My AC doesn't blow through the two center vents. Instead it comes through the defrost and side vents. Given that my car takes longer to shut off when the AC is on I figure I've got a vacuum leak somewhere in the system. I've manually pulled vacuum down in the car before with the AC off and it held pretty well so I think the problem is related to that system. Does anyone have any good tips trouble shooting this?

Thanks,

Frank

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'82 300SD

What you may have is a low refrigerant pressure that isn't opening the pressure switch anymore. Is your compressor actually running? I have the same problem, and when the pressure switch is bypassed, the compressor kicks on, and I also get air through the center vents, so it may not be a vacuum problem after all.

But then, the AC in your '79 300D may be quite different from my '82 300SD system...

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Scott Longston
Northern California Wine Country...
"Turbos whistle, grapes wine..."

It is cold and the compressor is working. It's been converted to R134A and actually had virtually the entire system replaced about 2 years ago to the tune of like 1500+ dollars. (The car was my dad's then) Ever since it was put back together I believe it's been this way.

As for the low refrigerant, I'm going to check that. Even though I know the new coolant shouldn't cool as effectively as freon, it seems to me it takes a while to cool down especially when in traffic. It eventually gets the car cooled down but it takes a while. I figure blowing the air across a hot windshield through the defrost vents isn't helping much. Aside from pumping more R134 in it, is there another way to check that valve?

Also, if I don't have a vacuum leak, why would my engine take longer to shut off when the AC is on? The engine shut off mechanism is vacuum controlled, the compressor should only encourage the engine to spin more slowly.

Thanks for your prompt response!

Frank.

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'82 300SD

Try testing the defrost damper actuator and the center vent damper actuator. At least on the 300D these two are tied together. The defrost damper act. has to shut while the center vent act. opens. Sounds like a vacuum leak in this particular system.

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Jim

If the compressor is working, then my suggestion is moot. And everything you describe (except the center vents) would be typical of a R134a retrofit blowing cold air on a hot windshield. I wouldn't add any R134a unless you have a set of guages, find that the system pressure is low, and are familiar with the correct pressures...

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Scott Longston
Northern California Wine Country...
"Turbos whistle, grapes wine..."

That makes sense. Now I don't suppose there is an easy way of testing this? I'm figuring I'll have to pull the dash apart but if I can get to it from under the dash, through the glove box, etc.. that would be nice. Otherwise I have vacuum gauges and a vacuum pump so I can leak down test the actuator when I find it.

I do also have the gauges for checking the R134 level. I'll check that maybe this weekend.

Thanks for your help.

Frank.

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'82 300SD

there are any differences between your '79 and my '85 but on the '85 you remove the cover underneath the steering wheel and look to the right of the gas (fuel?) pedal. The one closet to the floor is for the floor vents, the one closest to the windshield is for the defrost vent.
Underneath the pass. side cover and behind the glove box are the recir/outside damper actuator (the one closest to the side of the car and windshield) and the "main" damper actuator (which is staring you in the face when you remove the glovebox). The actuator for the center vents is behind the center vents under the center of the dash. I think the dash cover has to come off to replace it. You CAN see it if you look real hard from the pass. side feet area.

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Jim

vacumn line to center vents diaphragm is just left of glove box opening-should be red with black stripe on most 300d s .

I have the exact same problem in that I get no air through my center vents on my 79 300D.

The way to get to the vacuum diaphragm that actuates the center vents is to remove the glovebox shell (small plastic fasteners secure it inside the glovebox). Once you have the grey glovebox shell removed you'll be able to see the pink vacuum diaphragm on the center console. It should have a green vacuum line to it. Remove the line and attach a hand vacuum pump to it. Apply some vacuum and see if it holds.

My vacuum diaphragm did not hold. This leads me to a question about fastlane. What is this part called? Is it called the defroster nozzle flap control?

And does anyone have some suggestions on how to replace this part? It's kind of tight in there.

Well I heard this whining noise coming from the battery. It wasn't the battery though. When I took it out, I found that like 10 out of the 11 (I think) vacuum lines had like 8-10" of the harline missing. It appears the battery/vapors has disolved the hardline. I jumped them with rubber hose and now can hear a vacuum leak in the dash somewhere. I guess I'm a little closer but none of the dash accesories work. I think in the mean time I'm going to see if I can prop the center vent door open. I can reach in there and manually make the door move and I think I can get some thing to prom against the actuator and duct housing.

Frank.

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'82 300SD

Greetings Frank,

If you remove both kick panels then most vacuum actuators will be more accessable as are the door flap areas.

Charles

I know your pain

WHen diagnosing vacuum problems all appears lost.
Your center vents dont work because of a probable vacuum leak.
1 check first of all the vacuum pump by measuring vacuum pressure that comes off the vacuum line that goes to the brake booster (if you get 15 to 25 lbs of pressure then continue)
2 seal off all of the other vacuum functions that are not related to your climate control. (hook up only the green line to the vacuum line and seal off other connections, If no improvement then continue Your problem is a vacuum leak in the climate control system)
3 disconnect the battery as you dont want to cook any circuitry. Take off the side kick plates, and the center console so you can get at the climate control unit and the vacuum checkover valves. There are five valves with five vacuum circuits that control the opening and closing of the vents. Test each circuit to see if they can hold vacuum and perform their fucnction.
4 I would vacuum check each element by attacting your vacuum tester to each vacuum element to ensure their function. The center vent is the most difficult to get at and the hardest to repair. You should also have a check valve on the third circuit (center valve) check that too.

5. That should get you started. If you are short on time I would check the vacuum elements first as you do not have to take apart the center console and is most likely the source of your leak.


good luck

bob

Ok,
1. Vacuum pump ois 25-26 lbs.

2. While I didn't do that, I did leak down test the rest of the car, like the doors etc... and replaced the check valves under the hood. The car held tight and the check valves appear to be working. However, I couldn't get anything on the climate control side so a leak is certain.

3. Good idea, haven't done that yet.

4. Good idea also. I checked a few but I need to go through it again.

Thanks, for the tips. As soon as time permits, I'll dive in there and see what I can do. I still need to find out where these two rather short pneumatic hoses, one black they other white, go to. I accidentally pulled them out of that port by the firewall when I was reconnecting all that damaged hose. I'll just do it all at once.

Thanks again,

Frank.

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'82 300SD