First let me say I've had this problem since I've owned the car (will be a year this spring), I have looked into it/troubleshot it multiple times, done extensive reading on here.

1984 300SD 126

Here are my symptoms:
1-I only get heat on defrost mode.
2-Other buttons blow air, but no heat.
3-A/C has not worked since I have owned it (have not looked into this issue at all, there is a R134a sticker from 2001, but not sure if they replaced anything or just took out R12 and added R134a without replacing compressor, etc.)
4-Defrost vents always stay open.
5-A symptom that is no longer occuring that used to was the defrost would kick on regardless of any buttons depressed or not when the RPMs would get around 3000 or higher (acceleration or freeway speeds). This doen'st occur anymore. I think it stopped occuring when I unplugged the Aux water pump, but don't remember for sure.
6-I believe (although am not positive) I am getting air coming through the system at all times. When I close the center vents (for fresh air I believe) with the sliding tabs it seems the air is diverted to the defrost vents.

Here is what I have done so far.
1-Checked into the monovalve. Ordered the rebuild and installed it, no change. Since it is winter and my heat is marginal, I just keep it unplugged for full heat.
2-Took out the CCU and tried to resolder it. I fried a trace. I believe something jumped around the 9 pin and it shorted out and then did not work at all. Ordered a rebuilt one.
3-Installed rebuilt CCU. At the same time, checked Aux Water Pump and found drawing too many amps. Unplugged it for awhile, but recently took it out and put an elbow in there.
4-Footwell vents were not opening so I wedged a beer tab in the flap to keep it open permantly. (Although now that I think about it, they shouldn't have been opening on defrost, maybe they were opening on other functions, not sure)
5-Took a Mity Vac to the switchover valves in the dash. Pulled vacuum and see if it held. The first time I tried it the top one and third from the top did not hold, all others did. Went back and tried them all a second time and this time it seemed they all held vacuum. I ran out of time and stopped. So I don't know if I simply wasn't getting a good seal the first time or what. I'll check into this again tonight. I think I was doing it correctly, but started second guessing myself when the results changed.


I get heat when the car warms up and running on defrost, but I am certain it could be hotter and I really want to get it functioning like it should as I plan on getting the air up and running this summer but want to make sure the controls are functioning as they should before I get into putting new components into the a/c system.

Lately I have been reading up on the dashpods. I will need to retest them. I plan on doing that tonight or tomorrow night to see if they hold vacuum. If they don't hold I will order new pods and rebuild?? (suggestions here appreciated)

So what do you think? I know I wrote a lot but I wanted to be sure I covered as many symptoms and covered as much of what I've done as possible. Is there anything else I should look into?

Thanks in advance everyone!
-Ryan

Heater temp. sensor is OK? I know it's a cheap part, but they do go bad.
Take a look at this thread and see if it gives you some ideas.

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- an official MB shop manual is invaluable for this. There are excellent pictures of the location and schematics of all the vac pods and relays.

- the shop manual also includes instructions on how to test the heat sensors (one by the interior mirror and two in the heater plenum behind the CCU) with nothing more fancy than a simple electric test meter. They are available from the dealer or probably from Fastlane.

- where did you get the re-built CCU? The first "re-built" one I got did not work properly as it removed the old problems I was suffering but introduced others. GDL Online seem to be recommended as the best re-builders.

- there are many threads on this subject if you search here or on the other MB forums as well. It is a common problem with an SD that has lacked good maintenance.

I have gotten both rebuilt CCUs and cruise control components from GDL, they have a good product and are very helpful. BTW, they will not accept cores that have been subject to DIY "repairs" so resist the temptation to "fix" it yourself before you exchange it.

Thanks everyone.

TX,
I read the thread. I have wondered about the temp sensor before. Next time I order parts I will order it since it is cheap. Could this be why I get heat only when it is forced in default mode (defrost) and not others? Hmmm....

Toronto,
I do have the CD manual. I have used it much. In fact I looked at it to check which switchover valves were which (for the ones that at the time seemed not to hold vacuum). I have also done extensive searches on here which is how I got all the valuable information on the things I have already done. As I continued to research it seemed I could not find any problems that were similar to mine.

I will check into the manual to see if it has anything on testing the temp sensor.

Craig and Toronto,
I don't remember where I ordered my CCU. I'll have to do some checking. I know I didn't send in my old one as it is still on my shelf. But I know it works better than my old one! After I fried my old one nothing worked. I don't really want to mess around with the CCU again since I fried the first one beyond repair.


Can anyone tell me why I only get heat on defrost? Could this be because of the temp sensor? Could any of this be related to the fact my a/c doens't work?

Also, am I using my MityVac correctly to diagnose the pods? I unhooked the black connectors from the switchover valves on the passenger floor side. Then I hooked my Mity Vac up to each valve tube, pulled vacuum to see if it held. Is this correct? Anything more to it?

As I remember, the monovalve fails to the full heat position. In other words have to apply voltage to it to stop the flow of hot water.

So, disconnect the monovalve electrical connector and go for a drive. Regardless of the CCU setting you should be roasted. Simply try all CCU buttons with the fan on max.

If not, you have an interesting problem not related to temperature control.

Gotta get back to work now

Thanks for all the input and the thread Toronto!

The monovalve has been disconnected for all of winter. At least 2 months in my case, perhaps a lot longer as I don't remember specifically when I disconnected it but it has been off for awhile.

When I am up to temperature and driving, I get heat out of defrost. When I switch to the other modes and the fan on high, it starts blowing cold (I'm assuming outside, nonheated vs. a/c) air.

What can anyone tell me about the heater core as seen here?http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=27M0UEZHE29T0TUHPV&year=1984&make=MB&model=300-SD-002&category=R&part=Heater+Core

When you press the defrost button, it goes into "default" mode, high fan and high heat. So it would seem your heat is OK, but the CCU is not "telling" the system correctly to provide heat elsewhere.
Can't help with the heater core.

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83 SD

84 CD

There is no "default" involved. That is just how the defrost mode is wired.

Yup, when they are failing that seems to be the last function that still works. Mine limped along like that for a while until it died completely.

300sd's, CCU's, and the meaning of life...

My 300D heat is much more than adequate at 20F and can certainly keep me warm enough in below 0F temperatures so I don't usually wear a coat while driving.