I recently bought a 85 300 TD with 128k miles. AC was working fine, but recently the AC air that should be coming through the center vents is coming through the defroster vents. Also, on cool mornings the heat only seems to blow warm when the car is stationary. The vents seem to move to their proper position in the heat mode.

I found an old thread that discusses replacing a vacuum control valve by accessing it via the glove box, but I wondered if the additional problem of the heat blowing cold while moving could indicate a different issue?

Also, in the above mentioned thread, someone referred to propping the flap open allowing the air to enter via the center vents, but did not elaborate on how to do it.

My questions are:

1. Has anyone encountered a similar issue?
2. What was the fix?
3. How do I prop open the flap to get some AC for the center vents in the hot Tucson weather until I can fix it properly?

Thanks,
Dave Newsome

First, check your coolant level. If you don't have enough coolant, you won't have enough heat.

Second, screwball AC temps can be attributed to the monovalve. It's in a box mounted on the engine firewall, passenger side. Four screws and it's easily replaced - I think it's $60 or so? The seal around the valve deteriorates and it makes the ac temps screwy.

Lastly, remove the center vents (can't remember how, but it was easy), then take the glovebox out and reach around to the center vents. Pull back the rubber sheathing to expose the problem. Through the center vents, you'll see the flap and an arm that control the flap via vacuum. In one of my old w123s, the arm had simply become detached from the flap - one zip tie later and presto it worked. If you find that the arm is still connected to the flap, then the vacuum pod may be dead. You can prop that flap open permanently in any way you see fit. That'll get you through the summer.

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- Brian


1989 500SEL Euro
1966 250SE Cabriolet
1958 BMW Isetta 600

Remember that the center vent does not get used for heat, only for cooling.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I've heard you can reroute a couple of vac lines behind CCU area, and the center flaps will stay open even with heat. You'll need to pull said unit to gain access, good luck.

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83 SD

84 CD

Thanks for the advice. I enede up taking it to a shop. They diagnosed the problem as the vacuum pod, said it would be a $500 fix, and told them to forget it. (After $95 diagnostic fee) Feeling screwed!

WHAT?!? You can buy a used engine for that price...
There's DIY threads detailing fixing the vacuum pods.
Spend a day and replace all of them behind the dash for $100 or less.

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