Im baking like a pizza in an Oven
 

I think it could be the ACC, but I've swapped it with a known good, and the problem is still there. I have run out of ideas on this one. Here's what I know.

Heat is uncontrollable, is extremely hot, temp is almost unbearable coming out of the vents. Voltage going to the monovalve is 13v, but the ACC will not change the voltage. Pulled the ACC, rotated the wheel, and get 1.5k of resistance top to bottom in minimum, varying to 8k in maximum. Same measurements on pins 1 and 2 off of the board on the left, where it plugs into the harness. I can hear relays clicking when I go to min and max when it's energized. Blower fan works on high and low, auto doesn't seem to matter a whole lot.
Does it sound like I need a new ACC? I have one torn apart on the bench, and there are no burnt traces or anything.
TIA

The monovalve is always on or off. System voltage or no voltage.

Hope that helps some.

In car temperature sensor......
 

Have you checked your "In car temperature sensor" ?

If the Klima Control thinks it's too cold in your car.....all you will get is heat...

Temperature Sensor Values.....Resistance
+15 C..............15.7 +- 0.5 k ohms
+25 C..............10.0 +- 0.5 k ohms
+35 C...............6.0 +- 0.5 k ohms
+60 C...............2.5 +- 0.5 k ohms

from the test values.....it looks as if you have an open circuit, or high resistance in an electrical contact/connection....your car will always heat.....regardless......

SB

That does help. I thought I knew how it operated, looks like I was wrong. I was under the impression that the amount the monovalve would open or close was directly tied to what temp you selected. Then when you go to min or max, the relays would close or open and max or zero the voltage.

Help me understand then, what does the temp dial do? Does it control the amount of cold air that gets mixed with the hot air from the heater core??
Thanks!!!

SB: I have not checked the tempomat sensor yet. thanks for the measurements.

The ACC will not change the voltage because it's desperately trying to close the monovalve because the driver is roasting.

The monovalve, however, is not cooperating.

So, pull the monovalve and look for obvious signs of binding of the valve or damage to the valve. If you can't find anything obvious, then the solenoid is probably toast and you must replace the entire valve.

I doubt the problem is a bad diaphragm as evidenced by the presence of maximum heat.

The one other possibility is the plug. You may measure 13V with your meter, but, if the sockets have corrosion, the valve is not getting the 13V. Try putting 12V and ground directly to the valve with jumper wires and see if you can get the valve to close.

Brian's comments re monovalve
 

Brian

Thank you for these comments. One more question for you.

I have the same problem with my 83 300sd and have replaced the monovalve insert and the cabin temperature sensor with no result. In my case I get swings of very hot air with periodic blasts of hot air out of the dash vents and no hot air at any time out of the foot well vents.

makes no difference if the CCU is in AC or ECONOMY setting. Temp still randomly roasts occupants. At speed will get sometimes get hot air out of dash vents even with CCU switched off.

Is this more likely to be a monovalve, CCU or a vacuum problem or can't you tell without further diagnosis?

Check the foam tube that directs the car's cabin air temp to the sensor.

IIRC Shorebilly did a post regarding replacing a dry rotted out tube.

Does the SD have the foam tube? I never figured out where it's located. I was of the opinion that the sensor was right behind the small grille........???

With the CCU funtioning properly, you can never get hot air out of the dash vents........no matter what the setting. So, with that tidbit, the CCU is definitely the culprit.

But, it may not be the only culprit. With no air out of the footwells, the problem could be the CCU, but is more likely to be a vacuum switchover valve , the vacuum pod, or the linkage from the pod to the footwells. Right now, the SD has no driver's footwell air.......but it does have passenger footwell air........go figure.......

w126 no foam tube!
 

the 300sd has a rubber or silicon plastic tube of some type from the temp sensor. It's a different set up from the w123 chassis.

In my case the tube is in fine shape in the roof space. Don't know where the other end is (glove box?).

Yep, that makes sense. Nobody usually has issues on an SD. The tube goes back to the blower motor housing in some fashion so that the blower can draw a bit of air across the sensor. My memory was failing me for a moment.

I have replaced the foam tube on mine, so that shouldn't be it. Brian, I'll jumper 12v directly and see if I can feel anything. This is the third complete monovalve, but you never know. Thanks again.

BTW, I"m glad nobody caught or noticed I said tempomat sensor,, which would have nothing to do with the heat. I thought about it all the way home, lol. I meant temp sensor.

The 12V signal to the valve must close it. You'll hear a "bump" inside the valve if it's functioning.

Three complete monovalves..........doesn't bode well for the "bad monovalve" theory.........:o

I'm Broke,

Any update? Am experiencing same issues as you.

My SD has the same no heat driver, heat in passenger footwell. So far can live with it. Will dig at it later.
But, it may not be the only culprit. With no air out of the footwells, the problem could be the CCU, but is more likely to be a vacuum switchover valve , the vacuum pod, or the linkage from the pod to the footwells. Right now, the SD has no driver's footwell air.......but it does have passenger footwell air........go figure.......[/quote]