The definitive Heat / AC / Blower Thread...
08-01-2008, 06:53 PM
#1
Burning Brakes
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The definitive Heat / AC / Blower Thread...
After reading tons of threads from doing searches and Adrian's book, I am somewhat smarter, but now have more questions...
Can some of the experts chime in here so we all have a place to refer..?
Rear (Electric) Heater Blower - I understand that it runs intermittently based on temperature read by the NTC sensor in the blower outlet tube. Can someone explain the logic? Is it to blow out heated air from the engine compartment? Or only to add volume to air going through the heater boxes?
What is the logic behind having the Heater Blower draw from engine cooling air instead of ambient air? It just makes sense to me that if you have an aux source of air for cockpit heating, all of the engine fan air could (should) be utilized for engine cooling...
Adrian's book says the rear blower aids engine cooling - does that only apply to the phase of operation after the engine is shut down and the temp (if temps are high enough) causes the blower to come on?
The small resistor downstream from the NTC - it causes the blower to run at slower speeds, can someone provide more detail as to the logic? If it is simply temp based, won't the speeds just continually cycle from high to low?
What I don't understand (clearly, there's plenty..) - If the heater blower kicks on when the temp goes UP, doesn't that also draw cooling air away from the engine (driven) fan? The air for the Heater Blower comes from inside the cooling cowling, no? When it's hot out, that should be bad, right?
I also read (over and over) that the two front fans' operation is tied to the rear blower's operation. That is, when the temperature control **** is set to anything other than full cold ("Blue Dot"), that these fans will only operate if the rear fan is operating. So if the rear fan is switched off by the NTC control while driving, full control of the cockpit fans is lost - unless the **** is rotated full cold. That doesn't make sense...
Here's what happened that's caused my confusion -
I've had my car for abt 6 months, and it seemed to be working fine. The heat and AC blower seemed weak (adjusting the blower speed WOULD increase the fan speed - just not by much), but I took this to be a "Porsche-ism". Would stay cool when warm, and was warm enough in the winter.
While doing other work on the car last night, I checked the rear fuses (for the first time), and discovered the #1, 30A fuse was blown. It may have been blown from day one. The oil temp has reached 9 o'clock maybe once - generally runs cool.
So I replace the fuse. About two mins into a short test drive (of the other items fixed), I'm hit in the face with a blast of air. Tremendous amount of air coming out of the vents - consistent with a real car's fan power.. Then it's gone, then it comes and goes. Checking the rear blower, it's running (didn't re-blow the fuse
). Why does the high speed mode of the front fans operate intermittently?
If there is a definitive thread, or reference - I didn't see it. Apologies for the long post...
Can some of the experts chime in here so we all have a place to refer..?
Rear (Electric) Heater Blower - I understand that it runs intermittently based on temperature read by the NTC sensor in the blower outlet tube. Can someone explain the logic? Is it to blow out heated air from the engine compartment? Or only to add volume to air going through the heater boxes?
What is the logic behind having the Heater Blower draw from engine cooling air instead of ambient air? It just makes sense to me that if you have an aux source of air for cockpit heating, all of the engine fan air could (should) be utilized for engine cooling...
Adrian's book says the rear blower aids engine cooling - does that only apply to the phase of operation after the engine is shut down and the temp (if temps are high enough) causes the blower to come on?
The small resistor downstream from the NTC - it causes the blower to run at slower speeds, can someone provide more detail as to the logic? If it is simply temp based, won't the speeds just continually cycle from high to low?
What I don't understand (clearly, there's plenty..) - If the heater blower kicks on when the temp goes UP, doesn't that also draw cooling air away from the engine (driven) fan? The air for the Heater Blower comes from inside the cooling cowling, no? When it's hot out, that should be bad, right?
I also read (over and over) that the two front fans' operation is tied to the rear blower's operation. That is, when the temperature control **** is set to anything other than full cold ("Blue Dot"), that these fans will only operate if the rear fan is operating. So if the rear fan is switched off by the NTC control while driving, full control of the cockpit fans is lost - unless the **** is rotated full cold. That doesn't make sense...
Here's what happened that's caused my confusion -
I've had my car for abt 6 months, and it seemed to be working fine. The heat and AC blower seemed weak (adjusting the blower speed WOULD increase the fan speed - just not by much), but I took this to be a "Porsche-ism". Would stay cool when warm, and was warm enough in the winter.
While doing other work on the car last night, I checked the rear fuses (for the first time), and discovered the #1, 30A fuse was blown. It may have been blown from day one. The oil temp has reached 9 o'clock maybe once - generally runs cool.
So I replace the fuse. About two mins into a short test drive (of the other items fixed), I'm hit in the face with a blast of air. Tremendous amount of air coming out of the vents - consistent with a real car's fan power.. Then it's gone, then it comes and goes. Checking the rear blower, it's running (didn't re-blow the fuse
). Why does the high speed mode of the front fans operate intermittently?If there is a definitive thread, or reference - I didn't see it. Apologies for the long post...
08-01-2008, 07:52 PM
#2
Burning Brakes
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Rear (Electric) Heater Blower - I understand that it runs intermittently based on temperature read by the NTC sensor in the blower outlet tube. Can someone explain the logic? Is it to blow out heated air from the engine compartment? Or only to add volume to air going through the heater boxes?
Both - if there is a demand for the air through the heater boxes thats where it will go, if not it gets dumped out thru the wheel arches.
What is the logic behind having the Heater Blower draw from engine cooling air instead of ambient air? It just makes sense to me that if you have an aux source of air for cockpit heating, all of the engine fan air could (should) be utilized for engine cooling...
Technically, it is ambient air, as the engine fan sucks it in from the outside world. The engine fan provides a small amount of flow up to the cabin, the heater fan is more of a booster fan than anything.
Adrian's book says the rear blower aids engine cooling - does that only apply to the phase of operation after the engine is shut down and the temp (if temps are high enough) causes the blower to come on?
it does, as the engine cooling fan blows over the cylinders to cool, the heater fan blows air thru the collectors, effectively cooling them. if the heat is not needed in the cabin, it is dumped out the arches as mentioned before.
The small resistor downstream from the NTC - it causes the blower to run at slower speeds, can someone provide more detail as to the logic? If it is simply temp based, won't the speeds just continually cycle from high to low?
The resistor i think is what allows it to run at high speed, the voltage is shared between the fan and resistor. its sited where it is as it gets hot, so the flowing air keeps it cool.
What I don't understand (clearly, there's plenty..) - If the heater blower kicks on when the temp goes UP, doesn't that also draw cooling air away from the engine (driven) fan? The air for the Heater Blower comes from inside the cooling cowling, no? When it's hot out, that should be bad, right?
no as i mentioned in the above that it also cools the engine by blowing air over the collectors
I also read (over and over) that the two front fans' operation is tied to the rear blower's operation. That is, when the temperature control **** is set to anything other than full cold ("Blue Dot"), that these fans will only operate if the rear fan is operating. So if the rear fan is switched off by the NTC control while driving, full control of the cockpit fans is lost - unless the **** is rotated full cold. That doesn't make sense...
i dont have a rear fan or bypass and my front fans still work...
Here's what happened that's caused my confusion -
I've had my car for abt 6 months, and it seemed to be working fine. The heat and AC blower seemed weak (adjusting the blower speed WOULD increase the fan speed - just not by much), but I took this to be a "Porsche-ism". Would stay cool when warm, and was warm enough in the winter.
While doing other work on the car last night, I checked the rear fuses (for the first time), and discovered the #1, 30A fuse was blown. It may have been blown from day one. The oil temp has reached 9 o'clock maybe once - generally runs cool.
So I replace the fuse. About two mins into a short test drive (of the other items fixed), I'm hit in the face with a blast of air. Tremendous amount of air coming out of the vents - consistent with a real car's fan power.. Then it's gone, then it comes and goes. Checking the rear blower, it's running (didn't re-blow the fuse). Why does the high speed mode of the front fans operate intermittently?
If there is a definitive thread, or reference - I didn't see it. Apologies for the long post...
oil cooler fan \ cooling is a totally seperate control... independant of the CCU
Both - if there is a demand for the air through the heater boxes thats where it will go, if not it gets dumped out thru the wheel arches.
What is the logic behind having the Heater Blower draw from engine cooling air instead of ambient air? It just makes sense to me that if you have an aux source of air for cockpit heating, all of the engine fan air could (should) be utilized for engine cooling...
Technically, it is ambient air, as the engine fan sucks it in from the outside world. The engine fan provides a small amount of flow up to the cabin, the heater fan is more of a booster fan than anything.
Adrian's book says the rear blower aids engine cooling - does that only apply to the phase of operation after the engine is shut down and the temp (if temps are high enough) causes the blower to come on?
it does, as the engine cooling fan blows over the cylinders to cool, the heater fan blows air thru the collectors, effectively cooling them. if the heat is not needed in the cabin, it is dumped out the arches as mentioned before.
The small resistor downstream from the NTC - it causes the blower to run at slower speeds, can someone provide more detail as to the logic? If it is simply temp based, won't the speeds just continually cycle from high to low?
The resistor i think is what allows it to run at high speed, the voltage is shared between the fan and resistor. its sited where it is as it gets hot, so the flowing air keeps it cool.
What I don't understand (clearly, there's plenty..) - If the heater blower kicks on when the temp goes UP, doesn't that also draw cooling air away from the engine (driven) fan? The air for the Heater Blower comes from inside the cooling cowling, no? When it's hot out, that should be bad, right?
no as i mentioned in the above that it also cools the engine by blowing air over the collectors
I also read (over and over) that the two front fans' operation is tied to the rear blower's operation. That is, when the temperature control **** is set to anything other than full cold ("Blue Dot"), that these fans will only operate if the rear fan is operating. So if the rear fan is switched off by the NTC control while driving, full control of the cockpit fans is lost - unless the **** is rotated full cold. That doesn't make sense...
i dont have a rear fan or bypass and my front fans still work...
Here's what happened that's caused my confusion -
I've had my car for abt 6 months, and it seemed to be working fine. The heat and AC blower seemed weak (adjusting the blower speed WOULD increase the fan speed - just not by much), but I took this to be a "Porsche-ism". Would stay cool when warm, and was warm enough in the winter.
While doing other work on the car last night, I checked the rear fuses (for the first time), and discovered the #1, 30A fuse was blown. It may have been blown from day one. The oil temp has reached 9 o'clock maybe once - generally runs cool.
So I replace the fuse. About two mins into a short test drive (of the other items fixed), I'm hit in the face with a blast of air. Tremendous amount of air coming out of the vents - consistent with a real car's fan power.. Then it's gone, then it comes and goes. Checking the rear blower, it's running (didn't re-blow the fuse
). Why does the high speed mode of the front fans operate intermittently?If there is a definitive thread, or reference - I didn't see it. Apologies for the long post...
oil cooler fan \ cooling is a totally seperate control... independant of the CCU
08-02-2008, 03:30 AM
#4
Three Wheelin'
For a data point - I removed the rear blower on mine for a day while working on the rear shocks. If I turned the temp dial ~10 degrees off the blue dot, then both front fans would stop entirely and I got zero air through the vents. Otherwise, the ccu worked as normal.
My dash blowers have both been replaced and I recently found that the hose to the left-most vent was unattached under the dash. With it attached, the volume of air moving through the vents is comparable to a modern car. It just doesn't blow anywhere near my body, no matter what the vent angle is!
My dash blowers have both been replaced and I recently found that the hose to the left-most vent was unattached under the dash. With it attached, the volume of air moving through the vents is comparable to a modern car. It just doesn't blow anywhere near my body, no matter what the vent angle is!
08-02-2008, 08:00 AM
#5
Burning Brakes
Nick - I have the same problem. It is 95 degrees outside these days and cannot get the vents to point to me. So I have to lean to the right to get a/c air to my face.
08-02-2008, 02:21 PM
#6
Three Wheelin'
There are a lot of questions here and Matt (Demonfish) has covered most of them correctly although there are one or two discrepancies.
The rear blower fan is operated solely and entirely by the CCU. The CCU makes the decisions about when the rear blower (and, indeed, the cabin fans and the oil cooler and air con condenser fan too). The CCU will switch the ballast resistor into circuit when the slow speed is required (the ballast resistor reduces the voltage applied to the fan motor, reducing the speed). It is switched out of circuit when the high speed is required.
The NTC sensor has nothing to do with the operation of the rear blower for normal heating purposes and is not functional when the engine is running. It's only purpose is to monitor the temperature of the engine cooling air (i.e. that coming from the belt driven fan on the alternator shaft via the engine cylinder cooling chamber) when the ignition is turned off. The NTC sensor is sampled by the CCU and if the air temperature is above a set limit the CCU will activate the rear blower fan for (I think) 15 minutes to aid the engine cooling without the ignition being switched on.
The cabin fans will normally be switched off (by the CCU) if there is insufficient air flow from the rear heat exchangers. If the mixing chamber flaps are closed the build up of air pressure in the plenums will cause the pressure differential flaps to flip over to divert unwanted hot air from the heat exchangers to the rear wheel wells.
Nobody said the 964 heating system wasn't complex!!!
Regards
Dave
The rear blower fan is operated solely and entirely by the CCU. The CCU makes the decisions about when the rear blower (and, indeed, the cabin fans and the oil cooler and air con condenser fan too). The CCU will switch the ballast resistor into circuit when the slow speed is required (the ballast resistor reduces the voltage applied to the fan motor, reducing the speed). It is switched out of circuit when the high speed is required.
The NTC sensor has nothing to do with the operation of the rear blower for normal heating purposes and is not functional when the engine is running. It's only purpose is to monitor the temperature of the engine cooling air (i.e. that coming from the belt driven fan on the alternator shaft via the engine cylinder cooling chamber) when the ignition is turned off. The NTC sensor is sampled by the CCU and if the air temperature is above a set limit the CCU will activate the rear blower fan for (I think) 15 minutes to aid the engine cooling without the ignition being switched on.
The cabin fans will normally be switched off (by the CCU) if there is insufficient air flow from the rear heat exchangers. If the mixing chamber flaps are closed the build up of air pressure in the plenums will cause the pressure differential flaps to flip over to divert unwanted hot air from the heat exchangers to the rear wheel wells.
Nobody said the 964 heating system wasn't complex!!!
Regards
Dave
