Climate Control Temperature Sensor
 

I'm still a newbie on this forum and haven't mastered all the search details. I'm sure this is here somewhere. Can someone direct me or reply to this question.
How does the dash mounted temperature sensor connect to the CCU? I'm putting a hard plastic cover over my badly cracked dash and want to move the sensor to a shaded spot to avoid the intense summer sun in Texas. I've found in other threads that there is a foam tube that could go bad and should be replaced by a rigid plastic tube. Apparently there is airflow through this sensor and tube to where? How long can I make the tube? Newer MB had the sensor above the rearview mirror - was that an air tube or electrical connection to the CCU? I found a thread that showed a picture of a replacement with a rigid tube - found it by accident and now I can't find again. That poster could probably answer my questions.

I'm spending hours searching and reading - got to be a better way. I just caught on to thread subscription but that's not always given as a choice, why? I need a forum tutorial.

Charles

Here's the rigid tube thread. http://www.peachparts.com/shopforum/tech-help/70866-ac-temp-sensor-foam-tube.html?highlight=aspirator+tubing

There is a temp sensor in there, and the tube draws the cabin air over it with a small fan under the dash, so it senses cabin air, and hopefully not the hot dash. If your sensor is on the dash, you will have to move it, or cut some access for air to flow through there for it. Several small drill holes, or a couple of slots?

Also some info about your car, madel year etc. Might help those more familiar than I, with your specific car

Car details
 

The car is a 1983 300D Turbo with 201k+ miles. I bought it new in 1983 and it's been well cared for but now needs a paint job and a new dash. The AC has always been confused by the hot sun directly on the sensor. Most people complain of being too cold.

New dash is too expensive so I plan to do a hard plastic cover. That would cover the old sensor hole and I could relocate to a shady spot like just above the switch panel for antenna, roof, etc.

Thanks for the help. Is the fan prone to failure? Is the measureing end part of the CCU or wired to it?

You can move the sensor but why. The sensor is about 2inches deep and 3/4 inches round. It has worked fine in the Texas summer for me. Besides in Texas your going to have the temp sensor set at full cold with the fan at full high. So it's not going to "regulate" anything . A combination of the Ac in the car and the Texas heat is a little to much for the system. So during the summer its full AC.
The other models had a small fan associated with the sensor. But the assembly was larger.
The tube attached to your sensor goes from the sensor to the blower motor box

http://www.peachparts.com/Wikka/W123Evaporator

Picture #64 shows the tube.

Dave

Thanks Dave, That was the picture I needed. It is reversed right? The tube goes from the dash center to the left as shown or to the right?
I've actually had the opposite problem. Mine has always been hard to regulate the temperature even when new in 1983. I've never used the max settings. Set at about 22 C the AC will quickly send you looking for a sweater or turning the dial to a higher number and then quickly too hot. Maybe I've always had a bad CCU unit even when new? I had convinced myself that the unit was getting the temperature of the dash surface rather than the ambient air. Wrong?

Since I plan to cover the badly cracked dash with a hard plastic cover I had thought it would also look better to elimate that hole and find a shady spot. That was confirmed when the newer models had the sensor overhead instead of on the dash. One of the plastic cover vendors had recommended just leaving the sensor where it is and covering it over. That would mean even less air flow.

Other links have convinced me to pull and resolder the CCU. Maybe I should do that first and see how the AC responds. I don't want to lose the ability to get cold fast like I can now and that's why I've stuck to R12 and smiled when others conplain of being too cold when its 110 outside.

Looks like we are close neighbors. I'm in Denton.
Charles

Perhaps a spider built a home in the tube, or something else is restricting flow?

Take the sensor out and test it. It should give a certain Ohm resistance based on the ambient air temperature. You can move the sensor from outside to inside to see if the unit is working correctly based on varying temperatures.
If you do chose to move it you could place it under the passenger knee bolster. That is the location of the tube connection to the blower motor. a flexible hose could be used and mount the unit in a hole you will make in the under dash panel. The air would still be drawn over the sensor via the tubes you would connect to the current blower motor setup. Drop the passengers under dash panel and figure out where you would like to mount it. To access the sensor unit remove the glove box interior and pull the blower motor tube as well as the 2 connecting wires. then the unit is pulled out of the dash towards the rear view mirror

Yes the picture shows the windshield side at the top and the black tube moves from the center of the dash to the passenger A pilar area.

Dave

Thanks,
When I get the project started, I'll do the ohm test on the sensor. Don't know what numbers to look for but I guess the point is the resistance should change with temperature. If it's doesn't, the sensor is bad and it might have been that way since new. I had the climate control checked a couple of times under warranty in 1983 and they never found a problem.

Data
 

Hello there,

here you are data for the overhead temp sensor for a W124. I think the thermistor is more or less the same for your model.

Ambient temp (C) kOhm
.................................................................

+10 / 18.3-21.5
+15 / 15.2-17.2
+20 / 11.5-13.5
+25 / 9.5-10.5
+30 / 7.5- 8.5

For A/C to operate correctly (on a 124) there is also an outside temp sensor (under wiper) and an evaporator temp sensor (in the evaporator box) and a heat exchanger sensor (in the same box).

There is no fan on a W123.

Thanks for the information. Even if the 123 numbers are different, I should probably see similar variations based on temperature. I think I'll probably need to pull the dash to do this right and I might as well do some of the modifications addressed in other threads:
1. Block open the center vent position,
2. Increase the max AC mix from 80% recirculate to 100%
3. Anything else while I'm there?

Charles

it doesn't have the little aspirator fan, but the other end of the hose attaches to the main fan housing on the back side and allows for the same effect

Not hardly.