No heat from floor vents?

[bnaquin]

I have zero heat coming from the floor vents or back seat vent in my '82 300SD. Any ideas on the cause?

[PagodaLover]

Use the search function, it will yield a wealth of information that will either be a little discouraging if the problem is within the dash and expensive climate units, or if you are simply not using the correct setting try another button, there is one button that doesn't send heat to the footwells, in my memory.

[MarkM]

I may be way off base here, but I would assume that the 82 SD is not all that different than the 123's with automatic climate control. If so, your most likely culprit is the vacuum pod that controls the flap that opens to allow hot air to the footwell ducts. If the rubber diapham is leaking air, the flap does not open ("closed" is the default position under the spring tension in the vac. pod). Locate the pod, pull off the vac. line, and use a vac. tester (mity vac) to pull a vacuum and see if it holds.

The next most likely culprit is one of the electronic vacuum valves behind the push button unit in the dash. To test this, attach a vac. gauge to the line you removed from the pod, then, while the car is running and in economy AC mode, rotate the temp. wheel up and down to see if you see vacuum at high temp. and no vacuum at low temp. If you see vacuum here when you rotate the wheel to hi temp. then you verify that the vacuum pod is bad. If you dont see vacuum here, you need to test the electronic vacuum valve behind the pushbutton unit. You pull out the pushbutton unit and you will see a series of these valves. I believe the one on the left end is the one that controls foot well vac. pod. While the car is running and in economy climate control mode rotate the temp. wheel up and down to see if this switch clicks on and off. If it clicks on and off, then you know its getting voltage from the electronic controller (pushbutton unit, wheel), but the internals must not be working as evidenced by the failure to see vacuum at the pod. So you need to replace the vac. valve. If the vac. valve doesn't click when you rotate the temp. wheel, then you want to see if the fault is with the valve, or with the control unit (wheel, pushbuttons). To do so, pull off the elect. connecter to the vac. valve and apply 12V (run leads directly from the battery). If the valve works, then you know the fault is with the pushbutton unit. If not, you know the valve must be replaced. Alternately, you can use a volt meter to test if you get 12 volts at the connecter to the valve. If you do, you know the push button unit is good; if not, you probably need to preplace the pushbutton unit.

I think it is most likely that the vacuum pod is bad...this is the simplest fix. However, in my own experience since owning two 300Ds (123) since 1997, I have had a pushbutton unit go bad and have had to replace a couple of electronic vacuum valves on my first car. On my currrent car, I recently had no heat to the foot wells and fixed it by replacing the vac. pod....everything else was ok.

It is possible that the specifics of your SD are very different from the 123s, but the general operation is very likely similar and I hope the above info. is helpful.

Good Luck

Mark

[Jim H]

Originally Posted by bnaquin

I have zero heat coming from ...back seat vent...

Rear console vents are outside or cooled air only, never heated air. That much is 'normal' on your car.

As others have said, check for vacuum pod leak, or disconnected linkage.

[Fisherman]

The vacuum valves are easy to get to (relatively speaking) on the SD. I have posted about htois a few times before. The vacuum valves are down the passenger side of the center console, you do not need to get behind the acc controller for these tests.
That being said however the pod for the foot vents is located directly behind the radio and is a real pain to get to. You can test the pod from teh vacuum valve connection though which is much simpler than getting to the pod itself. If you can't find any of the previous posts let me know and I'll try to find them. For my .02 I think it is a bad vacuum valve. I have found that these may respond to you jumping them at 12 volts but that the controller does not necessarily put out enough power to switch the valve. That being said, this is about as far as my little brain has gotten me at this time. One of these days I'll put some more power into thought and try to figure out exactly what the spec should be etc but in the meantime I have found some interesting ways to change controller leads to pods and rearrange vacuum lines so that everything works "pretty much" as it should...