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1980 300SD 116 Interesting Center Air Flap Question

This car has auto climate control and the center dash air outlets recently started blowing air during heat mode. In heat mode in either Auto Hi or Low, the air flap for the center dash should be closed.

According to the service manual, there is only a single white vacuum line that goes directly from the flap actuator to the climate servo. Under vacuum, the flap is supposed to allow air out, with no vacuum, it closes. What appears to be happening, is the servo applies a full vacuum, about 20 inchs when in cooling mode, as it should. When it switches to heating mode, the pressure drops to 2.5 inchs, as if the servo isn't fully releasing the vacuum, so the flap never closes, which is the problem. If I manually vent the line, it does close fully.

Now, here's the interesting part. From the service manual, it would appear the servo provides or removes the vacuum and the line goes straight to the actuator. However, if I disconnect the line from the servo that goes to the actuator and connect a vacuum gauge to the end of the line, somehow, full vacuum is being developed on the end of the disconnected line.

So, my question is, does anyone know if the center air flap actuator only has a single vacuum line connected as the manual indicates? Is a second line connected to it? Or is something else not shown in the vacuum diagrams in between the servo and the actuator? I traced the white line back, and it disappears headed into the upper dash, in the area where the actuator is located.

For the life of me, I can't figure out how vacuum is appearing on this disconnected line. The problem is either the vacuum is slightly too much for the servo, or the servo has too much air resistance to completely relieve the vacuum. I tried putting in a T fitting in the line and introducing a pin hole leak, and voila! it works perfectly again. Yet, I'm left wondering what is really going on, but don't want to pull the whole dash to find out.

Thanks for any help!

Reply to
Chet Hayes
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I congratulate you on following the convolutions of this crazy climate control system. I've owned one of these '80 SDs since new and have learned that the "amplifier" is more often than not the cause of strange and erratic behavior.

In this instance it appears your servo may not be controlling the vacuum quite as it should.

Look at it this way - summer is on the way and soon you'll be using the center vents full time!

Reply to
T.G. Lambach

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Reply to
Richard Sexton

Thanks for the compliment. This is an amazingly complex contraption and I'll probably never understand it completely. I agree, that it's very possible that the servo is not fully releasing the vacuum. However, the servo is doing everything else it is supposed to do perfectly, so it's kind of a strange failure mode.

But the mystery is where is the vaccum coming from? According to the manual, only a single vacuum line runs between the center actuator and the servo and that would seem to make sense. Yet, when disconnected at the servo, the line still generates a full vacuum. Fron the servo, I've traced the vacuum line back with good visibility and the line isn't connected to anything else, at least as far as I can see it, which is where it disappears into the upper dash, maybe a foot away from the actuator and it's headed straight towards the area the actuator is in. My thought is that it must be connected to something else, or the actuator has another line connected, etc, and that could also be the problem.

Reply to
Chet Hayes

I looked at my chassis manual and agree that its schematic vacuum diagram shows a direct connection between the servo and the center vent actuator.

So, that leaves only a leaking servo, sorry to say.

Reply to
T.G. Lambach

Thanks for the response, but I think you're missing my central point.

I DISCONNECTED the white line at the servo, which, as you've seen from the diagrams, runs straight to the actuator. I connected a vacuum gauge to the end of the DISCONNECTED line and it consistently generates 18 inchs of vacuum. How is this possible, if the other end of the line goes straight to the actuator? Where is the vacuum coming from? There should be no vacuum. This means something must not be as shown in the diagrams and causes me to suspect that it may be something other than the servo which is causing the problem.

Has anyone seen what the center flap actuator looks like? Does it by chance have another vacuum connection on it? I was hoping to figure out what else is going on here before I have to either get a new servo or pull the whole dash out to see the actuator.

Reply to
Chet Hayes

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