HVAC Mystery - Center Vent Air Always Warm
03-27-2008, 06:05 PM
#1
Intermediate
Thread Starter
HVAC Mystery - Center Vent Air Always Warm
Hi everyone. I may need some help from the HVAC gourous.
93GTS. I seem to be getting warm air from the center vent regardless of the temperature slider position; to be more precise if the temperature slider is set just a bit more than 18°C (65F I believe for most of you) I can only get very warm air. A bit too binary to my taste.
Today I tried something a bit too obvious. I closed the center vent and realized all the other vents were reacting well to the temperature slider change ie at 20°C I get some warm air and gradually moving up the temperature produces warmer air.
I have been going through most of the posts dealing with HVAC issues which helped me in eliminating the obvious culprits such as the heater valve etc. Has my center vent decided to live a life of its own???
I would greatly appreciate if someone could direct me where I should be looking.
Thanks in advance.https://rennlist.com/forums/images/icons/icon7.gif
Michel
93GTS. I seem to be getting warm air from the center vent regardless of the temperature slider position; to be more precise if the temperature slider is set just a bit more than 18°C (65F I believe for most of you) I can only get very warm air. A bit too binary to my taste.
Today I tried something a bit too obvious. I closed the center vent and realized all the other vents were reacting well to the temperature slider change ie at 20°C I get some warm air and gradually moving up the temperature produces warmer air.
I have been going through most of the posts dealing with HVAC issues which helped me in eliminating the obvious culprits such as the heater valve etc. Has my center vent decided to live a life of its own???
I would greatly appreciate if someone could direct me where I should be looking.
Thanks in advance.https://rennlist.com/forums/images/icons/icon7.gif
Michel
03-27-2008, 06:32 PM
#2
Rennlist Member
A BIG hello gige02 and welcome.... !
Here are two sites that have very good troubleshooting guides.
DaveK9
http://members.rennlist.com/pirtle/svc.html
https://www.928gt.com/t-wallyhvac.aspx
Here are two sites that have very good troubleshooting guides.
DaveK9
http://members.rennlist.com/pirtle/svc.html
https://www.928gt.com/t-wallyhvac.aspx
03-27-2008, 06:39 PM
#3
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Perhap remove the row of solinoids that control the flow of vac. to the various pod actuators and check them?? On my S4 its just below and aft of the radio.....its where all the vac lines lead to.
Welcome aboard the board!
Welcome aboard the board!
03-27-2008, 06:55 PM
#4
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I think you are probably mistaken about the relative control - probably all the temp control is screwed up... there is no seperate temp adjustment for the center vent - but the airflow paths are a bit different.
It is probably either the water valve or the mixing stepper motor that are not working correctly. I'd check the water valve first since its easiest (under the air intake in the engine bay). may be the vacuum actuator - test by wiring it closed (non-default position).
Alan
It is probably either the water valve or the mixing stepper motor that are not working correctly. I'd check the water valve first since its easiest (under the air intake in the engine bay). may be the vacuum actuator - test by wiring it closed (non-default position).
Alan
03-27-2008, 07:07 PM
#5
Intermediate
Thread Starter
Thanks for the speedy responses and your wellcomes appreciated.
To DaveK9: Wally's write up is going to be my er...late bed time reading.
To Alan: You may be right about my senses...maybe wishful thinking. Spent too much time on this without any tangible results.
Checked the water valve. It is open when engine shut and closes once engine starts. Opens again when you increase the temperature setting from 18°C onwards (engine running.Hate to do that without the air filterby the way but could not figure any other way. There does not seem to be any vacuum issues there but I have a question. Does the water pump actually work on an all open or all closed way or is it supposed to partially open depending on the temperature setting?
To DaveK9: Wally's write up is going to be my er...late bed time reading.
To Alan: You may be right about my senses...maybe wishful thinking. Spent too much time on this without any tangible results.
Checked the water valve. It is open when engine shut and closes once engine starts. Opens again when you increase the temperature setting from 18°C onwards (engine running.Hate to do that without the air filterby the way but could not figure any other way. There does not seem to be any vacuum issues there but I have a question. Does the water pump actually work on an all open or all closed way or is it supposed to partially open depending on the temperature setting?
03-27-2008, 07:40 PM
#6
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The water valve is an on/off type, it may cycle (slowly) based on heater core temps but its not a partial activation thing - sometimes they leak water through even when closed by the actuator - ideally see if you can see if its hot downstream of the valve with the car on. With the car cold - put the temp on min -you can idle to warm up with the intake tubes & air box top off (and bottom unscrewed) but airbox & filter on. Stop the engine - then turn the ignition back on instantly. Pull the airbox & filter & feel the outlet temp on the water valve - hot or cold?
With just the ignition on - no fan - can you hear the mixing flap motor move when you change the temp setting - does it strain in some positions? The motor drives 2 flaps that control how much air gets diverted though the heater core Vs. bypassing it.
Sometimes one flapgets detached and always allows some air to go though the heater core - depending on which one it is - more hot air may go the the center vent when at least 2 outlets are open (e.g. with defrost or footwell modes).
There are also 2 limit switches one for full cooling and one for full heating at either end of the slider scale - these disable the closed loop temperature control and just go to unrestricted full heating or full cooling modes. If the full heating mode switch sticks you get full heating as soon as the slider moves off the full cooling position - could be that?
Can you hear the micro switches tripping? (ignition can be off for this)
Your temp sensor loop - external in the alternator intake hose in drivers front fender & internal behind the dash vent may be disconnected - similar sysptoms
Alan
With just the ignition on - no fan - can you hear the mixing flap motor move when you change the temp setting - does it strain in some positions? The motor drives 2 flaps that control how much air gets diverted though the heater core Vs. bypassing it.
Sometimes one flapgets detached and always allows some air to go though the heater core - depending on which one it is - more hot air may go the the center vent when at least 2 outlets are open (e.g. with defrost or footwell modes).
There are also 2 limit switches one for full cooling and one for full heating at either end of the slider scale - these disable the closed loop temperature control and just go to unrestricted full heating or full cooling modes. If the full heating mode switch sticks you get full heating as soon as the slider moves off the full cooling position - could be that?
Can you hear the micro switches tripping? (ignition can be off for this)
Your temp sensor loop - external in the alternator intake hose in drivers front fender & internal behind the dash vent may be disconnected - similar sysptoms
Alan
03-28-2008, 10:13 AM
#7
Intermediate
Thread Starter
Alan thanks. I will try your procedure on the water valve this evening. One should not trust a piece of plastic which lives most of its life in one of the hottest places of the 928. Worrying the fuel line is so close as well!!!
However I can report on a few other things. The mixing motor: This I assume is the plastic black box just in front of the brake pedal under the dash? It is functional but acts in a binary way; when the temp is set at 18°C it is closed, and increasing the temperature only slightly opens it fully. Any movement of the temp slide between maximum heat and something slightly higher than 18°C does not seem to have any effect; full open or full closed is what it does. Given your confirmation about the water valve I am thinking that this is the only piece of machinery that will actually influence on the air temperature.
I also remember checking the outside temp sensor about two weeks ago. It responded to roughly the resistance levels described elsewhere in the forum. So I am assuming it is fine. I have more trouble with checking the status of the inside temp sensor as I am not in favour of taking apart the center console. (My car actually never got dismantled except for installing a new headset). However I did take the head unit out and checked that there were two yellow wires going to where the sensor should be and that these were properly connected just behing the headset. And nothing seems to be contracted behind the headset either ie no torn electrical wires or vacuum lines.
So provided my water valve passes the test per your procedure (a big achievement for such cheap plastic) the only two things to check would be resistance of the temperature loop and its behaviour when the temp slide is moved and if that also succeeds then the liitle black box aka the mixing motor needs to leave the car. Right?
Is there any way to check the resistance of the temperature loop on the connectors leading to the mixing motor? I see there is a white connector which seems to have a couple of wires.
As always much appreciate your time and help on this.
However I can report on a few other things. The mixing motor: This I assume is the plastic black box just in front of the brake pedal under the dash? It is functional but acts in a binary way; when the temp is set at 18°C it is closed, and increasing the temperature only slightly opens it fully. Any movement of the temp slide between maximum heat and something slightly higher than 18°C does not seem to have any effect; full open or full closed is what it does. Given your confirmation about the water valve I am thinking that this is the only piece of machinery that will actually influence on the air temperature.
I also remember checking the outside temp sensor about two weeks ago. It responded to roughly the resistance levels described elsewhere in the forum. So I am assuming it is fine. I have more trouble with checking the status of the inside temp sensor as I am not in favour of taking apart the center console. (My car actually never got dismantled except for installing a new headset). However I did take the head unit out and checked that there were two yellow wires going to where the sensor should be and that these were properly connected just behing the headset. And nothing seems to be contracted behind the headset either ie no torn electrical wires or vacuum lines.
So provided my water valve passes the test per your procedure (a big achievement for such cheap plastic) the only two things to check would be resistance of the temperature loop and its behaviour when the temp slide is moved and if that also succeeds then the liitle black box aka the mixing motor needs to leave the car. Right?
Is there any way to check the resistance of the temperature loop on the connectors leading to the mixing motor? I see there is a white connector which seems to have a couple of wires.
As always much appreciate your time and help on this.
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03-28-2008, 11:18 AM
#8
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No what you describe sounds like just another vacuum actuator but this time for a mixer flap. The mixing flap motor is inside the console - deep.. and is continuously variable - in fact it can only move slowly. Unfortunately this one is complicated to resolve and mine has never failed so I only know what other have said on the topic.
Test the temp sensor loop and microswitches first - since they are much easier.
Alan
Test the temp sensor loop and microswitches first - since they are much easier.
Alan
03-28-2008, 03:59 PM
#9
Rennlist Member
Michel,
I'd be very sure that there is not a leak in the vacuum system before I went too much futher. Pull a vacuum (Mighty Vac is used here in the US) on the line running from the 4 way near the brake booster intoto the cabin (may be black, goes through the firewall. Pull a vacuum on it, and see if the system holds a vacuum for 10 minutes plus. Do the same with the lead to the water valve, etc. If ANYTHING going to the valve or into the cabin does not hold a vacuum, you'll need to find the source of the leak.
The rubber elbows behind the solenoids/vacuum distributor in the console, (about even with the top of the accelerator pedal) are notorious for developing cracks and leaks (had one leaking on my '94, 2 years ago). These leaks can disrupt all the automatic temp & air flow control processes.
Also, as Wally Plumley recommends, you can cut the tubes going from this vacuum distributor, pull a vacuum on each one to see if any of the actuator diaphragms are leaking. Re-connect with a short piece of hose after the check.
The vacuum distributor is not too easy to get at, but the repairs, if leaks are found is easy and cheap (except for some of the actuator diaphragms).
Gary Knox
West Chester, PA
'94 GTS, '89 S4
I'd be very sure that there is not a leak in the vacuum system before I went too much futher. Pull a vacuum (Mighty Vac is used here in the US) on the line running from the 4 way near the brake booster intoto the cabin (may be black, goes through the firewall. Pull a vacuum on it, and see if the system holds a vacuum for 10 minutes plus. Do the same with the lead to the water valve, etc. If ANYTHING going to the valve or into the cabin does not hold a vacuum, you'll need to find the source of the leak.
The rubber elbows behind the solenoids/vacuum distributor in the console, (about even with the top of the accelerator pedal) are notorious for developing cracks and leaks (had one leaking on my '94, 2 years ago). These leaks can disrupt all the automatic temp & air flow control processes.
Also, as Wally Plumley recommends, you can cut the tubes going from this vacuum distributor, pull a vacuum on each one to see if any of the actuator diaphragms are leaking. Re-connect with a short piece of hose after the check.
The vacuum distributor is not too easy to get at, but the repairs, if leaks are found is easy and cheap (except for some of the actuator diaphragms).
Gary Knox
West Chester, PA
'94 GTS, '89 S4
03-28-2008, 06:03 PM
#11
Intermediate
Thread Starter
Sounds like I’m going to have a busy weekend.
For starters MityVacs are sort of unkown of here. Worrying specially since our beloved neighbors, the Germans, were at the origin of this marvelous vacuum system. The only gizmo that creates vacuum in France is used to seal non-finished wine bottles!!!
I have seen the vacuum line to the water valve function properly. So for starters I will follow Alan's recommendation and check if the valve operates without any leaks. Next will be the micro switches on the temp slider amongst other things mentioned.
I will then take the head unit out to re check again if things are getting squeezed back there. Whilst at it I will try to find the vacuum console and try to make a visual inspection.
I’ll report back once I have checked everything except Gary’s ideas which will need a MityVac Français. Something to get on one of my next trips when I cross the pond.
Stay tuned.
Michel
For starters MityVacs are sort of unkown of here. Worrying specially since our beloved neighbors, the Germans, were at the origin of this marvelous vacuum system. The only gizmo that creates vacuum in France is used to seal non-finished wine bottles!!!
I have seen the vacuum line to the water valve function properly. So for starters I will follow Alan's recommendation and check if the valve operates without any leaks. Next will be the micro switches on the temp slider amongst other things mentioned.
I will then take the head unit out to re check again if things are getting squeezed back there. Whilst at it I will try to find the vacuum console and try to make a visual inspection.
I’ll report back once I have checked everything except Gary’s ideas which will need a MityVac Français. Something to get on one of my next trips when I cross the pond.
Stay tuned.
Michel
03-28-2008, 06:22 PM
#12
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The sensor loop is between pin 16 on the removable head unit connector and Pin 4 on the control unit (main HVAC block in the console). The control unit has 2 plugs but only one has a connection to its pin4 (Yellow wire). This same Yellow wire runs to one of the outside temp sensor connections via a 4 pin connector in the console - (look for 2 x Yellow & 2 x Violet/Yellow wires) - you need the yellow wire nearest the middle of the connector.
Alan
Alan
03-28-2008, 06:25 PM
#13
Drifting
Bonsoir Michel!
Closer to home there are always lots of MityVacs for sale on eBay UK.
Here's one which will ship to you:
http://cgi.ebay.co.uk/mityvac-profes...QQcmdZViewItem
It's about twice as expensive as the basic Mityvac which I got, but it does come with a very complete and useful range of adapters.
Bon chance
Closer to home there are always lots of MityVacs for sale on eBay UK.
Here's one which will ship to you:
http://cgi.ebay.co.uk/mityvac-profes...QQcmdZViewItem
It's about twice as expensive as the basic Mityvac which I got, but it does come with a very complete and useful range of adapters.
Bon chance
03-29-2008, 01:58 PM
#14
Intermediate
Thread Starter
I think I have found it !!! it is the usual suspect ⇒ The Water Valve.
I have noticed that whilst the engine was warming up that the lever was leaking where it joins the main body of the valve. This went on until the engine was up to running temperature and then stopped. Reason why I have not noticed any water loss. I am assuming that this in itself can not be normal as leaking fluids are a specialty of British automotive engineering not German (my way of saying merci beaucoup to StratfordShark for his valuable ebay link).
However during the whole process the temperature of the water hoses downstream as well as upstream were identically hot. So I have proper vacuum (as the valve was properly activated and closed) but a leaking or dead valve body.
For Alan : Micro switches are ok on the slider. I can hear them and there does not seem to be anything unusual on the vacuum console. I have done a visual inspection only though.
So next step is getting a new (two ?) water valve and see whether this solves the issue.
Any ideas where to get one ? Saw 926 International carries them but the shipping charge is basically the same or more than the price for the plastic. Another Google search challenge awaits…
Will report back in a few days (weeks) after changing the valve. By the way whilst doing this should I expect to be covered with coolant or is there a « clean » method of replacing the valve without spreading the coolant where it should not go ?
Thanks to all for your help. I have been on this for a while now.
I have noticed that whilst the engine was warming up that the lever was leaking where it joins the main body of the valve. This went on until the engine was up to running temperature and then stopped. Reason why I have not noticed any water loss. I am assuming that this in itself can not be normal as leaking fluids are a specialty of British automotive engineering not German (my way of saying merci beaucoup to StratfordShark for his valuable ebay link).
However during the whole process the temperature of the water hoses downstream as well as upstream were identically hot. So I have proper vacuum (as the valve was properly activated and closed) but a leaking or dead valve body.
For Alan : Micro switches are ok on the slider. I can hear them and there does not seem to be anything unusual on the vacuum console. I have done a visual inspection only though.
So next step is getting a new (two ?) water valve and see whether this solves the issue.
Any ideas where to get one ? Saw 926 International carries them but the shipping charge is basically the same or more than the price for the plastic. Another Google search challenge awaits…
Will report back in a few days (weeks) after changing the valve. By the way whilst doing this should I expect to be covered with coolant or is there a « clean » method of replacing the valve without spreading the coolant where it should not go ?
Thanks to all for your help. I have been on this for a while now.
03-29-2008, 03:26 PM
#15
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Changing the water valve is easier if you drain some coolant out of the rad; easy to do but be careful as the drain screw is plastic.
Also, the water valve is connected to the engine with a short piece of hose; good idea to replace this with the new valve.
When you've installed the new valve and refilled the coolant check operation of the system; its basically a comparison of where the temp lever is set to the outside and inside air temps.........if the head unit senses the inside temp is cooler than the temp lever setting it then opens the water valve while at the same time compares the outside temp..........if outside is warmer it opens the recirc flap so the system also receives warm air from that source.
If the inside car temp is warmer than the temp lever setting the opposite should occur; water valve closes and depending on the outside temp closes or opens the recirc flap. These basic openings/closings are further adjusted depending on the a/c switch or the direction lever set to defrost:
Also, the water valve is connected to the engine with a short piece of hose; good idea to replace this with the new valve.
When you've installed the new valve and refilled the coolant check operation of the system; its basically a comparison of where the temp lever is set to the outside and inside air temps.........if the head unit senses the inside temp is cooler than the temp lever setting it then opens the water valve while at the same time compares the outside temp..........if outside is warmer it opens the recirc flap so the system also receives warm air from that source.
If the inside car temp is warmer than the temp lever setting the opposite should occur; water valve closes and depending on the outside temp closes or opens the recirc flap. These basic openings/closings are further adjusted depending on the a/c switch or the direction lever set to defrost: