Captain_Slow

I've observed a strange behavior with the tell-tale tapping sound that usually points to a hole in the center flap actuator diaphragm - It pretty much only taps like mad in the summer or on warm/hot days, becoming less frequent in the fall - until the engine and firewall are very warm and the tapping increases. But the strangest observation is the tapping never happens during cold weather - and I mean from start to finish of a 45 minute drive the tapping is gone. But I know it will be back in spring...first sporadically, just as the daffodils are emerging, and becoming progressively more frequent (more episodes), and with a higher frequency (rate) of tapping as the days get hot.

I've got new silicone diaphragms to replace the center and footwell flap actuators (thanks Roger). It's been my plan to do this job over winter (but I'm still driving the car because El Nino is keeping us warm). Next week I'm putting it up on the LiftBars for the winter...

Is there anything about what I've described above that would lead anyone to think there is something other than the HVAC vent-flap actuator diaphragms needing replacing?

davek9

No

jerybak

Well that is disappointing for me as I had the same noise a few months back, and it has since disappeared.....got cold here in STL also..... this spring will be my first with the car so I guess it will be back.

Captain_Slow

Dave - Sure you don't want to elaborate? Thanks...full speed ahead on removing the console, etc.

jerybak - Dave's definitive response is actually good news. There are times when a symptom on a 928 can have multiple causes. You have to go after them one by one, starting with the most probable. In this case I was wondering if this might have been a problem with the vent solenoids in the HVAC control manifold. The solenoids are the electrically activated gate keepers that determine which actuator diaphragms have vacuum applied (depending on where you set the slider on the HVAC control).

Now for the really good news: There are very good step-by-step DIY repair guides available. I will use Dwayne's extremely well documented procedure. He includes steps for testing the solenoids. Fair warning...your jaw will drop when you see the amount of work involved. Use this as an opportunity to evaluate stereo wiring, clean, etc. as you go.

CLICK

dr bob

The solenoids don't tap. A split diaphagm in the center vent, the 'comb flap', is the most probable cause of the noise. Colder weather causes the setting motor to allow the comb flap to stay closed, porting less fresh outside air out the vent, more warm air that's passed through the heater box.

The definitive test is a vacuum test on each of the actuator circuits. A Mitivac or similar hand pump is connected to each actuator via its hose connection at the solenoids in the console. There are plenty of write-ups with details. The basics are hat the side covers for the console come off, three screws come out to allow you to pass the solenoids towards the drivers footwell. Then one by one, the hoses connected to each solenoid get removed and connected to the little hand pump. Once connected, you pump the air out out and see if it holds. You'll hear each actuator moving as you pump the air out.

Captain_Slow

Dr. Bob,
Thanks for clarifying the cause and temperature dependent tapping behavior. I've never read anything similar in other related threads that fully explained it as you have.

davek9

Hi sorry I was on my cell (no signature in my reply

What usually happens is the vacuum starts to pull closed the Actuator in question, however it is pulling against the spring inside the actuator and can not hold the actuator closed as there is a small hole in the rubber, thus it releases and starts once again.

Results is the click click click or thump thump thump as it continues to try to fight the spring.
This also requires vacuum to actually get to the correct actuator via the solenoids, good news is that that is happening however the entire system depends on all parts being able to hold a vacuum.

As Doc states a good vacuum test tool is the easiest way to test the HVAC system

I recently had a tiny hole in the center actuator and it produced a hi pitched wined like an audio signal, drove me nuts on a 8 hr drive

Hows that

Dave

Captain_Slow

Dave - Thanks for adding more detail on how the actuators work, and additional examples of symptoms of failure. Best of all, I now understand how the tap tap tap signals only the failure of the diaphragm and confirms the solenoid is working!