What Is It?

Microswitch in Center Vent

Robert Smallwood, 86.5; wrote:
>Anyone know what's the purpose of the little microswitch in the
>centre heater/AC vent that activates when you close the center
>vent to divert airflow to the side and defrost vents?

Wally Plumley replied:
>It operates the vacuum solenoid (thru the control box) that operates
>the bulkhead flap (the toothed flap behind the center vent). See page
>87-109 in the shop manual.

I wondered myself exactly what this switch does. (BTW, you can see the switch by removing the center vent grill by pulling it straight out.) After looking at the shop manual, I did some testing and came to the following (cold weather) conclusions:

1. With the temperature slider set "cold," the "comb flap" in the center vent stays open, allowing cool air to enter via the center vent

2. When the temperature slider is set far enough to the right to trigger the "heating" mode, the comb flap closes, causing warm air to flow through the center vent.

3. The previous observation was with the center vent "open". If you close the vent it closes the microswitch and immediately opens the comb flap. (On my car, this allows a little bit of cool air to leak around the seal on the center vent.) I do not know exactly why the climate control system has this particular mode, but I'm sure there must be a good answer for it. Anyone know?

4. The only other time the comb flap closes is when the selector is switched to "defrost." In my car there is a 3 second delay between selection of defrost mode and when the comb flap actually closes. As before, pressing the microswitch causes the comb flap to reopen.

5. Also, on rare occasion, my climate control goes "bonkers" and actuates the comb flap on/off/on/off rapidly for about a second, resulting in a chattering sound from behind the center vent. I assume this has something to do with being at a borderline temperature decision point for the control circuitry.

So, the short answer to the question "what is the purpose of the microswitch?" is:
When you close the center vent it causes the comb flap remain open always.
I'm not sure exactly why, but that's what it does.

+Greg Nichols
Alexandria, Virginia
'87 928S4, 5-speed, Venice Blue Metallic



The solenoid that overrides the flap position, as set by the user via micro-switch, is activated from the setting motor which comprises printed circuit tracks on a circular board which is moved by the setting motor as it changes the hot/cold mix in response to inside/outside temp and the users slider. This is fed with battery volts from the control unit. My setting motor occasionaly cycles rapidly back and forth over a small range, this could cause the chattering you mention, though I have not heard this particular phenomenon in my car (I have other oddities).

About the system taking 3 seconds to close the centre vent flap when defrost is selected - this delay is the setting motor cranking to the hot end.

Also, if you slowly move the lever that operates the micro-switch you will notice there is two positions, half and full. On mine these are very close togther. (you may not get both positions of the flap if the temp is cold).

Rex McIntosh
'81 928S Euro - GPWhite