air conditioning pressure question
07-12-2007, 01:04 AM
#1
Racer
Thread Starter
air conditioning pressure question
I'm refocusing on my air conditioning issues. my question is does this system have the standard 134a pressures? which I'm told is around 40 low side 200 high side at idle, possibly another 20 psi at speed.
I've been having on going a/c problems; had two head units put in; second time needed a new solenoid which I was told fits into the compressor and blew the first head unit. It worked for a week or two then lost cooling capacity.
finally took it to a good general mechanic who does alot of a/c work;
the pressures were good. he thought that maybe the system wasn't drained and vaccumed which might have caused some air in the system.
he pulled a deep vaccum, added some fluoresent oil to see if there are any leaks, then refilled to 40 or 45 and 200. The system went from just a few degress lower than ambient before he did it, to much more. during the mid day houston (90 plus, I think) heat, it bottomed out at around 60-62 degrees, but it took some tome to get there. he said it should get to 55. It's not nearly as cold as other cars I have, and doesn't seem as cold as it used to be.
he thinks the compressor might be alittle weak, based on some movement of the pressure when the gages are on it.
the pressure would indicate that there are no restrictions, and it's working adequately, but not really well. any suggestions for other things to check?
I've been having on going a/c problems; had two head units put in; second time needed a new solenoid which I was told fits into the compressor and blew the first head unit. It worked for a week or two then lost cooling capacity.
finally took it to a good general mechanic who does alot of a/c work;
the pressures were good. he thought that maybe the system wasn't drained and vaccumed which might have caused some air in the system.
he pulled a deep vaccum, added some fluoresent oil to see if there are any leaks, then refilled to 40 or 45 and 200. The system went from just a few degress lower than ambient before he did it, to much more. during the mid day houston (90 plus, I think) heat, it bottomed out at around 60-62 degrees, but it took some tome to get there. he said it should get to 55. It's not nearly as cold as other cars I have, and doesn't seem as cold as it used to be.
he thinks the compressor might be alittle weak, based on some movement of the pressure when the gages are on it.
the pressure would indicate that there are no restrictions, and it's working adequately, but not really well. any suggestions for other things to check?
07-12-2007, 01:29 AM
#2
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The refrigerant pressures vary greatly with ambient temps...
You really need to look at pages 87-130 and 87-131 in the back of Vol IV.
Just as an example, the low-side pressures are closer to 15 psig than 40 psig, and the central outlet temps are close to 35 - 40 deg F.
This is on a system with R-134a.
If your system is in good mechanical condition, sounds as if you might be over-charged.
You really need to look at pages 87-130 and 87-131 in the back of Vol IV.
Just as an example, the low-side pressures are closer to 15 psig than 40 psig, and the central outlet temps are close to 35 - 40 deg F.
This is on a system with R-134a.
If your system is in good mechanical condition, sounds as if you might be over-charged.
07-12-2007, 05:51 AM
#3
Three Wheelin'
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Rick
Do you have the rear A/C option? With 134a i have heard this can be a problem.
Other issues that can kill the A/C is heat getting into the cabin. I had some heat coming from a small hole in the firewall and a lot of heat coming in the shifter. On a mildly warm day the A/C could not keep up on full. I was beginning to doubt the new charge. I then took some spray foam insulation and blocked off the heat leaks. Be careful not to use too much. I also made sure the hood gasket was on the fire wall and connecting with the hood and that the two rubber stoppers were in the fenders by the fire wall. Check the heater value to insure it is closing all the way.
After these changes the A/C drives me out even on very hot days. I have to cycle it between no fan and position one. Any higher and I freeze. Eliminating the heat from the shifter made a huge difference.
As far as filling the system I put in freon until the bubbles stop in the sight glass and then add a half can. I evacuate the system and keep track of the amount I put in matching what the required amount required. Yet a half can after the bubbles stopped came out to be the same as the required amount. I am running the stock 85 compressor with 150K miles and do not see any degradeation in the output. Maybe some day i will use a thermometer to measure the output temperature. Low side pressure seemed to be about 30 when not running if i remember. I am using R12.
Dan the Pod Guy
Do you have the rear A/C option? With 134a i have heard this can be a problem.
Other issues that can kill the A/C is heat getting into the cabin. I had some heat coming from a small hole in the firewall and a lot of heat coming in the shifter. On a mildly warm day the A/C could not keep up on full. I was beginning to doubt the new charge. I then took some spray foam insulation and blocked off the heat leaks. Be careful not to use too much. I also made sure the hood gasket was on the fire wall and connecting with the hood and that the two rubber stoppers were in the fenders by the fire wall. Check the heater value to insure it is closing all the way.
After these changes the A/C drives me out even on very hot days. I have to cycle it between no fan and position one. Any higher and I freeze. Eliminating the heat from the shifter made a huge difference.
As far as filling the system I put in freon until the bubbles stop in the sight glass and then add a half can. I evacuate the system and keep track of the amount I put in matching what the required amount required. Yet a half can after the bubbles stopped came out to be the same as the required amount. I am running the stock 85 compressor with 150K miles and do not see any degradeation in the output. Maybe some day i will use a thermometer to measure the output temperature. Low side pressure seemed to be about 30 when not running if i remember. I am using R12.
Dan the Pod Guy
07-12-2007, 12:31 PM
#4
Chronic Tool Dropper
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What Wally said...
The partial pressure of air in the system would add 15 or so PSI to the suction pressure, so your mechanic is thinking along the correct lines.
Many home AC techs charge the system while looking a bubbles in the sight glass. That method gives you a charge that's OK at that particular condition, but it may not be right all the time. With lower airflow than you'd see driving the car, and typically having the engine/compressor at idle or close to idle speed when charging, you'll almost always end up with the wrong amount of refrigerant in the system if you are just looking at bubbles, depending on ambient conditions of course. Air in the system will ALWAYS show as bubbles in the sight glass, since the air never condenses. So the moral of the story is to charge by weight if you can. If you have R-134a, you'll be charging to 90-95% of the R-12 charge weight for your car, typically. Your AC guy will know this.
The partial pressure of air in the system would add 15 or so PSI to the suction pressure, so your mechanic is thinking along the correct lines.
Many home AC techs charge the system while looking a bubbles in the sight glass. That method gives you a charge that's OK at that particular condition, but it may not be right all the time. With lower airflow than you'd see driving the car, and typically having the engine/compressor at idle or close to idle speed when charging, you'll almost always end up with the wrong amount of refrigerant in the system if you are just looking at bubbles, depending on ambient conditions of course. Air in the system will ALWAYS show as bubbles in the sight glass, since the air never condenses. So the moral of the story is to charge by weight if you can. If you have R-134a, you'll be charging to 90-95% of the R-12 charge weight for your car, typically. Your AC guy will know this.
07-12-2007, 01:26 PM
#5
Racer
Thread Starter
Originally Posted by Podguy
Rick
Do you have the rear A/C option? With 134a i have heard this can be a problem.
Other issues that can kill the A/C is heat getting into the cabin. I had some heat coming from a small hole in the firewall and a lot of heat coming in the shifter. On a mildly warm day the A/C could not keep up on full. I was beginning to doubt the new charge. I then took some spray foam insulation and blocked off the heat leaks. Be careful not to use too much. I also made sure the hood gasket was on the fire wall and connecting with the hood and that the two rubber stoppers were in the fenders by the fire wall. Check the heater value to insure it is closing all the way.
After these changes the A/C drives me out even on very hot days. I have to cycle it between no fan and position one. Any higher and I freeze. Eliminating the heat from the shifter made a huge difference.
As far as filling the system I put in freon until the bubbles stop in the sight glass and then add a half can. I evacuate the system and keep track of the amount I put in matching what the required amount required. Yet a half can after the bubbles stopped came out to be the same as the required amount. I am running the stock 85 compressor with 150K miles and do not see any degradeation in the output. Maybe some day i will use a thermometer to measure the output temperature. Low side pressure seemed to be about 30 when not running if i remember. I am using R12.
Dan the Pod Guy
Do you have the rear A/C option? With 134a i have heard this can be a problem.
Other issues that can kill the A/C is heat getting into the cabin. I had some heat coming from a small hole in the firewall and a lot of heat coming in the shifter. On a mildly warm day the A/C could not keep up on full. I was beginning to doubt the new charge. I then took some spray foam insulation and blocked off the heat leaks. Be careful not to use too much. I also made sure the hood gasket was on the fire wall and connecting with the hood and that the two rubber stoppers were in the fenders by the fire wall. Check the heater value to insure it is closing all the way.
After these changes the A/C drives me out even on very hot days. I have to cycle it between no fan and position one. Any higher and I freeze. Eliminating the heat from the shifter made a huge difference.
As far as filling the system I put in freon until the bubbles stop in the sight glass and then add a half can. I evacuate the system and keep track of the amount I put in matching what the required amount required. Yet a half can after the bubbles stopped came out to be the same as the required amount. I am running the stock 85 compressor with 150K miles and do not see any degradeation in the output. Maybe some day i will use a thermometer to measure the output temperature. Low side pressure seemed to be about 30 when not running if i remember. I am using R12.
Dan the Pod Guy
also I'm sticking the a/c temp probe into the vent, so it's not reading cabin temp, but temp inside the vent.
apart from the probe, it just doesn't feel as cold as other cars.
07-12-2007, 04:08 PM
#6
Chronic Tool Dropper
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Rick--
Besides air in the system, which will cost you 20-30 degrees of center vent temp worst case, the temp is limited by a few things.
-- Evaporators get fouled with dust and dirt, which turns into a mud cake when the evaporator starts condensing moisture from the air flowing through it. Cleaning is possible through the blower housing under the hood. Remove the hood, remove the blower boot and the blower. Clean with a vacuum cleaner first. Then use a mild spray detergent and a gentle spray of water. All that stuff needs to go down through the condensate drain tube or get vacuumed out the top.
-- There's an updated expansion valve with the R-134a pressure/temperature curve programmed. All the current new expansion valves are good for this service. New expansion valves are less than $30. Good for maybe 5 degrees.
-- Condenser needs to be clean. Leaves and dirt/mud/bugs pile up in front of the condenser, but I've seen several inches of crud accumulate between the radiator and the rear of the condenser. Pull the fan console and fans out, and you can tilt the radiator back and see. If yopu can pull the radiator out completely, lay it on the ground and hose-wash it from back to front. Do the condenser in places, same back-to-frant wash. You'll be impressed with how much crud was hiding in there. Tour engine may run a bit cooler too after this little exercise.
-- The HVAC vaccum system needs to be working right. No leaks, and the doors, flaps and heater valve need to be working right. A small vacuum leak may allow the heater valve to open slightly, adding some engine heat to your cabin air.
-- The heater valve itself has an annoying habit of leaking as it ages. Even when full closed, hot coolant may sneak through. These valves are almost PM items, due for replacement with every TB change. Easy to change, along with the hoses at the back of the head there that are always getting cooked in place.
-- The anti-freeze switch is supposed to cut off current to the compressor when evap temps drop below 35 degrees. This prevents ice formation on the coils, so air will continue to flow through/between them. It's possible that the switch is opening too soon, and the coils are not allowed to get as cold as they should. You can test this --after-- the other steps, especially the cleaning steps.
-- The avaialble evaporator temp is a function of how low the compressor can get the suction pressure. Look at your gauges and yoiu'll see that about 30 PSI is a good target pressure for freezing the evaporator. If your compressor is worn excessively, it may not be able to pull that kind of low-side pressure while still compressing enough to feed liquid to the expansion valve. Your AC guy will tell you all this, since ambient conditions and the condition of the two exchangers are critical to these pressures also.
Expectations: The clean evaporator should be able to drop the air temperature about 40 degrees, give or take. If the air on the passenger's floor is 80 degrees, don't expect the center vent temp to get much below 40 degrees, for instance. Vent temps should be measured with the engine speed no less than 1500 RPM's, with the fan at position 2. Use a recording digital thermometer for your measurements. Kitchen gadget stores have them for less than $20, they record min and max temps for you. Youll need to know ambient temp, footwell/evap inlet temp, and center vent temps to properly analyze performance.
----
I converted my black S4 to R-134a 9 years ago, the first summer after I bought it. The freeze switch didn't work. After the conversion, I could pull 20 degree vent temps easily at 1500 RPM standing still. Cruising at 70 MPH in 90 degree SoCal heat (low humidity), it would go as low as 17 degrees. On low fan speed, it would freeze my fingers on the wheel. Karen would gripe that it was too cold. So I just slide the temp lever over to 70-75 and let the system work. Getting into a heat-soaked black car, that 40 degree limitation still gets us, though. It takes a while to cool everything in the car down enough to make ice for a martini. You can limit the heat-soak effects a little by using a tie-wrap to hold the heater valve closed during AC season. Otherwise the heater valve opens after the engine has been shut down a while, bleeding hot coolant into the heater. On restart, you get a facefull of really hot air until the AC strips out all that added heat. Lock the valve closed and avoid at least some of that effect.
Besides air in the system, which will cost you 20-30 degrees of center vent temp worst case, the temp is limited by a few things.
-- Evaporators get fouled with dust and dirt, which turns into a mud cake when the evaporator starts condensing moisture from the air flowing through it. Cleaning is possible through the blower housing under the hood. Remove the hood, remove the blower boot and the blower. Clean with a vacuum cleaner first. Then use a mild spray detergent and a gentle spray of water. All that stuff needs to go down through the condensate drain tube or get vacuumed out the top.
-- There's an updated expansion valve with the R-134a pressure/temperature curve programmed. All the current new expansion valves are good for this service. New expansion valves are less than $30. Good for maybe 5 degrees.
-- Condenser needs to be clean. Leaves and dirt/mud/bugs pile up in front of the condenser, but I've seen several inches of crud accumulate between the radiator and the rear of the condenser. Pull the fan console and fans out, and you can tilt the radiator back and see. If yopu can pull the radiator out completely, lay it on the ground and hose-wash it from back to front. Do the condenser in places, same back-to-frant wash. You'll be impressed with how much crud was hiding in there. Tour engine may run a bit cooler too after this little exercise.
-- The HVAC vaccum system needs to be working right. No leaks, and the doors, flaps and heater valve need to be working right. A small vacuum leak may allow the heater valve to open slightly, adding some engine heat to your cabin air.
-- The heater valve itself has an annoying habit of leaking as it ages. Even when full closed, hot coolant may sneak through. These valves are almost PM items, due for replacement with every TB change. Easy to change, along with the hoses at the back of the head there that are always getting cooked in place.
-- The anti-freeze switch is supposed to cut off current to the compressor when evap temps drop below 35 degrees. This prevents ice formation on the coils, so air will continue to flow through/between them. It's possible that the switch is opening too soon, and the coils are not allowed to get as cold as they should. You can test this --after-- the other steps, especially the cleaning steps.
-- The avaialble evaporator temp is a function of how low the compressor can get the suction pressure. Look at your gauges and yoiu'll see that about 30 PSI is a good target pressure for freezing the evaporator. If your compressor is worn excessively, it may not be able to pull that kind of low-side pressure while still compressing enough to feed liquid to the expansion valve. Your AC guy will tell you all this, since ambient conditions and the condition of the two exchangers are critical to these pressures also.
Expectations: The clean evaporator should be able to drop the air temperature about 40 degrees, give or take. If the air on the passenger's floor is 80 degrees, don't expect the center vent temp to get much below 40 degrees, for instance. Vent temps should be measured with the engine speed no less than 1500 RPM's, with the fan at position 2. Use a recording digital thermometer for your measurements. Kitchen gadget stores have them for less than $20, they record min and max temps for you. Youll need to know ambient temp, footwell/evap inlet temp, and center vent temps to properly analyze performance.
----
I converted my black S4 to R-134a 9 years ago, the first summer after I bought it. The freeze switch didn't work. After the conversion, I could pull 20 degree vent temps easily at 1500 RPM standing still. Cruising at 70 MPH in 90 degree SoCal heat (low humidity), it would go as low as 17 degrees. On low fan speed, it would freeze my fingers on the wheel. Karen would gripe that it was too cold. So I just slide the temp lever over to 70-75 and let the system work. Getting into a heat-soaked black car, that 40 degree limitation still gets us, though. It takes a while to cool everything in the car down enough to make ice for a martini. You can limit the heat-soak effects a little by using a tie-wrap to hold the heater valve closed during AC season. Otherwise the heater valve opens after the engine has been shut down a while, bleeding hot coolant into the heater. On restart, you get a facefull of really hot air until the AC strips out all that added heat. Lock the valve closed and avoid at least some of that effect.
