HVAC 'Comb Flap' what does it do exactly
09-29-2007, 07:21 PM
#1
Electron Wrangler
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HVAC 'Comb Flap' what does it do exactly
I'm digging into understanding the HVAC and think I understand all the features except for exactly how the internal flaps within the airbox work.
These are the Variable mixing flaps (before/around the heater core), which I mostly understand - the comb flap which I don't.
In the wiring diagrams the comb flap actuator seems to be called: 'Impact Wall' - is that the correct one?
I'm also not now sure of the exact function of the center nozzle micro-switch (I used to think I understood it...)
Does it change the comb flap position when the center nozzle is closed? to what end? Seems it should also open either the footwell or defrost vents (or both) too - but I don't see how it can actually do that.
Anyone understand this part of the system.
Alan
These are the Variable mixing flaps (before/around the heater core), which I mostly understand - the comb flap which I don't.
In the wiring diagrams the comb flap actuator seems to be called: 'Impact Wall' - is that the correct one?
I'm also not now sure of the exact function of the center nozzle micro-switch (I used to think I understood it...)
Does it change the comb flap position when the center nozzle is closed? to what end? Seems it should also open either the footwell or defrost vents (or both) too - but I don't see how it can actually do that.
Anyone understand this part of the system.
Alan
Last edited by Alan; 09-29-2007 at 07:37 PM.
09-29-2007, 07:26 PM
#2
Drifting
I understood that it does very little aside from diverting some air to the vents in the doors, which is why I haven't bothered with the exquisite agony of trying to change my leaking comb flap actuator and have just sealed that vac line.
09-29-2007, 07:42 PM
#3
Chronic Tool Dropper
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I think that the task is more for diverting some of the available center-vent air to the windscreen and footwell vents when those modes are selected. Visit the 'truth table' for solenoids and you'll see what I mean.
09-30-2007, 01:29 AM
#4
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I have all of Jim's material - but frankly the AC materials are difficult to follow through the evolutionary flow & are lacking documentation on S4+. I think the quality of the material is pretty variable here too...
Plus I am convinced some of the details in one of the later documents are just wrong. They ascribe functionality that I don't see is possible - like mixing flap percentage controlling the recirculation flap position... how?
Best I can tell the comb flap is used to control the degree of mixing of cool air (fresh or AC cooled) with mixed heated air going to the door and console vents. The fully mixed heated air goes to the footwells and when the comb flap is open it also goes to the door/console vents - when closed only a lesser amount of heated air is mixed to keep the face directed air a slight differential coooler than the rest of the cabin air. It seems to me the defrost vents always get the same mixing as the face vents based on the arrangement in the diagrams.
There also seems to be a mechanism on cold start to close the footwell vent flap (actually probably a secondary flap) to avoid chilly air to the footwells until the heater core reaches ~40C. Not sure how that is done exactly.
Any illumination on these items...? anyone have a S4 HVAC document?
Alan
Plus I am convinced some of the details in one of the later documents are just wrong. They ascribe functionality that I don't see is possible - like mixing flap percentage controlling the recirculation flap position... how?
Best I can tell the comb flap is used to control the degree of mixing of cool air (fresh or AC cooled) with mixed heated air going to the door and console vents. The fully mixed heated air goes to the footwells and when the comb flap is open it also goes to the door/console vents - when closed only a lesser amount of heated air is mixed to keep the face directed air a slight differential coooler than the rest of the cabin air. It seems to me the defrost vents always get the same mixing as the face vents based on the arrangement in the diagrams.
There also seems to be a mechanism on cold start to close the footwell vent flap (actually probably a secondary flap) to avoid chilly air to the footwells until the heater core reaches ~40C. Not sure how that is done exactly.
Any illumination on these items...? anyone have a S4 HVAC document?
Alan
09-30-2007, 01:43 AM
#5
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Alan,
Not to hijack your thread, but... where were you tonight??? Rich, Chuck and I were at the Pavillions with AZPCA. We needed more 928's! We were actually outnumbered by GT3's!!! By about double!!!
Not to hijack your thread, but... where were you tonight??? Rich, Chuck and I were at the Pavillions with AZPCA. We needed more 928's! We were actually outnumbered by GT3's!!! By about double!!!
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09-30-2007, 02:11 AM
#8
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Go back to the comb flap. I just cut and capped mine 'cause I'm tired of f^&ing with it. It was a constant problem in my 86.5 and when it started in th 87, I said "screw it" and cut and capped the vacuum line. Replacing the diaphram is on my list of projects, just not at this time.
09-30-2007, 09:24 AM
#11
Race Car
Not sure if this helps!
Control Unit/Flap Operation
The control unit is mounted left on the air box and has
2 electric plugs (Sl) and (S2).
The control unit houses an electronic control for
regulation of the solenoid valves as well as a servo
motor for the temperature mixing flaps.
The control unit can be checked with the control lever
or by alternately unplugging and bridging the plug to the
inside temperature sensor with the control lever at 75' F.
1.) Checking Power Motor
Turn ignition on and set selector lever to ECO, AUTOM, or
BI.
The temperature selector lever has microswitches at 6S" F
and 8S" F end positions, which are closed in these positions
and short circuit the control potentiometer. This means
that in pos.
in pos.
65OF air conditioner provides max. cooling and
850 F max. heating. In 65 F position the power
motor of the control unit must run to its lowest position
(cooling) and at 85O F to its highest position (heating).
The position of the temperature mixing flaps can be checked
through the center vent in the heating position. They
should be opened fully.
If servo motor is not running, check plug (S2) term. 3 and
11 for voltage. If sensor circuit is okay, then replace
control unit.
Note: The microswitch for 850 F pos. (max. heating) is
only standard
Control Unit/Flap Operation
The control unit is mounted left on the air box and has
2 electric plugs (Sl) and (S2).
The control unit houses an electronic control for
regulation of the solenoid valves as well as a servo
motor for the temperature mixing flaps.
The control unit can be checked with the control lever
or by alternately unplugging and bridging the plug to the
inside temperature sensor with the control lever at 75' F.
1.) Checking Power Motor
Turn ignition on and set selector lever to ECO, AUTOM, or
BI.
The temperature selector lever has microswitches at 6S" F
and 8S" F end positions, which are closed in these positions
and short circuit the control potentiometer. This means
that in pos.
in pos.
65OF air conditioner provides max. cooling and
850 F max. heating. In 65 F position the power
motor of the control unit must run to its lowest position
(cooling) and at 85O F to its highest position (heating).
The position of the temperature mixing flaps can be checked
through the center vent in the heating position. They
should be opened fully.
If servo motor is not running, check plug (S2) term. 3 and
11 for voltage. If sensor circuit is okay, then replace
control unit.
Note: The microswitch for 850 F pos. (max. heating) is
only standard
09-30-2007, 11:13 AM
#12
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Tampa - thanks for the info - but that part I already understand - and it relates only to the temperature mixing flaps.
The documents are consistent and detailed about the temperature mixing function - but very vague on comb flap (even its name) and the cold start feature.
Alan
The documents are consistent and detailed about the temperature mixing function - but very vague on comb flap (even its name) and the cold start feature.
Alan
Good luck!
11-18-2007, 05:50 PM
#14
Drifting
I've just replaced the actuator on the Comb flap, as well as the Defrost and Footwell. I have just driven in total winter comfort for the first time ever.
The temperature differential between feet and face is now perfect. I am truly very happy. I don't think I can help answer your question, but I can confirm that it makes a hell of a difference now it's working.
The temperature differential between feet and face is now perfect. I am truly very happy. I don't think I can help answer your question, but I can confirm that it makes a hell of a difference now it's working.
11-21-2007, 12:55 AM
#15
Alan,
I noted your questions and the replies received, so I have done some investigation of the documentation that I have and I add the following to the descriptions already given.
Unfortunately the various documents, drawings and schematics do not use consistent terminology nor do they use consistent alphanumeric descriptors.
The system is basically a fully automatic air conditioning system based on temperature differentials for control to maintain the cabin temperature to the selected value regardless of the outside temperature, however, the system can also operated without the refrigeration compressor heat pump in operation.
Fundamentally, if the air conditioner system (HVAC) is without power or vacuum supply, the defrost flap will be open, the circulating air flap set to fresh air and the heater valve will also be open. All other flaps will be closed. This is therefore the ‘at rest position’ when the ignitions is switched off, so the mixing flap (comb flap) will be in the closed position (the position of this mixing flap can be seen through the centre vent as the ‘comb’ flap will be visible), the circulating air flap will be closed to the fresh air supplied position, the defrost flap will be opened, the foot well flap will be closed , the temperature control flaps will be closed and the heater valve will be opened.
The ‘at rest’ position is the reason why hot air is blown into the cabin on re-start when the car‘s engine jacket water is still hot, as the jacket water is thermocycling through the engine jackets through the heating valve into the cabin heating system with the hottest part being the cabin heater core as it is the highest point in the system.
If there is an electrical fault which effects the closure of the compressor clutch or setting motor fault or temperature control flaps out of adjustment, or low or high freon pressure, or large vacuum leakage etc., the system will revert to its ‘at rest position’ and the air blown into the cabin will be continuously hot when the car is operated and the engine is at operating temperature.
As the centre nozzle is used for the basic ventilation of the passenger compartment, and the air volume can be regulated infinitely with the lever on the side of this nozzle (this was changed from a double vacuum controller in MY84 via a sixth selonide valve).
The centre nozzle control flap can be fully closed with this lever if the inlet cabin air is found to be annoying and the air will be distributed as per the setting of the air distribution panel slide ****.
The micro switch (newly installed from MY84) is connected to the central nozzle control (shut off) flap and is fitted to open or close the central nozzle mixing flap (comb flap). When the central nozzle control flap and micro switch is closed a circuit is made via the Control Unit Temperature (cpu) to the AC Control Switch to the MV3 solenoid valve, which in turn activates the vacuum controller of the mixing flap, loacted in the centre nozzle, to close it (refer to electrical drawing sheet for “Engine Cooling, Heater and Air Conditioner (for MY1990 it is sheet 5). Im not sure for your MY1994GTS.
If the AC switch turned off, fresh air (not cooled) can also be distributed individually as well. The air temperature in the passenger compartment can be increased via the temperature selection slide via the temperature control slide **** (this is accomplished via the temperature control vacuum operated valve (heater valve) and temperature control flaps located in front of the heater core (controlled by the setting motor via the differential temperature electronic control system).
As mentioned previously, the position of the centre nozzle flap can be regulated manually with the side control lever thereby regulating the air flow volume to the required amount, however if the centre nozzle flap is shut off the micro switch is closed which in turns closes the mixing flap (comb flap). When closed, the mixing flap has the task of mixing cold or fresh air with heated air again in front of the centre nozzle outlet, to avoid great differences in the outlet air temperature from the remaining nozzle or outlets that are selected to be open.
The only time the mixing flap is closed is when the slide control **** is moved to the defrost position, the central nozzle is shut off or the system is at rest (ignition off).
I hope this helps, took quite a bit of research. I hope that some one comes up with the AC Training Manual for the S4/GT/GTS.
Tails 1990 928S4 Auto
I noted your questions and the replies received, so I have done some investigation of the documentation that I have and I add the following to the descriptions already given.
Unfortunately the various documents, drawings and schematics do not use consistent terminology nor do they use consistent alphanumeric descriptors.
The system is basically a fully automatic air conditioning system based on temperature differentials for control to maintain the cabin temperature to the selected value regardless of the outside temperature, however, the system can also operated without the refrigeration compressor heat pump in operation.
Fundamentally, if the air conditioner system (HVAC) is without power or vacuum supply, the defrost flap will be open, the circulating air flap set to fresh air and the heater valve will also be open. All other flaps will be closed. This is therefore the ‘at rest position’ when the ignitions is switched off, so the mixing flap (comb flap) will be in the closed position (the position of this mixing flap can be seen through the centre vent as the ‘comb’ flap will be visible), the circulating air flap will be closed to the fresh air supplied position, the defrost flap will be opened, the foot well flap will be closed , the temperature control flaps will be closed and the heater valve will be opened.
The ‘at rest’ position is the reason why hot air is blown into the cabin on re-start when the car‘s engine jacket water is still hot, as the jacket water is thermocycling through the engine jackets through the heating valve into the cabin heating system with the hottest part being the cabin heater core as it is the highest point in the system.
If there is an electrical fault which effects the closure of the compressor clutch or setting motor fault or temperature control flaps out of adjustment, or low or high freon pressure, or large vacuum leakage etc., the system will revert to its ‘at rest position’ and the air blown into the cabin will be continuously hot when the car is operated and the engine is at operating temperature.
As the centre nozzle is used for the basic ventilation of the passenger compartment, and the air volume can be regulated infinitely with the lever on the side of this nozzle (this was changed from a double vacuum controller in MY84 via a sixth selonide valve).
The centre nozzle control flap can be fully closed with this lever if the inlet cabin air is found to be annoying and the air will be distributed as per the setting of the air distribution panel slide ****.
The micro switch (newly installed from MY84) is connected to the central nozzle control (shut off) flap and is fitted to open or close the central nozzle mixing flap (comb flap). When the central nozzle control flap and micro switch is closed a circuit is made via the Control Unit Temperature (cpu) to the AC Control Switch to the MV3 solenoid valve, which in turn activates the vacuum controller of the mixing flap, loacted in the centre nozzle, to close it (refer to electrical drawing sheet for “Engine Cooling, Heater and Air Conditioner (for MY1990 it is sheet 5). Im not sure for your MY1994GTS.
If the AC switch turned off, fresh air (not cooled) can also be distributed individually as well. The air temperature in the passenger compartment can be increased via the temperature selection slide via the temperature control slide **** (this is accomplished via the temperature control vacuum operated valve (heater valve) and temperature control flaps located in front of the heater core (controlled by the setting motor via the differential temperature electronic control system).
As mentioned previously, the position of the centre nozzle flap can be regulated manually with the side control lever thereby regulating the air flow volume to the required amount, however if the centre nozzle flap is shut off the micro switch is closed which in turns closes the mixing flap (comb flap). When closed, the mixing flap has the task of mixing cold or fresh air with heated air again in front of the centre nozzle outlet, to avoid great differences in the outlet air temperature from the remaining nozzle or outlets that are selected to be open.
The only time the mixing flap is closed is when the slide control **** is moved to the defrost position, the central nozzle is shut off or the system is at rest (ignition off).
I hope this helps, took quite a bit of research. I hope that some one comes up with the AC Training Manual for the S4/GT/GTS.
Tails 1990 928S4 Auto