81 300SD W126 no floor heat

[SD300]

I got this old W126 300SD it has no floor heat,and heat seem to be on high temp all the time.

Has it got to do with vacuum pump in trunk not working?
As door locks not working right now.


Merc is the only car that hot water doesn't get circulated when idling.
It relies on an electric water pump, with my luck it doesn't work.

[piccolovic]

There are servos in the dash, and the hardest one to get to which requires dissassembly of the dash, is the one that controls the floor vents. If bypassed, which can be done without disassembly, heat will come out through the center vents at all times. Unfortunately, that is a decision I made, and in Minnesota, which is where I am now, my feet freeze while the rest of the car is like the inside of a hair dryer!!! Good luck.

Torie

[SD Blue]

The vacuum/pressure pump in the trunk is strictly for the door locks. The drivers key lock is the "switch".

Does your ACC system seem to work in any other modes besides defrost? If not, I would suspect the push-button unit. Inside are 3 circuit boards arranged in an "H" pattern and the solder joints tend to crack where they meet. Someone handy with a soldering iron can touch them up in no time at all.

If you seem to have switching between modes but just no floor heat, then it may be a vacuum pod. Do you have a mightyvac? You will find it indespensable in working on the MB vacuum systems. Replacement diaphragms are available at www.autoluxmbz.com The pod for the footwell flap is located behind and just below the pushbutton unit. It's really not bad to access on the 126, as it only requires removal of the ashtray, radio and then two screws on the underside of the wooden trim.

[SD300]

two screws on the underside of the wooden trim

The other day I un did 2 screws. But wood panel didn't want to come out,
I'll try it again sometimes.

Thanks.
The top of the car is toasty warm, feet freezes.

[300sdToronto]

I have the same problem in my 83 300sd over here in Toronto. Warm air to the proper dash vents, nada to the feet.

Can someone please confirm that the proper diagnosis steps are

1. remove wood trim, radio and ccu.

2. check floor vent flaps to see if they are jammed (can this be done another way without step #1

3. check vacuum actuator for floor vent flaps and replace if necessary

4. suspect CCU if 1, 2 and 3 check out OK


Cheers

[SD Blue]

Which vents are you describing as the proper dash vents? If you are referring to the center dash vents, then you have a center flap that is not actuating properly. This will also result in less air to the footwells. Have you tried manually shutting the center vents to see if it changes? Another actuator that will cause less air is the diverter flap. (It's vacuum pod is on the drivers side of the tranny tunnel.)

All of the actuators can be checked from the switchover valves located behind the tranny tunnel kick panel on the passenger side. Follow an imaginary line from the dash to the floor and their location is behind that panel. I believe there is only one, maybe two, screws holding the kick panel. Slide it forward to remove.

(Switchover valves top-to-bottom should be Defrost/Footwells/Center/Diverter/Fresh-Recirc.)

[300sdToronto]

Hi SD Blue

I do not know which vents I am talking about as I am just learning about the climate system.

All I can tell you is that I have warm air coming out of the side vents on the dash and a very small "trickle" of cool air coming from the centre dash vents. The sliders on all the dash vents are in the open position. In summer using AC I get cold air to the centre dash vents in large quantities - which is all good, correct?

No warm air at all to the floor.

I was under the impression from other postings that there was a separate vacuum pod which controlled the vents that allowed warm air to reach your feet... hence the question.

Thanks for posting a reply and I look forward to more comments.

[SD Blue]

From what you tell me about the dash center vents, I believe the pods that actuate center flap and diverter flap are working as they should. That's good news.

Yes there is a separate pod that actuates the footwell flaps (and air to the rear seats). It is located behind the radio and CCU pushbutton unit. I believe removal of the radio (and ashtray) is all that is needed. Check it with a mightyvac and it should be the culprit. This is one I had to repair also.

New vacuum diaphragms are available at www.autoluxmbz.com However, I don't know what the shipping requirements might be for Canada. At least you may be able to temporarily rig the flap open until you can get it repaired.

Let me know what you find.