BS911

Hi everyone. I'm trying to sort out some vacuum leak issues and I was wondering if something should be attached to the end of the elbow coming off the top of the piece shown below. This is the solenoid/actuator/thingamajig that is mounted on the heater blower housing in the engine bay just in case my picture doesn't give enough perspective. You can see the rubber elbow come up from the top and bend towards the front of the car. What you can't see is that it is an open ended elbow - no termination. Is this correct or should something attach to that.



BTW: My key problem is if in recirc mode I get good airflow to the cabin but it cuts off under accelleration. If I go from recirc to fresh, it sounds like the flap doesn't close all the way and the air almost completely cuts off.

Thanks for your help.

viperbob

Brian,

That is normal. That selenoid is designed to bleed pressure. It does look like there is supposed to be something connected, but that is not the case...

My car does the same under acceleration when the AC is running. I have tried to track it down a number of times, and can not find anything wrong. No vacuum lines disconnected that I can find. Oh well, I'll keep searching.

Good luck with getting it fixed....

BS911

Thanks for the quick answer Bob. I was afraid of that but was clinging to hope. I knew an easy fix wasn't going to be in the cards. Oh well, the hunting shall continue.

Thanks again.

Terry Adams

Your problem may lie in that Mercedes emblem on the connector plug. I thought the a/c compressor was supposed to shut off under acceleration, and could this be related to that?

BS911

Interesting thought. To expand on the shut down, it does it under load based accelleration, not just rpm. If I push on it at even around 2500 (not that I ever let my rpms dip that low ) it chokes it out. Is this perhaps a normal operation?

Out there somewhere is someone in a Mercedes asking why his fan keeps kicking ON for no reason - completely unaware that I've got the second half of his solenoid.

nile13

Many cars turn the A/C off at WOT to not rub power from the engine. I don't know for sure that 993 does it, but I would not be surprised.

KINGSRULE

My selenoid doesn't look like this.. 97C4S. I beleive both hoses are attached

Randall G.

Hey Brian,

If you kept your 964, you wouldn't have this problem. The 964 doesn't use vacuum control for its HVAC system. LOL!

Seriously, the 993 uses vacuum control for the "bypass air flap," which is a misleading name for the cabin air intake. When you're in recirc mode, the (servo-motor controlled) fresh air-intake at the base of the windshield is closed, and the interior fan draws on the cabin (and/or, the heat-exchangers, for heated air). When you accelerate under load, the bypass air-flap can close if there's inadequate vacuum. Thus, no air flow.

Similarly, when running the A/C, the system switches to recirc. mode automatically, and you get the same symptoms.

I don't think what you're experiencing is normal. Several people have managed to fix this problem. For example, I'm pretty sure Ray C. replaced the valve you've pictured.

Good luck!

BS911

Kept the 964??? DOH! It's always one set of problems for another. Hopefully they even out in the end.

Thanks for the info. Sounds like I'm definitely losing vacuum somewhere. At least I'll have a fun project here solving this problem before I get back to my no-AC-in-real-hot-weather problem.

ksimpson

I had a vacuum leak which resulted in the air flow cutting out under hard acceleration (which I do occasionally).

It eventually manifested itself into a more annoying problem - when I turned the A/C on, the system went into recirc and I got no air at all. I could hear the fan, but no air.

I left it into the OPC and they traced the problem down to a seized bypass flap. They took it out and greased it and now everything is fine. However, the bad news was that it took three hours labour to trace the fault.

BS911

Ok, so I just finished "learning" more about what hoses go where for what vacuum and I have a better ... er ... more detailed description of my problem - which apparently starts before the vacuum problem even kicks in.

Vacuum pressure seems good all around and when I hit the recirc (activates vacuum through to bypass flap) the flap will what I'll call open to recirc mode giving me nice air flow. Release the recirc button which releases the vacuum and the recirc flap closes with a nice solid thump. (read: full closure) At this point, there is very little air flow but I can hear the two front blower motors straining to get air. In fact, if I lift the cover to the pollen filter, exposing the fans, they start breathing through that open cover and have plenty of air. So the problem is the fresh COLD air source seems to be "blocked" somehow. Of course, if I hit the a/c, it forces a recirc mode and I get nice a/c air flow.

Now, if I turn to heat, the rear blower kicks on and I get air flow whether in recirc or not.

Problem #2 comes in when I'm in recirc, getting good air flow, accellerate to load the engine and apparently the recirc flap loses some of its vacuum and closes down - thereby shutting down my air flow.

So if I'm looking at this right there are 2 distinct problems at work here. Where do I go next? Why am I not getting fresh cold air? Does this match with what some other folks have?

Thanks again.

BS911

Oh yeah... Good point on the bypass flap, I'll have to check it.... do you know where it might be? Or is it that same flap under the ccu unit?

nile13

Brian, not to hijack the thread, but a qustion for you: Does your rear fan turn on with the car not running?
Mine does not and as per Randall G that is my problem with lack of hot airflow. Seems to make sense, I just wanted to conform (and figure out, perhaps, how to better test the damn fan).

BS911

My fan does not run (that I am aware) with the engine and ignition turned off. However, I think it will when either the engine is REAL hot after I turn it off in the summer, or if I disconnect the temperature sensor from the tube coming out of the rear fan which - might simulate the same hot or open condition from that circuit. I'll be doing some more work this weekend on this system and will try and work out the exact scenarios mine kicks in for.

JohnJL

Sorry I can't help you with your problem, but let me get this straight...you're diagnosing AC problems in MD in February? How did you even notice this?

You are indeed a thorough fellow.

best,
john