I know I read the answer to this question some years ago, but I haven't been
able to find it, so I am coming to the collective wisdom of this group for some
help. When I first start the car, I hear a steady clicking sound from behind the
center of the dash. It lasts for several minutes and then switches to an
intermittent clicking noise, then finally goes away after several minutes. The
ventilation system does not blow much air and changing the fan speed changes the
fan noise, but not the amount of air being blown out. This is true for cooling
and for heat, and it doesn't matter where the control is set. The sound is as if
the fan is blowing against a closed door. Any ideas?
89 red, white and blue
The ventilation system is a combined effort of a stepper motor and some vacuum actuators. The clicking sound may be the stepper positioning one flap. The fact that none of the controls seems to open the other flaps in the air ventilation would be explained by a severe vacuum leak. There's a bit of info on that on my web. Here is a direct link:
http://jenniskens.livedsl.nl/Technical/Tips/T_Climate.htm look under subject ventilation. Diagrams and procedures are in there. A Mityvac helps diagnosing and finding leaks. You'd be amazed how well the system works if all leaks are fixed.
I've been doing repairs in my 1992 gts, but still have a leaking footwell actuator left to repair. I'm waiting for a good moment to tackle that one bolt residing close to the pass. side airbag .... The one you can't reach. So if anyone is able to give a brilliant tip....?
Dick: be glad you don't have the airbag in the way and the new style dash layout ;-)) Yours is relatively easy to fix if needed.
1992 928gts Midnight Blue
1988 928s4 Cherry Red (Sold in 2006)
The noise is coming from a failed vacuum actuator on the center vent.
You can test this with a MityVac - purchased cheap at Harbor Freight.
Remove the lower console panel on the driver's side. This is the piece of carpet that goes below the center console. It is held in place with a plastic clip in the rear and a Phillips metal screw about half way back near the bottom. The carpet piece drops down and slides out to the rear.
Locate the Orange plastic line that runs to the actuator. There should be a rubber coupler to disconnect. You can tell the end going to the actuator by starting the car briefly. The end drawing a vacuum comes from the engine.
The other end is the one to test. It should hold a vacuum forever. It will probably not, but you should hear the same clicking. If you plug the end of the orange tube with the vacuum the clicking will go away.
Fixing the center flap is a bit more work. You will have to remove the center console. There are some screws under the center vent, which just pries out. A couple down by the back end and one sticky one that can only be accessed by removing the plastic plug in the glove box - use a magnet.
Once the console is pulled away you will see the orange tube running to an actuator. The actuator body is welded to the HVAC unit, but the back of it will unclip revealing a rubber diaphragm. It is the diaphragm that gets old and starts to leak.
The diaphragm is connected to a metal arm to operates the flap. The arm has a plastic clip that plugs into the flap and has to be pried off. You can then disassemble the new actuator you bought from Jim Bailey at 928 International thus voiding the warranty and use the new diaphragm to replace the old one. You will need to line up the new arm with the flap from inside the glove box. Once lined up it will snap back in place with a push.
I cannot remember if the door and interior of the glove box have to come out. By the time I am working at this level the dash is already removed.
It is best anyway to remove the glove box door before you break the two plastic arms that limit its opening position. Finally use the new cap to close the actuator and hook up the vacuum pump to see if the vacuum really holds. You might need to reseat the diaphragm to get a good seal.
Put everything back together again and you are ready to chase down all of the electrical problems you have now created. This is not the only actuator that can go bad, but is the most common. When you are testing the orange tube, you can also test the rest of the system from the same point. The manual has a chart of the various hoses and what each color does. There could be other vacuum leaks. If the orange tube tests fine then you will need to look for other leaks. However, from experience it is the center actuator that is most commonly the problem.
And you thought you were having fun.
Dan the Pod Guy
one thing that makes a decent clicking under the dash is the center console flap. When the actuator leaks a bit the flap tries to open a bit, suddenly leaks more than normal due to the movement of the arm, pops back and starts again to gain vacuum to activate the flap (activate = open) What you see in the center vent opening is a comb like flap that tries to open. You may need to take the grill out to see this,. The clicking is caused by the flap opening and closing in a 3hz cycle. Very annoying. An easy fix is to but a but of foam under the flap to make it stop annoying you by ticking so loudly,. but it does take a new actuator to really fix it.