climate control only producing heat
 

When I got my car it had no servo, so I got a used one that looked clean, installed it and got the climate control blowing again. The strange thing is that the first day I installed it, the temp dial worked -- I could turn it all the way down and get relatively cool air, center vents started blowing. The A/C even worked. Ever since that one day, I only get heat and it is really hot. Turning down the temp dial seems to make it just a little less hot. It hasn't been a problem since it's been cold out but I'm preparing for summer.

I just noticed that the tube going from under the glove compartment to the temp sensor in the speaker grill was gone, so I replaced it with pipe insulation, but that didn't seem to change anything. Anyone have an idea how I should go about looking into this?

Hmmm
 

Sounds like the climate control amplifier may have burnt out.
MB# 0008350057


The servo may be burnt out also.
MB# 0008300684
http://www.peachparts.com/shopforum/mercedes-parts-reference-library/187807-acc-servo-unit-%3D-heater-control-servo.html

I replace the heat box Aux. Water Pump Control Switch at the same time.

heat box Aux. Water Pump Control Switch
MB# 0028211851


You need to test the amp draw on the AUX water pump, before installing the new parts, replace it when draw is over one AMP.

These threads may help you.
300d part ID
http://www.peachparts.com/shopforum/diesel-discussion/207690-300d-part-id.html#post1699979

The evil servo is not so evil, but Norm is still A/C less!
http://www.peachparts.com/shopforum/diesel-discussion/240530-evil-servo-not-so-evil-but-norm-still-ac-less.html#post2052747

Aux. Water Pump takes out CCU
http://www.peachparts.com/shopforum/diesel-discussion/140870-aux-water-pump-takes-out-ccu.html#post1051832

Here is a reliable modern replacement unit, offered by AllPartsExpress.
ACC servo unit = Heater Control Servo
http://www.peachparts.com/shopforum/showthread.php?t=187807



Have a great day.

So I checked a few things out. I looked at the servo amp behind the glove compartment and did not see anything visibly burnt. I checked out my aux pump and it is working.

To check out the CCU, do I just take the control panel out of the dash and visibly inspect?

Strange thing happened the other day. I had been messing around under the hood and hadn't realized that I had pulled one of the vacuum lines loose and had also cut power to the aux pump.

Next time I drove it, I was getting cool air. When I turned off the car and it kept running, I realized something was wrong. So I hooked everything back up and have had only hot air since. My guess is that it had something to do with the aux pump?

Have you taken a look at your mono-valve?

Good idea. The aux pump and the monovalve are usually on the same circuit. Both of them share a hot lead that has power any time the car is "ON." The aux pump has a normal ground (brown wire to chassis somewhere) but the monovalve's ground is sent through the CCU where it is modulated as needed to turn the flow of hot coolant off. If that hot lead is interrupted, the monovalve stays wide open all the time so you get nothing but heat out of the vents.

I just started looking through old posts to find some info on monovalves and someone said that they didn't have monovalves until 81 or 82. Is this correct? It does sound like that would be my problem.

A little more info: When I said I had been "messing around under the hood," what I had been doing was hooking up a wire from the aux pump over to the light/radio fuse in order to wire the aux pump on when the car is on. I had not grounded that line properly, so the radio and the aux pump were not on, but I'm not sure how this would affect things.

Answer
 

That is correct.

so if there's no monovalve, is there nothing really controlling the flow to the heater core? could i bypass it? put some stop valves in and out of the firewall to turn off when i don't want heat?

No, your car has the chrysler system which has the valve on the passenger fender.

-J

Answer
 

Go back and read post# 2.

The servo controls flow.

so i guess my servo isn't working in that it isn't stopping flow to the heater core when it should? rather than buying the expensive replacement servo that I can't afford, could bypassing flow to the heater core witha stop valve during summer possible work?

Yes
 

Several local members have installed ball valves as a cheap alternative.