2002M3Drew

Hi all,

I'm trying to figure what this noise is, and how to fix it. I figure a picture (video in this case) is worth a thousand words, so here it is:


Any ideas or tips?

mike77

Mine is the same. To fix it I usually turn the fan on for a bit and then back off.

A permanent fix would be good.

jcorenman

When it is doing the clicking thing, does adjusting the temperature (upper slider) change it?

There is a temperature-adjusting motor on the left side of the header box, accessible via the left side (USA driver's side) footwell space. You will need to remove the parcel tray (pre-90) or knee-bolster (airbag cars) for access. It is a little rectangular gizmo that operates the temperature-mixing flaps via a little pushrod. When the adjusting motor gets funky it chatters, and sounds like squirrels running around behind the dash somewhere.

2002M3Drew

Thank you for the reply. As you can hear in the video, the sound is very mechanical and cyclical In other cars I have found this sound to be connected to the cabin fan wearing out, but apparently in the 928 there are various flaps controlled by servos that may be the culprit. I will be installing a new head unit soon, and I figured it would be a good time to tackle this project as well. Do you think that that box that you described could be the source of this?

Kevin in Atlanta

Remove the vent grill above the AC head and observe the comb as you run your test.

2002M3Drew

Do the vents simply pop out?

Captain_Slow

Oops...I just watched the video more carefully. That is not the same symptom caused by failed vacuum actuators

Captain_Slow

The center vent does. I haven't tried pulling out the vents in the doors.

dr bob

Yes.

There's a fairly simple and fast way to determine the source of the noise. Remove the left (driver's side on US cars, above the pedals and to the right...) side cover on the console area to expose the little metal console that holds the HVAC solenoids. Uing your MitiVac or similar, pull vacuum on each actuator. You are testing to see which will hold vacuum. Note the colors of the tubes as you test, and relate those colors to the diagram in the WSM to decide which ones leak and need to be replaced. Note also that the diaphragms are probably about the same age, so one failure is a telltale of others that will fail soon.

You'll need to remove the console for the comb flap actuator, and you might as well replace the diaphragm for the footwell flap actuator at the same time since both will be accessible. In Jon's picture above with the console removed, you can see the little black actuator canister for the comb flap to the right of the center vent area, facing forward om the car. The footwell flap actuator is below that, facing down.

The other actuators are the fresh air flap above the CE panel, and the defrost flap actuator. The defrost actuator needs at least the pod out of the way for replacement, much easier with the whole dash unbolted and pulled back. The fifth actuator is the heater control valve in the engine compartment.

---

My preliminary diagnosis based on your description of location and sound is that comb flap actuator, with a torn/split diaphragm lurking inside the little actuator canister. The actual air flow in the vacuum system is restricted by the size of the hose from the source at the brake booster, With a tear at a fold in the little rubber diaphragm, it will hold vacuum until the actuator pulls in far enough to open that tear/split, at which point all the vacuum on the canister is dumped rapidly. It relaxes, and the restricted vacuum source once again starts to pull on the diaphragm and move the flap. The clicking noise is from the rapid dump of air into the actuator where it's split.

2002M3Drew

Thank you, Dr. Bob! Very thorough...doesn't sound like an easy fix, but at least I know what I'm looking for.

dr bob

Several of the 928 Tips Pages have good write-ups on the replacement procedure. There's nothing exceptional except the time and patience needed. You'll un-pin the end of the little actuator arm, lift the diaphragm and actuator arm out of the canister. You'll do the same with the new one, so you can just stuff your new diaphragm and arm into the old canister housing. The comb flap actuator arm connects to a little arm on the flap/diverter, with a pin that's inside the airbox a little. Following the instructions, you'll do a little almost-blind surgery through the side at the glove box area where the cooling duct for the glove box attaches. Some folks have just hole-sawed an opening in the plastic airbox, then plugged or taped up the hole when the new arm is connected again. I did mine the "blind three-handed surgeon with a flashlight and a mirror" way, and it was a little bit of frustrating work getting the actuator arm onto the little comb flap actuator arm. If you aren't already tri-dextrous (three right hands...) the holesaw method might be appealing. I don't like using a saw in any regular repair, personally.

worf928

Sounds like the blower resistor pack going mad.

I'm going to put my chips on an iffy ground.

The Forgotten On

The easiest way to RnR the HVAC system with the least frustration is to take the dashboard off.

It allows access to all of the vacuum pods and you can take the front of the HVAC box off to get to anything inside and give everything a good clean and adjust the hvac motor without contorting yourself.

You also wont have to drill a hole into the box to deal with the comb flap valve.

corellian vette

OH MY GOD my car literally started doing this exact thing last week.

dr bob

The miracle for many is that the actuator diaphragms have lasted as long as they have. Replacing them is not so much a question of "if" as of "when". When the first one goes, the others are certainly not far behind, so there's a pretty good case for doing all of them at the same time. Of course there's no shortage of other WYAIT work once you commit to pulling the dash. The already-mentioned servo motor for the diverter snaps to mind. The doors in the air handler are "sealed" with rubber weatherstrip that has undoubtedly deteriorated a lot over the years. No shortage of fun available for the project. Do it during hibernation season so you don't sacrifice drive time.

Our vendors are able to supply replacement diaphragms for a couple of the actuators, rather than forcing us to buy the whole thing and toss the canister.

I will share that one vacuum leak will trash AC performance even if you secure the failed actuator. Having other things in the wrong positions due to low vacuum available wreaks havoc on temperature and airflow control. Good reasons to take care of everything once you decide to go in there.


Have fun!