this drain pipe.

Is the drain exactly in the center of the car on the transmission hump or is it to one side?

Thanks - starting to get a lot of sweat equity in the ole 300D.

Ken300D

It is exactly in the center of the transmission hump, but it's way up in there towards the front. Take out the passenger side kick panel and you can reach up in there. I would like to have used a better material than the original foam tube, but I could not come up with anything that worked better that I could make fit. It is the same foam material as the aspirator hose only with a coating on it. It is held on by a plastic twist tie. I was unable to re-install the twist tie thingey, so I held it on with a regular tiewrap. I would replace this for sure on an old car. The hose just crumbles and it will soak your carpets if it is leaking.

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE