No "Gradient" Temp w/Climate Control on '82 300D - PeachParts Mercedes-Benz Forum

cewyattjr · #1 05-18-2007, 09:07 PM

The symptoms are really consistent -- there is no "gradient" between the hot/cold extremes. If I turn the temp wheel to hot, I get hot. If I spin it back to the "click" on the cold setting, it will go back to the regular non-hot temp air. But nothing at all in between.

I'm not really expecting that it will cool, because I'm pretty sure that the freon is long gone.

I guess I'm kind of hoping that just replacing the whole unit would fix it. I've heard of something called "the dreaded ACC problem" and wonder if these symptoms fit?

Thanks folks,

Chuck

Brian Carlton · #2 05-18-2007, 09:16 PM

The problem is either the CCU or a faulty cabin temperature sensor.........or a faulty tube that provides the air to measure the temperature. Sometimes the tubes fall apart and the vehicle doesn't know it's own cabin temperature.

carnut · #3 05-18-2007, 09:38 PM

The primary cause is probably lack of freon. The climate control system has to mix refrigerated air with heated air to arrive at the setting on the rotating temperature dial. Yes, the tube or pipe going from the in car temp sensor has to be there and connected at either end, but without refrigerated air to measure, I would think the system would be erratic at best. You might leave it in econ mode to see if theres an improvement in comfort. Describe "nothing at all in between?" do you mean the air stays extremely hot or it goes to cool. This year of car is famous for the MONO valve diaphram tearing which will make the air temperature erratic. This valve is located on the firewall just to the right of the battery facing the engine. Remove the four screws holding its lid on (remove radiator cap first!) engine must be cold! and lift the internals up and out. Tug on the black rubber diaphram around its diameter, you will most likely find it torn.

Brian Carlton · #4 05-18-2007, 10:40 PM

Quote:

Originally Posted by carnut

The primary cause is probably lack of freon. The climate control system has to mix refrigerated air with heated air to arrive at the setting on the rotating temperature dial.

This is flatly incorrect.

The system does not mix any air whatsoever. In economy mode, it regulates flow from the monovalve and blower speed to maintain cabin temperature in colder weather. It regulates blower speed with outside airflow to maintain cabin temperature in hotter weather.

With the a/c selected, it regulates blower speed to maintain cabin temperature in hotter weather. The monovalve remains closed.

toomany MBZ · #5 05-19-2007, 02:20 AM

When in anywhere in between, does the fan speed change? Let it run for a while before concluding. Can you hear a change when turning temp wheel to extremes? The AC does not work in the SD, belt wrapped itself around pulley when I engaged. The AC in CD [do-da-do-da] does work, fan speeds change in both. The CCU is famous for failing, but check the foam tube first.

cewyattjr · #6 05-19-2007, 12:11 PM

The fan speed doesn't change at all when I rotate the temp wheel. What happens is that if I'm too cool, I rotate the temp control until the "click" on the hot side. Only then will it start blowing hotter. And it stays hot until rotated back to the far end of the "cool" spectrum... If I click it there, it will go back to the cooler air.

I'm running in Econ mode constantly, because I know that there isn't enough freon in it to get any kind of compressed cool.

Thanks for the tips on the MONO valve, I can check that out.

-Chuck

carnut · #7 05-19-2007, 01:59 PM

Well, I guess I stand corrected! I no longer own my 300CD or manuals to verify no blending of air.

toomany MBZ · #8 05-19-2007, 05:53 PM

Quote:

Originally Posted by cewyattjr

The fan speed doesn't change at all when I rotate the temp wheel. What happens is that if I'm too cool, I rotate the temp control until the "click" on the hot side. Only then will it start blowing hotter. And it stays hot until rotated back to the far end of the "cool" spectrum... If I click it there, it will go back to the cooler air.

I'm running in Econ mode constantly, because I know that there isn't enough freon in it to get any kind of compressed cool.

Thanks for the tips on the MONO valve, I can check that out.

-Chuck

You'll have to leave the temp wheel in place for a little while, it'll build up to high speed, then when it gets to the selected temp, it should slow fan speed. Sounds like the temp sensor is doing it's job. Econ will not let the AC come on. I have a used mono valve, that I took out of the basket case, pretty new, you can have for a song. Doesn't sound like your problem tho. I would go with an AC "refreshing". When checking mono valve, do so on a cold engine...unless you'd like a trip to the burn unit.

Richard Wooldridge · #9 06-02-2007, 02:14 AM

Be aware that your monovalve will not work properly unless your auxiliary water pump is running. The monovalve requires a certain amount of water pressure to operate. The solenoid valve controls a small bleed that uses the water pressure to move the main valve, and if the aux. water pump is bad, the monovalve operation will be erratic, especially at lower engine rpms. I replaced the (good) diaghram in my monovalve twice before discovering this fact. Now I have replaced both the monovalve and auxiliary water pump with a vacuum controlled heater valve ($17.00 from NAPA, 1974 Ford) and a switchover valve ($38.00 from Performance Products, 1990 Mercedes 300D). This setup works at all engine speeds, cuts the heater water flow completely off at all speeds, and gives excellent modulation all through the temp wheel range. It also looks very much like the original setup.

Brian Carlton · #10 06-02-2007, 02:21 AM

Quote:

Originally Posted by Richard Wooldridge

Now I have replaced both the monovalve and auxiliary water pump with a vacuum controlled heater valve ($17.00 from NAPA, 1974 Ford) and a switchover valve ($38.00 from Performance Products, 1990 Mercedes 300D). This setup works at all engine speeds, cuts the heater water flow completely off at all speeds, and gives excellent modulation all through the temp wheel range. It also looks very much like the original setup.

Richard, this sounds very interesting.

Can you expound on it a bit. Details of the operation and the required connections.........including how the CCU ground signal operates these valves...........would be invaluable.

PatricdeBoer · #11 06-02-2007, 02:33 AM

I had the same problem with my 300sd. There is a temperature sensor on the dash of my sd, but it is up on the roof of most. There is a tube that leads down to the blower, so whenever the blower is on it is drawing air across the sensor. The usual problem with this is that under the dash, the tube is made of some type of foam. It is usually completely disentigrated. I used a peice of garden hose (classy, I know) to replace the foam tube. Now I have precise adjustability of cabin temperature.

Richard Wooldridge · #12 06-02-2007, 02:35 AM

Brian- Sure, I'll be happy to. I will make up a little vacuum diagram and take some pictures of what I have along with the part numbers of the stuff involved tomorrow, and post it all. I have a feeling that lots of auxiliary water pump failures are being attributed to the Monovalve in error. My setup eliminates the problem altogether. I got the idea from the Unwired Tools digital servo kit that I installed in my '79 Mercedes 450SL, they use the same setup to control the heater water flow, getting the control signal from the electronics they provide. Basically, the signal that feeds the monovalve is just wired to the 12v solenoid on the vacuum switchover valve, and vacuum is applied to the heater valve through the solenoid. I found it necessary to provide a "leak" to the heater valve side so the valve could return to the open position, and did this by means of a three way connector with a piece of wire stuck in the third hole, ala the original ACII setup in my '79. (I actually installed this in my '82 380SL.) As I said, I'll come up with some pix and a diagram which should make it all very clear.