Problem: My 1980 300SD's heater does not work properly. The Defrost works but the Lo-Hi & Bi levels do not. Some times the temp is around 70 degrees comming out of the L-h-B levels but some times it is just cold air. Sometime the Defrost doesn't even work until I give the Aux. water pump a few taps. I know that I have a minor vacumme leak somewhere because my door locks won't work (unlock) after it sits for a while. However. I can hear the doors etc operating when I Hit the different L-H-B buttons and it changes where the air comes out and the fan speeds are correct.
I would appreciate any suggestions.
Thanks
Slim

1980 300SD 210,000
1995 C220 185,000
1991 525I 187,000
1985 GTI 160,000

the climate control servo on this model is known to be problematic.the bottom half of the unit contains the hot water valve.the top half the fan speed and flap control.sounds like your aux water pump is bad as well.i would just bypass the coolant hose past the pump and servo until you can locate a replacement servo

hello-if you need a servo, also replace the amplifier. they tend to crap out when the servo starts drawing too many amps. good luck, chuck.

What is the procedure for checking the servo and or the amp?
Slim

a special tool or a good wiring diagram and a 6 volt battery. the electrical connection at the servo has 1 pin that moves the servo from ac to heat and it moves at 4v, i think. apply 12 and the thing fries. the amp drops the voltage from 12 to 4. most times, one can hear the servo trying to park after the car is shut off. you need 2 people, 1 to run the key, the other to listen/feel the servo while it's shutting down. but, if it's frozen, then it won't move and therefore, you get stuck, ac or heat.

run the car 'till it's warm. hopefully the system will work correctly for this test. try the heat, then ac. IF it changes, open the hood, put your hand on the servo. shut the car off while holding the servo. after the key is off, the servo should park about 1 minute or so later. you may hear it, also. it'll whine for about 5-10 seconds. it you hear it park, things are ok now. the next time the ac\heat won't change, do the same thing. if it parks, look elsewhere for your problem. if it doesn't park, change the servo and the amp.

sometimes, when the servo's lock, they also break the outer case and leak a little coolant. good luck, chuck.

Thanks Chuck
I had to replace the servo about 7 years ago and it did have a hairline crack on the bottom of it, no leakage though. And come to think of it I have not heard the sound of it parking lately. Its my wifes car and I had forgotten about that sound because I don't drive it that often. I my just need to by pass the servo's hot water line value and put a manual valve in the line and turn it on in the winter and off in the summer. If I replace the servo and the amp I would be looking at $500+- for the parts and I am not a big fan of climate control anyway for just this reason.

I've had the same problem for several years. I have also hand a friend of mine, a trained Mercedes mechanic look at it and work on it with no positive results. Is it possible that the heater core is plugged.
Did you ever get yours fixed?
Rev Wallace Schulz
ST. Louis, Mo

Open the hood while the car is running. When you shut it off walk to the servo and you will definitely hear it. It runs fro about 25 seconds or so. I bought a used one and that fixed my problem. You can tell if teh aux. pump is working by looking at the top and seeing whether or not the brass piece is turning. It's hard sometimes by looking so just stick your finger on the top of it and push down and you should feel it turning. Sometimes I could disconnect the power cords to the servo and plug them back in and it would work. I took my old one apart and am still mystified as to how the thing works. You can see somewhat but nothing will just stick out as to why it's not working.
You need to feel the hoses coming out of the servo to see iof their hot. If they're cold water's not getting out of the servo obviously. If it's hot then check the hose coming from the core and see if it's hot and that would tell you as to whether or not the core's plugged.
My mechanic told me also that sometimes the gears inside get old and get gummed up. I belive there are 3 holes half-way down that you need to spray WD-40 into to try and free them up. I did that and it worked for awhile. After it quit again I opened it and it didn't seem like the gears were messed up.
If you do try to take the thing apart pay close attention to how it all came apart. Even then, there is a good chance you will end up with a couple extra pieces that you won't think were needed anyways in the beginning.
If you determine that you need a servo, e-mail Jim Harris here and I think he has some w116 parts cars.
-David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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I forgot, it could also be the push-button unit inside the cabin. I bet that piece is pretty pricey new, but try Jim if you need any good used parts.
You could try to resolder the connections inside the amplifier behind the glovebox I believe. I've heard it's a pretty simple job since it's such old technology regarding the solder.
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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DAve:

Thanks for the info. I will try to get back to you. This heater ephisode has been one of the most troubling in my life. God bless you for the words of help. Rev Wallace Schulz

Reverend, do you ever get any heat at all? Does just defrost work? Does it not hold temp like it is suppose to? Does the blower motor work?
Thanks
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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Yes, I do get some heat, but minimal. And, it take a long time in cold weather to get any significant heat. I have been thinking of taking off the radiator and get it professionally flushed out. Does that make sence? I did pick up some acid additive for the radiator but I am not sure whether this would be as good as getting a radiator specialist to flush it. Since I plan to drive the 300SD for some years, I do want to fix this problem in the correct way. Rev Schulz

Wallace if you unplug the amplifier behind the glovebox and then turn on the defrost and heat comes out (hot) then the heater core is clear. I don't know why but when the amplifier is disconnected the defrost comes on full blast.
I have been considering puting in a manual heater valve in place of the servo's lower workings. (I believe that a 74 Ford Pick Up with a 460ci eng. has a heater valve that is the correct dia.) You would have to turn it on and off manually or you could run a cable into the car and operate it from inside. (Like a choke cable)