Fresh air blower ( HVAC ) always on high even when switch is off SOLVED
12-13-2009, 09:41 AM
#1
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Fresh air blower ( HVAC ) always on high even when switch is off SOLVED
This has been bothering me for a while, and getting worse and worse. The HVAC blower would suddenly go on full speed no matter what position any control was on. I know Alan and Wally P could instantly diagnose this, but I futzed around for a day or two trying to solve it myself. Finally, staring at the wiring diagram this morning I realized only one thing could cause this...
Relay, relay, relay. Specifically, #8 - defroster. Fixed. Hope this saves time for someone else.
Relay, relay, relay. Specifically, #8 - defroster. Fixed. Hope this saves time for someone else.
12-13-2009, 12:42 PM
#4
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My problem was turn on the ignition key and the fan was on full blast (intermittently) no other position of fan speed, temp, a/c, or flap position would change it. Now that I'm thinking about the problem though it might be more than the relay, because the relay had to be getting a signal from somewhere. Back to the drawing board.
Last edited by M. Requin; 12-13-2009 at 12:43 PM. Reason: clarity
12-13-2009, 12:45 PM
#5
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If you want to defrost the windshield set temp to max, AC off and fan to max and air flaps to defroster windshield vent only and close the center console vent flap (if manual). This gets you max windshield flow at max heating
The DEF mode on the other hand is in case the windshield suddenly fogs on the inside. Select DEF and you get max fan (automantically), non-recirculate (automatically), max heating (automatically) and AC on (automaticaly).
Since in DEF you have both max heating and AC on - you will not get the max temperatue possible for defrosting... what you do get is maximum de-humidification - that is what this mode if for - and it really works - really fast.
If you have ever had the windshield fog rapidly at high speed you will understand why this is hugely valuable... and needs to be quick & easy to select - you have lots else to do... like triangulating out the windows to try to stay on the road...
To the original poster - its quite likely you haven't solved your issue. To me its unlikely the DEF relay caused this problem (possible but unlikely)... much more likely on your vintage 928 is the magic blower syndrome - the thermal bypass mechanism built into the blower resistor pack... once it starts it usually gets worse - but is often quite intermittent initially. It may mean your blower motor/fan is clogged with leaves or is generally faling OR it could just be the resistor pack is damaged.
Test - if you pull the resistor pack does it still happen? if not its the resistor pack - if yes it was indeed the DEF relay.
Alan
12-13-2009, 12:49 PM
#6
Rennlist Member
Martin,
Roger has a resistor pack that should fix this. Its for the later cars, so you will need to switch positions of a couple of wires in the harness.
Its one of the easier repairs to perform on these cars. Location is accessed from the engine compartment.
Roger has a resistor pack that should fix this. Its for the later cars, so you will need to switch positions of a couple of wires in the harness.
Its one of the easier repairs to perform on these cars. Location is accessed from the engine compartment.
12-13-2009, 01:45 PM
#7
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Thread Starter
Its actually not a DEFroster its a DEFogger (despite what some Porsche literature says). How can you tell - based on what it does...
If you want to defrost the windshield set temp to max, AC off and fan to max and air flaps to defroster windshield vent only and close the center console vent flap (if manual). This gets you max windshield flow at max heating
The DEF mode on the other hand is in case the windshield suddenly fogs on the inside. Select DEF and you get max fan (automantically), non-recirculate (automatically), max heating (automatically) and AC on (automaticaly).
Since in DEF you have both max heating and AC on - you will not get the max temperatue possible for defrosting... what you do get is maximum de-humidification - that is what this mode if for - and it really works - really fast.
If you have ever had the windshield fog rapidly at high speed you will understand why this is hugely valuable... and needs to be quick & easy to select - you have lots else to do... like triangulating out the windows to try to stay on the road...
To the original poster - its quite likely you haven't solved your issue. To me its unlikely the DEF relay caused this problem (possible but unlikely)... much more likely on your vintage 928 is the magic blower syndrome - the thermal bypass mechanism built into the blower resistor pack... once it starts it usually gets worse - but is often quite intermittent initially. It may mean your blower motor/fan is clogged with leaves or is generally faling OR it could just be the resistor pack is damaged.
Test - if you pull the resistor pack does it still happen? if not its the resistor pack - if yes it was indeed the DEF relay.
Alan
If you want to defrost the windshield set temp to max, AC off and fan to max and air flaps to defroster windshield vent only and close the center console vent flap (if manual). This gets you max windshield flow at max heating
The DEF mode on the other hand is in case the windshield suddenly fogs on the inside. Select DEF and you get max fan (automantically), non-recirculate (automatically), max heating (automatically) and AC on (automaticaly).
Since in DEF you have both max heating and AC on - you will not get the max temperatue possible for defrosting... what you do get is maximum de-humidification - that is what this mode if for - and it really works - really fast.
If you have ever had the windshield fog rapidly at high speed you will understand why this is hugely valuable... and needs to be quick & easy to select - you have lots else to do... like triangulating out the windows to try to stay on the road...
To the original poster - its quite likely you haven't solved your issue. To me its unlikely the DEF relay caused this problem (possible but unlikely)... much more likely on your vintage 928 is the magic blower syndrome - the thermal bypass mechanism built into the blower resistor pack... once it starts it usually gets worse - but is often quite intermittent initially. It may mean your blower motor/fan is clogged with leaves or is generally faling OR it could just be the resistor pack is damaged.
Test - if you pull the resistor pack does it still happen? if not its the resistor pack - if yes it was indeed the DEF relay.
Alan
) problem returned. Not the relay. I'm taking a closer look at the circuit diagram, and it looks like I need to see why pin 11 on the A/C control unit is going hot when it shouldn't. My working hypothesis (fancy for guess) is that a diode has gone fail-short in this control, maybe the blocking diode from pin 7 (from the A/C push button switch circuit), and that was triggering relay VIII. Another reason for suspecting this is that before it failed completely, when the blower would only occasionally go WOT, I could stop it by turning on the A/C circuit (push button switch) and turning it back off. So actually I might have two things going on, bad switch and failed diode.
So here's where I am: resistor pack not the problem, taking relay VIII out blower does not operate, therefore something is causing relay to operate, looks like it could be a diode in the control unit, which is next step. If the freezing rain we are having here stops I'll feel more like going outside and pulling it, in any case I'll report when I think I know something.
Alan, thanks for your input, if you have any other ideas after reading this please post them! And your explanation of how the defogging (I agree- best term) works - and it is just about the best/fastest I have ever used -is why I need to get this darned thing fixed!
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10-21-2021, 12:14 AM
#8
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What was your solution?
That's what I looked at first, by taking the resistor pack out so I could observe if it was the problem as I turned things on and off - it wasn't. And I rushed to judgment that it was the relay because when I pulled it, the problem went away. Put a known good relay in (after I posted of course ) problem returned. Not the relay.
I'm taking a closer look at the circuit diagram, and it looks like I need to see why pin 11 on the A/C control unit is going hot when it shouldn't. My working hypothesis (fancy for guess) is that a diode has gone fail-short in this control, maybe the blocking diode from pin 7 (from the A/C push button switch circuit), and that was triggering relay VIII. Another reason for suspecting this is that before it failed completely, when the blower would only occasionally go WOT, I could stop it by turning on the A/C circuit (push button switch) and turning it back off. So actually I might have two things going on, bad switch and failed diode.
So here's where I am: resistor pack not the problem, taking relay VIII out blower does not operate, therefore something is causing relay to operate, looks like it could be a diode in the control unit, which is next step. If the freezing rain we are having here stops I'll feel more like going outside and pulling it, in any case I'll report when I think I know something.
Alan, thanks for your input, if you have any other ideas after reading this please post them! And your explanation of how the defogging (I agree- best term) works - and it is just about the best/fastest I have ever used -is why I need to get this darned thing fixed!
) problem returned. Not the relay.I'm taking a closer look at the circuit diagram, and it looks like I need to see why pin 11 on the A/C control unit is going hot when it shouldn't. My working hypothesis (fancy for guess) is that a diode has gone fail-short in this control, maybe the blocking diode from pin 7 (from the A/C push button switch circuit), and that was triggering relay VIII. Another reason for suspecting this is that before it failed completely, when the blower would only occasionally go WOT, I could stop it by turning on the A/C circuit (push button switch) and turning it back off. So actually I might have two things going on, bad switch and failed diode.
So here's where I am: resistor pack not the problem, taking relay VIII out blower does not operate, therefore something is causing relay to operate, looks like it could be a diode in the control unit, which is next step. If the freezing rain we are having here stops I'll feel more like going outside and pulling it, in any case I'll report when I think I know something.
Alan, thanks for your input, if you have any other ideas after reading this please post them! And your explanation of how the defogging (I agree- best term) works - and it is just about the best/fastest I have ever used -is why I need to get this darned thing fixed!
10-21-2021, 01:05 AM
#9
Rennlist Member
Take a look at your blower resistor pack under the cowl. It's held in by two screws. If it goes bad you will only have high speed on your fan, which is the default position for the blower pack. Look for burned, broken coils or pieces falling out of it. I chased this issue at great expense recently all to find out it was a bum resistor pack that was only four years old.
10-21-2021, 01:17 AM
#10
Team Owner
NOTE the resistor pack is letting you know that the blower motor is taking more juice to run,
best to replace the blower motor before it destroys the resistor pack.
Roger has a mostly plug and play unit depending on the year,.
NOTE my 82 had the motor lower bushing seize,
this ripped out the bushing mount and cooked the resistor pack.
best to replace the blower motor before it destroys the resistor pack.
Roger has a mostly plug and play unit depending on the year,.
NOTE my 82 had the motor lower bushing seize,
this ripped out the bushing mount and cooked the resistor pack.
10-21-2021, 12:08 PM
#11
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Sure sounds like the resistor pack failure mode - I believe the old style resistor pack has been superseded - check with Roger. If so you can use the newer S4 one but need to change the connections slightly - see the threads on this with full details on how
At least check the blower motor as Stan says - if its clogged up with leaves fix that - if its noisy or doesn't run smoothly - replace. Check for other airflow issues - esp. evaporator fins all clogged up.
Alan
At least check the blower motor as Stan says - if its clogged up with leaves fix that - if its noisy or doesn't run smoothly - replace. Check for other airflow issues - esp. evaporator fins all clogged up.
Alan
