AC Evacuation and Charging issues
04-08-2019, 03:25 PM
#46
Rennlist Member
oh boy...… a long time ago, when I noticed mine did not cycle, I asked someone of much greater 928 knowledge and was told the clutch and compressor are always engaged and always on with the dash AC button depressed.
what component is responsible for cycling the compressor?
what component is responsible for cycling the compressor?
Dave
04-08-2019, 03:31 PM
#47
Three Wheelin'
Thread Starter
since my system is not cooling enough, perhaps my freeze switch is not being triggered, and thus, compressor always on?
04-08-2019, 03:45 PM
#48
Rennlist Member
In 25 years of ownership of a dual a/c 928,I too have found that the use of the rear unit decreases the efficiency of the front unit,increases discharge temp of the front unit and raises the operating head pressure.I too also find it of little actual use,except to increase the amount of refrigerant that I have to purchase to keep it full.IF I use the 928 on a sunny summer day(unlikely as my 400 dollar beater blazer blows ice cubes in a matter of a minute),I do use the rearr just to move more air thru evaporators to bring interior temps below 90 degrees and then switch off the rear so that the air discharge temp drops in to the mid 30's at the center vent.
This vehicle,like a lot of Porsche vehicles designed in the 70's have a/c that is really just an afterthought.Any one here ever see the head pressure on a air cooled 70's,80's 911 sitting still on a 90 degree day ?Frightening! 350 maybe 400 Lbs on the Hi side gauge.
This vehicle,like a lot of Porsche vehicles designed in the 70's have a/c that is really just an afterthought.Any one here ever see the head pressure on a air cooled 70's,80's 911 sitting still on a 90 degree day ?Frightening! 350 maybe 400 Lbs on the Hi side gauge.
That doesnt make sense, the fill is the fill. It's a connected system even if you dont use it, unless you entirely remove it. The the fill is "less" because the system volume is smaller.
If you have to "keep it full" then "fix the leak".
What you say about using the rear is true..agreed.
04-08-2019, 03:47 PM
#49
Rennlist Member
Hi Fixer,
You are right on both counts. The limiting factor is usually condenser capacity, which involves surface area, design and air flow exchange. The evaporator fan in the 911 is also undersized.
Both problems can be partially addressed with parallel flow heat exchangers.
Good luck,
Dave
You are right on both counts. The limiting factor is usually condenser capacity, which involves surface area, design and air flow exchange. The evaporator fan in the 911 is also undersized.
Both problems can be partially addressed with parallel flow heat exchangers.
Good luck,
Dave
Id buy it tomorrow...and I'd still stay on R12.
04-08-2019, 03:54 PM
#50
Rennlist Member
thanks dave. I installed new freeze switch (tested and adjusted) a few months back when I installed an external relay in my head unit. also installed an inline fuse holder, as the community prescribed, with 4 amp fuse.
since my system is not cooling enough, perhaps my freeze switch is not being triggered, and thus, compressor always on?
since my system is not cooling enough, perhaps my freeze switch is not being triggered, and thus, compressor always on?
Its -not- the freeze switch. Its not the thermostat, its not the Temp2 sender...stop, stop, stop.
Do you have a WSM, did you read the section about HVAC, have you researched any of the highly informative 928 HVAC WWW page how-to's? It's OK if you have not, but then you need to listen to us here.
We can only help as you as much as you stop inventing problems to discuss.
You don't have an accurate fill, do that, or find someone who can...and I'm SURE you will be fine. (DIY [you did that and it didn't go well and thats OK], or take it to a shop to evacuate, validate, and fill)
There is nothing to diagnose, you need to start over at the step labeled "Vacuum pump".
04-08-2019, 04:20 PM
#51
Rennlist Member
I agree with above!!
I just reread your original post. The charge of your system did not go well. You don't really know how much R134 you have. You should definitely have the system recovered, vacuumed and refilled by weight at 80% of the R12 amount. When this has been done, I believe you will see numbers similar to the WSM. Your freeze switch will start working, because now the system is cooling well. Most indy car repair places will perform an evac/recharge with an automatic machine for <$200. They will also be able to give a general idea of system function, leaks, etc. Charge by weight is definitely the way to go!
Good luck,
Dave
I just reread your original post. The charge of your system did not go well. You don't really know how much R134 you have. You should definitely have the system recovered, vacuumed and refilled by weight at 80% of the R12 amount. When this has been done, I believe you will see numbers similar to the WSM. Your freeze switch will start working, because now the system is cooling well. Most indy car repair places will perform an evac/recharge with an automatic machine for <$200. They will also be able to give a general idea of system function, leaks, etc. Charge by weight is definitely the way to go!
Good luck,
Dave
04-08-2019, 06:13 PM
#52
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Paul --
Run the engine at 1500-2000 RPM and note the ambient temp, the system pressures after 5 minutes, the center-vent temp at the same time, and the temperature at the passenger's feet. Interior fan speed set at 2, slider at full-cold, and your armrest vents closed. The rear system is off, and the freeze switch is jumpered. The RH or dew point number may help. Share those numbers here please.
Watch the sight glass as you run the car this way. A good charge level will have a few vapor bubbles visible in the glass during the test. The suction-side pressure should correspond to a temperature 40-50 degrees below the footwell temp unless it's really hot and humid. During hot-and-humid and with the car closed, interior humidity and therefore interior temp should drop over time. Those side-read thermometers are so you can see the readings with the doors closed.
The system is required to work under a wide range of operating conditions, with compressor speed changing with engine RPM, in the sun or the shade, cool or hot outside, one to four occupants, at varying condenser airflows. The WSM charge level is a good guide but is not absolute. finding the correct balance around all those operating variables starts with the WSM charge level, but may also require fine tuning. The expansion valve does a pretty impressive job of managing the system balance.
Undercharging limits available heat transfer from the evaporator, as it sees a partial flow of refrigerant that's already flashed/boiled to vapor before it gets there. Overcharging raises the suction pressure in the system, effectively raising the boiling point of the refrigerant and therefore offering a warmer evaporator and less heat removal capacity. Overcharging raises the pressures in the whole system, a particular concern for compressor efficiency and survival. Losing condenser heat removal capacity is limited by the high-side pressure switch, and ultimately a blowout plug in the drier. We don't depend on them as regular controlling elements; they are safety devices only.
Grab some readings and share them here. There are some system experts here that can help you find just the right balance.
Run the engine at 1500-2000 RPM and note the ambient temp, the system pressures after 5 minutes, the center-vent temp at the same time, and the temperature at the passenger's feet. Interior fan speed set at 2, slider at full-cold, and your armrest vents closed. The rear system is off, and the freeze switch is jumpered. The RH or dew point number may help. Share those numbers here please.
Watch the sight glass as you run the car this way. A good charge level will have a few vapor bubbles visible in the glass during the test. The suction-side pressure should correspond to a temperature 40-50 degrees below the footwell temp unless it's really hot and humid. During hot-and-humid and with the car closed, interior humidity and therefore interior temp should drop over time. Those side-read thermometers are so you can see the readings with the doors closed.
The system is required to work under a wide range of operating conditions, with compressor speed changing with engine RPM, in the sun or the shade, cool or hot outside, one to four occupants, at varying condenser airflows. The WSM charge level is a good guide but is not absolute. finding the correct balance around all those operating variables starts with the WSM charge level, but may also require fine tuning. The expansion valve does a pretty impressive job of managing the system balance.
Undercharging limits available heat transfer from the evaporator, as it sees a partial flow of refrigerant that's already flashed/boiled to vapor before it gets there. Overcharging raises the suction pressure in the system, effectively raising the boiling point of the refrigerant and therefore offering a warmer evaporator and less heat removal capacity. Overcharging raises the pressures in the whole system, a particular concern for compressor efficiency and survival. Losing condenser heat removal capacity is limited by the high-side pressure switch, and ultimately a blowout plug in the drier. We don't depend on them as regular controlling elements; they are safety devices only.
Grab some readings and share them here. There are some system experts here that can help you find just the right balance.
04-08-2019, 07:21 PM
#53
Rennlist Member
Paul --
Run the engine at 1500-2000 RPM and note the ambient temp, the system pressures after 5 minutes, the center-vent temp at the same time, and the temperature at the passenger's feet. Interior fan speed set at 2, slider at full-cold, and your armrest vents closed. The rear system is off, and the freeze switch is jumpered. The RH or dew point number may help. Share those numbers here please.
Watch the sight glass as you run the car this way. A good charge level will have a few vapor bubbles visible in the glass during the test. The suction-side pressure should correspond to a temperature 40-50 degrees below the footwell temp unless it's really hot and humid. During hot-and-humid and with the car closed, interior humidity and therefore interior temp should drop over time. Those side-read thermometers are so you can see the readings with the doors closed.
The system is required to work under a wide range of operating conditions, with compressor speed changing with engine RPM, in the sun or the shade, cool or hot outside, one to four occupants, at varying condenser airflows. The WSM charge level is a good guide but is not absolute. finding the correct balance around all those operating variables starts with the WSM charge level, but may also require fine tuning. The expansion valve does a pretty impressive job of managing the system balance.
Undercharging limits available heat transfer from the evaporator, as it sees a partial flow of refrigerant that's already flashed/boiled to vapor before it gets there. Overcharging raises the suction pressure in the system, effectively raising the boiling point of the refrigerant and therefore offering a warmer evaporator and less heat removal capacity. Overcharging raises the pressures in the whole system, a particular concern for compressor efficiency and survival. Losing condenser heat removal capacity is limited by the high-side pressure switch, and ultimately a blowout plug in the drier. We don't depend on them as regular controlling elements; they are safety devices only.
Grab some readings and share them here. There are some system experts here that can help you find just the right balance.
Run the engine at 1500-2000 RPM and note the ambient temp, the system pressures after 5 minutes, the center-vent temp at the same time, and the temperature at the passenger's feet. Interior fan speed set at 2, slider at full-cold, and your armrest vents closed. The rear system is off, and the freeze switch is jumpered. The RH or dew point number may help. Share those numbers here please.
Watch the sight glass as you run the car this way. A good charge level will have a few vapor bubbles visible in the glass during the test. The suction-side pressure should correspond to a temperature 40-50 degrees below the footwell temp unless it's really hot and humid. During hot-and-humid and with the car closed, interior humidity and therefore interior temp should drop over time. Those side-read thermometers are so you can see the readings with the doors closed.
The system is required to work under a wide range of operating conditions, with compressor speed changing with engine RPM, in the sun or the shade, cool or hot outside, one to four occupants, at varying condenser airflows. The WSM charge level is a good guide but is not absolute. finding the correct balance around all those operating variables starts with the WSM charge level, but may also require fine tuning. The expansion valve does a pretty impressive job of managing the system balance.
Undercharging limits available heat transfer from the evaporator, as it sees a partial flow of refrigerant that's already flashed/boiled to vapor before it gets there. Overcharging raises the suction pressure in the system, effectively raising the boiling point of the refrigerant and therefore offering a warmer evaporator and less heat removal capacity. Overcharging raises the pressures in the whole system, a particular concern for compressor efficiency and survival. Losing condenser heat removal capacity is limited by the high-side pressure switch, and ultimately a blowout plug in the drier. We don't depend on them as regular controlling elements; they are safety devices only.
Grab some readings and share them here. There are some system experts here that can help you find just the right balance.
If its fine, who cares what the inside is if it's not all healthy, its just another huge distraction from getting -this- part right.
??
04-08-2019, 08:06 PM
#54
Three Wheelin'
Thread Starter
Paul --
Run the engine at 1500-2000 RPM and note the ambient temp, the system pressures after 5 minutes, the center-vent temp at the same time, and the temperature at the passenger's feet. Interior fan speed set at 2, slider at full-cold, and your armrest vents closed. The rear system is off, and the freeze switch is jumpered. The RH or dew point number may help. Share those numbers here please.
Watch the sight glass as you run the car this way. A good charge level will have a few vapor bubbles visible in the glass during the test. The suction-side pressure should correspond to a temperature 40-50 degrees below the footwell temp unless it's really hot and humid. During hot-and-humid and with the car closed, interior humidity and therefore interior temp should drop over time. Those side-read thermometers are so you can see the readings with the doors closed.
The system is required to work under a wide range of operating conditions, with compressor speed changing with engine RPM, in the sun or the shade, cool or hot outside, one to four occupants, at varying condenser airflows. The WSM charge level is a good guide but is not absolute. finding the correct balance around all those operating variables starts with the WSM charge level, but may also require fine tuning. The expansion valve does a pretty impressive job of managing the system balance.
Undercharging limits available heat transfer from the evaporator, as it sees a partial flow of refrigerant that's already flashed/boiled to vapor before it gets there. Overcharging raises the suction pressure in the system, effectively raising the boiling point of the refrigerant and therefore offering a warmer evaporator and less heat removal capacity. Overcharging raises the pressures in the whole system, a particular concern for compressor efficiency and survival. Losing condenser heat removal capacity is limited by the high-side pressure switch, and ultimately a blowout plug in the drier. We don't depend on them as regular controlling elements; they are safety devices only.
Grab some readings and share them here. There are some system experts here that can help you find just the right balance.
Run the engine at 1500-2000 RPM and note the ambient temp, the system pressures after 5 minutes, the center-vent temp at the same time, and the temperature at the passenger's feet. Interior fan speed set at 2, slider at full-cold, and your armrest vents closed. The rear system is off, and the freeze switch is jumpered. The RH or dew point number may help. Share those numbers here please.
Watch the sight glass as you run the car this way. A good charge level will have a few vapor bubbles visible in the glass during the test. The suction-side pressure should correspond to a temperature 40-50 degrees below the footwell temp unless it's really hot and humid. During hot-and-humid and with the car closed, interior humidity and therefore interior temp should drop over time. Those side-read thermometers are so you can see the readings with the doors closed.
The system is required to work under a wide range of operating conditions, with compressor speed changing with engine RPM, in the sun or the shade, cool or hot outside, one to four occupants, at varying condenser airflows. The WSM charge level is a good guide but is not absolute. finding the correct balance around all those operating variables starts with the WSM charge level, but may also require fine tuning. The expansion valve does a pretty impressive job of managing the system balance.
Undercharging limits available heat transfer from the evaporator, as it sees a partial flow of refrigerant that's already flashed/boiled to vapor before it gets there. Overcharging raises the suction pressure in the system, effectively raising the boiling point of the refrigerant and therefore offering a warmer evaporator and less heat removal capacity. Overcharging raises the pressures in the whole system, a particular concern for compressor efficiency and survival. Losing condenser heat removal capacity is limited by the high-side pressure switch, and ultimately a blowout plug in the drier. We don't depend on them as regular controlling elements; they are safety devices only.
Grab some readings and share them here. There are some system experts here that can help you find just the right balance.
will report all pertinent numbers in a few days; for my clarification, the freeze switch is designed to "freeze" and then it shuts down the compressor? and then "thaws" and the cycle starts again?
can't be a good thing that my compressor was constantly running? I had '67 Lincoln continental, and if I remember correctly, that compressor never cycled, that's why I never questioned the 928.
04-08-2019, 08:10 PM
#55
Rennlist Member
Like everyone is saying, the car won't be nice and cool on hot days, unless the refrigeration system and the air distribution system are both working. So far I have been addressing the refrigeration system, since this thread started with a recharge. Once that is sorted out, we can move on to fan, vents, doors and the heater valve.
Good luck,
Dave
Good luck,
Dave
04-08-2019, 08:12 PM
#56
Rennlist Member
for my clarification, the freeze switch is designed to "freeze" and then it shuts down the compressor? and then "thaws" and the cycle starts again?
can't be a good thing that my compressor was constantly running? I had '67 Lincoln continental, and if I remember correctly, that compressor never cycled, that's why I never questioned the 928.
can't be a good thing that my compressor was constantly running? I had '67 Lincoln continental, and if I remember correctly, that compressor never cycled, that's why I never questioned the 928.
If it never switches off because it doesnt freeze up, that's ok too.
04-08-2019, 08:21 PM
#58
Archive Gatekeeper
Rennlist Member
Rennlist Member
No.
04-08-2019, 09:10 PM
#59
Rennlist Member
dr bob, thanks for the info. I was able to find a few minutes and just hooked up my rented vacuum pump. much smaller than my buddies, which was the size of a microwave, but this one is 2 CFM and came with a fresh bottle of oil, so no limitation on time running. my plan is to work alone; sometimes that is best...lol. going to fill according to GTS WSM specs which is 37 oz. that is about 91% of factory fill spec of 40.5. if anyone things I should start with a lower percentage, let me know. strongly feel that the first recharge was in error, and 2nd time will be the charm. will be **** with the weight scale.
will report all pertinent numbers in a few days; for my clarification, the freeze switch is designed to "freeze" and then it shuts down the compressor? and then "thaws" and the cycle starts again?
can't be a good thing that my compressor was constantly running? I had '67 Lincoln continental, and if I remember correctly, that compressor never cycled, that's why I never questioned the 928.
will report all pertinent numbers in a few days; for my clarification, the freeze switch is designed to "freeze" and then it shuts down the compressor? and then "thaws" and the cycle starts again?
can't be a good thing that my compressor was constantly running? I had '67 Lincoln continental, and if I remember correctly, that compressor never cycled, that's why I never questioned the 928.
Also, I would charge at 80% of R12, then evaluate performance. It's easier to add by weight, than to remove by weight.
Good luck,
Dave
04-08-2019, 09:29 PM
#60
If the evaporator core runs at or below 32F for long periods of time in high humidity ice will form on the outside of the core.
When ice forms heat transfer decreases, vent temps rise. Your observation would be the perfect system pressures, all flaps working as they should, but terrible vent temps.
The only way to determine if the external core is frozen is knowing the evap core temp.
If the low side of the system (evaporator back to the compressor) runs at lower than normal pressures (say, below 15 psi) for extended periods of time
you can damage the compressor. When the compressor is on (pumping; moving parts) it requires a constant flow of refrigerant/oil.
The refrigerant carries the oil through the system (hence the need to use a refrigerant oil that is compatible with the refrigerant you are using; with R134a its PAG or Ester),
constantly lubricating the compressor's moving parts. When refrigerant flows are reduced (low low side pressures), say by having not enough refrigerant in the system or when system pressures drop very
low on the low side, the compressor does not receive enough lubrication. Not enough lubrication causes friction, wear and heat, all of which are not good for the compressor; just like your engine.
Things that cause excessive low side pressures are:
Not enough refrigerant.
A TEV(thermal expansion valve) "issue"; either system debris, ice, or a mechanically failing TEV.
An evaporator temp sensor that is not turning off the compressor when it should.
Things that cause a TEV to 'fail' are:
Moisture freezing inside the TEV, system debris, such as a piece of desiccant from the drier, piece of o-ring, compressor debris that got past the drier, 'snotty oil' (typically old mineral oil that has waxed), and finally
a mechanical failure in the TEV. However, most 928 block style TEV's fail due to leakage (they leak from the inside out).
On a side note, oil lubrication to the compressor is critical. Although the factory 6E171 Denso compressor is quite a beast (a design carried over from the Continental and still used on John Deeres), steel sleeves, teflon piston ring, rugged wobble plate, hefty reed plate system) it still needs lube like any other compressor. Sharks with rear air turn on the flow of refrigerant/oil to the rear evaporator coil via a solenoid valve; its either open or closed. Unfortunately, the rear air circuit can be a trap for refrigerant oil. If the solenoid valve is not completely closed, say from years of running R12 with moisture in the system, the valve mechanism develops some corrosion, its possible that refrigerant will pass through it however not at its designed rate, call it a trickle.
Oil tends to hang back in that situation and can collect in the rear circuit which will lead to oil starvation; hence a worn or debris tossing compressor.
And, Dave is spot on with the starting charge volume, "It's easier to add by weight than to remove by weight." A multi-billion business, Weight Watchers!
If you start off with a volume of refrigerant that is less than the know factory recommended weight, and you observe your high side pressures are higher
than they should be, the first thing you want to consider is residual air in the system.
With regard to charging the system you have 2 choices, either vapor (can up) or liquid (can down). In terms of how much are you are putting in remember that by charging by liquid
your service lines can hold quite a few ounces. A good rule of thumb is 1 oz liquid for every 3 feet of 1/4" ID service line; hence if you have a common 72" long line you could have 2 ounces of liquid. If your line from the refrigerant canister is to the service manifold gauge set is 72" and your high side line from the service manifold gauge set to the high side service port on the compressor is 72", then you got approx 4 ounces of refrigerant in the line charging by the liquid method. Hence the need to know when to zero out your weight scale.
When ice forms heat transfer decreases, vent temps rise. Your observation would be the perfect system pressures, all flaps working as they should, but terrible vent temps.
The only way to determine if the external core is frozen is knowing the evap core temp.
If the low side of the system (evaporator back to the compressor) runs at lower than normal pressures (say, below 15 psi) for extended periods of time
you can damage the compressor. When the compressor is on (pumping; moving parts) it requires a constant flow of refrigerant/oil.
The refrigerant carries the oil through the system (hence the need to use a refrigerant oil that is compatible with the refrigerant you are using; with R134a its PAG or Ester),
constantly lubricating the compressor's moving parts. When refrigerant flows are reduced (low low side pressures), say by having not enough refrigerant in the system or when system pressures drop very
low on the low side, the compressor does not receive enough lubrication. Not enough lubrication causes friction, wear and heat, all of which are not good for the compressor; just like your engine.
Things that cause excessive low side pressures are:
Not enough refrigerant.
A TEV(thermal expansion valve) "issue"; either system debris, ice, or a mechanically failing TEV.
An evaporator temp sensor that is not turning off the compressor when it should.
Things that cause a TEV to 'fail' are:
Moisture freezing inside the TEV, system debris, such as a piece of desiccant from the drier, piece of o-ring, compressor debris that got past the drier, 'snotty oil' (typically old mineral oil that has waxed), and finally
a mechanical failure in the TEV. However, most 928 block style TEV's fail due to leakage (they leak from the inside out).
On a side note, oil lubrication to the compressor is critical. Although the factory 6E171 Denso compressor is quite a beast (a design carried over from the Continental and still used on John Deeres), steel sleeves, teflon piston ring, rugged wobble plate, hefty reed plate system) it still needs lube like any other compressor. Sharks with rear air turn on the flow of refrigerant/oil to the rear evaporator coil via a solenoid valve; its either open or closed. Unfortunately, the rear air circuit can be a trap for refrigerant oil. If the solenoid valve is not completely closed, say from years of running R12 with moisture in the system, the valve mechanism develops some corrosion, its possible that refrigerant will pass through it however not at its designed rate, call it a trickle.
Oil tends to hang back in that situation and can collect in the rear circuit which will lead to oil starvation; hence a worn or debris tossing compressor.
And, Dave is spot on with the starting charge volume, "It's easier to add by weight than to remove by weight." A multi-billion business, Weight Watchers!
If you start off with a volume of refrigerant that is less than the know factory recommended weight, and you observe your high side pressures are higher
than they should be, the first thing you want to consider is residual air in the system.
With regard to charging the system you have 2 choices, either vapor (can up) or liquid (can down). In terms of how much are you are putting in remember that by charging by liquid
your service lines can hold quite a few ounces. A good rule of thumb is 1 oz liquid for every 3 feet of 1/4" ID service line; hence if you have a common 72" long line you could have 2 ounces of liquid. If your line from the refrigerant canister is to the service manifold gauge set is 72" and your high side line from the service manifold gauge set to the high side service port on the compressor is 72", then you got approx 4 ounces of refrigerant in the line charging by the liquid method. Hence the need to know when to zero out your weight scale.
Last edited by griffiths; 04-09-2019 at 07:48 AM. Reason: Just had to