Can someone point me to a decent HVAC diagnosis process?

Suddenly something in the system is making a sound like a card-in-spokes on a bicycle, that has a rate dependent upon vacuum.

 

Walley Plumley has one

 

Maybe foreign object in fan and maybe not related to vacuum, but voltage supply from the alternator.

Good maintenance to clean out fan inlet as well A/C condenser inlet surface. Write ups are on this forum.
Tails 1990 928 S4 Auto

 

Definitely vacuum related....... accelleration or hills (more open throttle) decreases frequnecy of noise...... coast-down increased frequency.

Moving air-flow control terminates noise until new position selection is complete, then the noise resumes.

Noise terminated by disconnecting vacuum source to HVAC under hood.

Switching fan speed has no effect.

Alternator output is nominal irrespective of load.Great!!

How do I find it??

 

I have the same situation but mine squeals instead of fluttering (see post below).
Ditto on where to find Plumley's info.

 

Wally's Write-up......... is in this site but the link will not post.

https://www.928gt.com

See under "Tips" / "Wally World" / "HVAC System"

 

https://www.928gt.com/t-wallyhvac.aspx is the fully link. Extremely comprehensive write up with lots of conceptual stuff too. Very helpful.

 

Based on the limited description, this is almost certainly the comb flap in the ventilator. It's a common problem brought on by vacuum leaks somewhere in the HVAC control. the system doesn't provide much volume, so even a very small leak anywhere in the vac control causes the actuators to work poorly.

I usually don't advise folks to search for stuff, but there's oodles of threads about this issue. Wally's write up is very useful. Roger (Rog100, 928sRUS)also has had several threads with photos and such. He can also supply all the parts needed to get it running smooth again. Once you start, it's best to do the whole vac system rebuild, cause problems in one area indicate problems will soon crop up elsewhere.

 

Brian
Wallys write up is very good....but Tony's and John Pirtles have pictures..which help me ALOT....

Start at the brake booster...I am 100% sure the blue-black check valve is bad.....then chances are the 4 way T is bad too...just order both....since they are cheap from 928intl (don't but the Porshce valve its big $$$).....then move inside to the HVAC solenoid strip behind the stereo....chances are the orange line (center flap) & Blue line (recirculation) are leaking.....if you don't have time to fix them just buy a couple 3mm vacumm caps and cap them....this will restore vacuum and everything else will work