AC Temperatures Kind of High During Hot Weather.
06-21-2020, 06:13 PM
#1
Racer
Thread Starter
AC Temperatures Kind of High During Hot Weather.
Hello guys,
Its been like 2 years since I last logged in. But I’m back with my black 87. Got it running again and this west Texas heat is doing a number on me while i drive. Cold air seems to be hotter at the weather gets closer to 100. What hacks are there to get my AC running cold when its that warm? AC is fine when its like 70-80 but then once the heat gets going it seems to exponentially go warmer inside the car. If its like 75-80 i can have AC blow 35-40 at 0 setting but once its closer to 100 outside i cant get it lower than 70. I just recently got a pusher fan from 928Motorsports but will install in the next week or so. Ive reached out to Greg Brown several times over emails about his higher capacity motor for fans and relay but hasn’t responded back. Next week I’ll be loosing at temps from 102-105. What tricks or products can some one recommend?
Thanks
Ricardo Vega
1987 928 S4 Black/Plum
125,XXX Miles.
Its been like 2 years since I last logged in. But I’m back with my black 87. Got it running again and this west Texas heat is doing a number on me while i drive. Cold air seems to be hotter at the weather gets closer to 100. What hacks are there to get my AC running cold when its that warm? AC is fine when its like 70-80 but then once the heat gets going it seems to exponentially go warmer inside the car. If its like 75-80 i can have AC blow 35-40 at 0 setting but once its closer to 100 outside i cant get it lower than 70. I just recently got a pusher fan from 928Motorsports but will install in the next week or so. Ive reached out to Greg Brown several times over emails about his higher capacity motor for fans and relay but hasn’t responded back. Next week I’ll be loosing at temps from 102-105. What tricks or products can some one recommend?
Thanks
Ricardo Vega
1987 928 S4 Black/Plum
125,XXX Miles.
06-21-2020, 07:14 PM
#4
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Diagnosis starts with checking system pressures, verifying vacuum system, verifying that the heater valve is closed -and- preventing coolant flow, and finally a peek at the sight glass in top of the dryer looking for bubbles.
06-22-2020, 12:47 PM
#5
Racer
Sorry Bother. I don't mean to hijack your thread but maybe there is some overlap...
Good Doctor. Is it possible to expand on the "vacuum system" idea you touched on...?
I replaced the receiver/dryer a few days back, charged the system (didn't read high pressure side during the filling but will). Clutch engages, air is cold but not cold enough... Fan kicks in, replaced refrigerant temp sensor (the little threaded section sheared when removing sensor from old receiver/dryer)... And that's about as far as I've gotten.
How can I know the recirculation flap is opening/working?
I'm experiencing the same thing as the OP... For one, I know my belt is too loose and the adjustment is bottomed out. Can't get it tighter. I think PO had placed the wrong belt, we'll see. I have the replacement on the way. The expansion valve is I don't know how old, and was thinking of replacing that as well???? Then, tint the back windows, lol... Kill the greenhouse effect...
Tell me what to do. I'm all yours.
06-22-2020, 02:29 PM
#6
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Things I've Discovered:
-- Getting the various actuators both sealed and working is important to vent temps. Specifically, my Heater Control Valve was leaking on the vacuum side but not on the coolant side. Living at the time in SoCal, the valve was tied closed unless the two weeks of "winter" happened to be particularly harsh. "Winter" includes those few days around New Years when daytime temps don't make it above about 55º. Anyway, even with the blend door working (electric servo motor) and no coolant passing through the heater, center vent temps wouldn't go much below about 50º. Pressures all said "cold", sight glass was liquid, everything seemed fine except things didn't get cold. Plugged the vacuum line to the HCV and suddenly everything goes to normal ice-cubes cold.
-- The fresh air flap is actuated by contacts at that blend door servo motor. On your wiring diagram it's the box on the left with the wiring to the vacuum solenoids. With everything electrical working correctly, the fresh air flap cycles based on error between cabin temp and outside temp, and the slider position. When I first start the engine cold, AC button depressed, with the slider set at its normal 70º mark, I can hear the fresh air flap go closed with a soft whump! when outside temp is warmer than the 70º slider setpoint and cabin is warm.
-- While driving with AC on, slider still at 70º, I can hear the blend door motor moving as heat load in the cabin changes. Various actuators move in addition to the fresh air flap, but generally just when the blend door servo reaches end of travel towards warming and the cabin is still too cold.
Bottom line: You really want the vacuum system tight, even if your HCV is tied closed. The default not-enough-vacuum positions for actuators is full heat to the windscreen. The actual flow of air through the vacuum system is limited at the connection to the check valve and brake booster, so even a small leak will cause actuators to move towards failure positions even if they don't go all the way. While the dash or floor vents don't seem that important to vent temps, allowing warm air there means less flow from the center and door vents. With the center vent diverter (comb flap) not moving all the way to open/cold, the airflow will be a mix that includes some that didn't go through the evaporator. In my situation with the vacuum leak, the pressures all said cold, but warm air was mixing with cold air.
We all talk about charging by weight, and for the most part it's the safest method. But... if you're using small cans and going by the stated contents as ounces, you'll be short on charge. Turns out that volume ounces on the can and weight ounces for the charge mass are different for refrigerants. Charging by cylinder rather than cans, with a refrigerant scale, is much better. Even then, I purge the manifold and hoses with refrigerant before connecting to the car, so that no air gets pushed in. After the purge, tare the cylinder and only then start charging and watching the mass going in. Per previous posts on the subject, this is all done with the engine off and cold, system evacuated.
For cars that need a maintenance top-off due to leakage, the situation changes. Decide what a typical ambient operating condition looks like, and plan to do your service when conditions are the same. Given a choice, err on the service conditions hotter side. Then charge to system pressures and watch the sight glass. System running, fan on speed 2, engine at 1500-2000 RPM. Ideally, the sight glass has no bubbles visible when operating. At the same time, any air in the system will show in the sight glass, so just adding refrigerant trying to get rid of all the bubbles may not be possible. Coldest system is at the lowest possible pressures that offer liquid to the expansion valve(s). With R134a in my car, I see 15-20 PSI low-side pressure and about 175 PSI high-side at 80º ambient test condition, and a clear sight glass. The actual pressures are dependent on compressor condition, expansion valve personality, cooling fans working, and whether you have any air in the system. Air hogs condenser capacity, interferes with proper expansion valve operation, makes the compressor work harder pumping, and the higher suction pressure at the evaporator raises the flash temperature of the refrigerant inside. Plus will contribute to corrosion. Do everything you possibly can to keep air out of the system when testing, charging, servicing, etc.
The system will make some seriously cold air if you let it.
-- Getting the various actuators both sealed and working is important to vent temps. Specifically, my Heater Control Valve was leaking on the vacuum side but not on the coolant side. Living at the time in SoCal, the valve was tied closed unless the two weeks of "winter" happened to be particularly harsh. "Winter" includes those few days around New Years when daytime temps don't make it above about 55º. Anyway, even with the blend door working (electric servo motor) and no coolant passing through the heater, center vent temps wouldn't go much below about 50º. Pressures all said "cold", sight glass was liquid, everything seemed fine except things didn't get cold. Plugged the vacuum line to the HCV and suddenly everything goes to normal ice-cubes cold.
-- The fresh air flap is actuated by contacts at that blend door servo motor. On your wiring diagram it's the box on the left with the wiring to the vacuum solenoids. With everything electrical working correctly, the fresh air flap cycles based on error between cabin temp and outside temp, and the slider position. When I first start the engine cold, AC button depressed, with the slider set at its normal 70º mark, I can hear the fresh air flap go closed with a soft whump! when outside temp is warmer than the 70º slider setpoint and cabin is warm.
-- While driving with AC on, slider still at 70º, I can hear the blend door motor moving as heat load in the cabin changes. Various actuators move in addition to the fresh air flap, but generally just when the blend door servo reaches end of travel towards warming and the cabin is still too cold.
Bottom line: You really want the vacuum system tight, even if your HCV is tied closed. The default not-enough-vacuum positions for actuators is full heat to the windscreen. The actual flow of air through the vacuum system is limited at the connection to the check valve and brake booster, so even a small leak will cause actuators to move towards failure positions even if they don't go all the way. While the dash or floor vents don't seem that important to vent temps, allowing warm air there means less flow from the center and door vents. With the center vent diverter (comb flap) not moving all the way to open/cold, the airflow will be a mix that includes some that didn't go through the evaporator. In my situation with the vacuum leak, the pressures all said cold, but warm air was mixing with cold air.
We all talk about charging by weight, and for the most part it's the safest method. But... if you're using small cans and going by the stated contents as ounces, you'll be short on charge. Turns out that volume ounces on the can and weight ounces for the charge mass are different for refrigerants. Charging by cylinder rather than cans, with a refrigerant scale, is much better. Even then, I purge the manifold and hoses with refrigerant before connecting to the car, so that no air gets pushed in. After the purge, tare the cylinder and only then start charging and watching the mass going in. Per previous posts on the subject, this is all done with the engine off and cold, system evacuated.
For cars that need a maintenance top-off due to leakage, the situation changes. Decide what a typical ambient operating condition looks like, and plan to do your service when conditions are the same. Given a choice, err on the service conditions hotter side. Then charge to system pressures and watch the sight glass. System running, fan on speed 2, engine at 1500-2000 RPM. Ideally, the sight glass has no bubbles visible when operating. At the same time, any air in the system will show in the sight glass, so just adding refrigerant trying to get rid of all the bubbles may not be possible. Coldest system is at the lowest possible pressures that offer liquid to the expansion valve(s). With R134a in my car, I see 15-20 PSI low-side pressure and about 175 PSI high-side at 80º ambient test condition, and a clear sight glass. The actual pressures are dependent on compressor condition, expansion valve personality, cooling fans working, and whether you have any air in the system. Air hogs condenser capacity, interferes with proper expansion valve operation, makes the compressor work harder pumping, and the higher suction pressure at the evaporator raises the flash temperature of the refrigerant inside. Plus will contribute to corrosion. Do everything you possibly can to keep air out of the system when testing, charging, servicing, etc.
The system will make some seriously cold air if you let it.
The following users liked this post:
Schocki (07-01-2021)
06-22-2020, 05:29 PM
#7
A quick question, with 134a should the sight glass be clear? An a/c guy is telling me that with 134a it doesn’t necessarily clear up like in the “Old Days”?
Thanks
Dan
Thanks
Dan
Trending Topics
06-23-2020, 03:21 PM
#8
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Hi Dan --
R134a system, especially conversions, are often slightly undercharged to limit overpressure risks at higher ambient and load conditions. So a sight glass may not be totally bubble-free, but needs to be almost all liquid. It shouldn't be seriously foamy/frothy. For the system to work, liquid is presented at the expansion valve. It's metered into the evaporator, where heat from the air flow is used to boil that liquid into vapor. Passing vapor into the evaporator means that no heat is borrowed from the airflow to boil the refrigerant, so cabin air doesn't get cooled as much.
The pressure readings you see correlate to temperatures, and there are some rules of thumb about what kind of heat transfer efficiency you might expect. Temp at the center vent is expected to be about 40ºF below the passenger floor air temp where the evaporator draws air in. Low suction-side pressure is ideal, so long as there's enough refrigerant mass available to carry calories/BTU's for that 40ºF drop. There's a similar balance done for the high-pressure side and temps across the condenser. Ambient temp is easy to find but getting a good after-condenser temp can be a challenge with that darn radiator in the way. So instead you get to find the condensing temp based on high-side pressure, And verify that heat is moving there.
R134a system, especially conversions, are often slightly undercharged to limit overpressure risks at higher ambient and load conditions. So a sight glass may not be totally bubble-free, but needs to be almost all liquid. It shouldn't be seriously foamy/frothy. For the system to work, liquid is presented at the expansion valve. It's metered into the evaporator, where heat from the air flow is used to boil that liquid into vapor. Passing vapor into the evaporator means that no heat is borrowed from the airflow to boil the refrigerant, so cabin air doesn't get cooled as much.
The pressure readings you see correlate to temperatures, and there are some rules of thumb about what kind of heat transfer efficiency you might expect. Temp at the center vent is expected to be about 40ºF below the passenger floor air temp where the evaporator draws air in. Low suction-side pressure is ideal, so long as there's enough refrigerant mass available to carry calories/BTU's for that 40ºF drop. There's a similar balance done for the high-pressure side and temps across the condenser. Ambient temp is easy to find but getting a good after-condenser temp can be a challenge with that darn radiator in the way. So instead you get to find the condensing temp based on high-side pressure, And verify that heat is moving there.
07-01-2021, 03:06 PM
#9
Rennlist Member
Hi Dan --
R134a system, especially conversions, are often slightly undercharged to limit overpressure risks at higher ambient and load conditions. So a sight glass may not be totally bubble-free, but needs to be almost all liquid. It shouldn't be seriously foamy/frothy. For the system to work, liquid is presented at the expansion valve. It's metered into the evaporator, where heat from the air flow is used to boil that liquid into vapor. Passing vapor into the evaporator means that no heat is borrowed from the airflow to boil the refrigerant, so cabin air doesn't get cooled as much.
The pressure readings you see correlate to temperatures, and there are some rules of thumb about what kind of heat transfer efficiency you might expect. Temp at the center vent is expected to be about 40ºF below the passenger floor air temp where the evaporator draws air in. Low suction-side pressure is ideal, so long as there's enough refrigerant mass available to carry calories/BTU's for that 40ºF drop. There's a similar balance done for the high-pressure side and temps across the condenser. Ambient temp is easy to find but getting a good after-condenser temp can be a challenge with that darn radiator in the way. So instead you get to find the condensing temp based on high-side pressure, And verify that heat is moving there.
R134a system, especially conversions, are often slightly undercharged to limit overpressure risks at higher ambient and load conditions. So a sight glass may not be totally bubble-free, but needs to be almost all liquid. It shouldn't be seriously foamy/frothy. For the system to work, liquid is presented at the expansion valve. It's metered into the evaporator, where heat from the air flow is used to boil that liquid into vapor. Passing vapor into the evaporator means that no heat is borrowed from the airflow to boil the refrigerant, so cabin air doesn't get cooled as much.
The pressure readings you see correlate to temperatures, and there are some rules of thumb about what kind of heat transfer efficiency you might expect. Temp at the center vent is expected to be about 40ºF below the passenger floor air temp where the evaporator draws air in. Low suction-side pressure is ideal, so long as there's enough refrigerant mass available to carry calories/BTU's for that 40ºF drop. There's a similar balance done for the high-pressure side and temps across the condenser. Ambient temp is easy to find but getting a good after-condenser temp can be a challenge with that darn radiator in the way. So instead you get to find the condensing temp based on high-side pressure, And verify that heat is moving there.
My 928 should be able produce very cold air because:
- HVAC box is in very good condition. 94 used unit with air filter after the evaporator in the old box sprung a leak.
- All sealing foam in the box is new
- New pollen filter and very clean evaporator
- All new diaphragms (either silicone from Roger or OEM)
- New receiver drier
- New expansion valve
- All new o-rings
- New MB heater valve
- No vacuum leaks
- New 10PA20 compressor from Roger
- No leaks or other issues
- 90° shut-off valve installed in the return line from the heat exchanger
Followed Dr. Bob's instructions and did a refrigerant top off with the sight glass and high/low pressure gauges connected. I'm using the regular cans, purged the feed line before filling. Initially the sight glass showed a constant stream of foam (no air bubbles visible). AC was not contaminated with air just low on refrigerant.
In order to hit the numbers according to the WSM, I zip tied the throttle to 2000 RPMs, fan on 2 and temp setting full cold and return line shut-off valve closed.
Temp in the garage at exactly 90° F. I ignored the pressure values initially and slowly topped off with R134-a until the sight glass was filled with liquid (no foam or bubbles visible).
After about 2 minutes the high pressure side stabilized at 17.5 bar or 255 PSI, low side at 1 bar or 15 PSI.
Center vent temperature dropped into the mid 30's. Great!
Moment of truth today: car was parked outside at 99° F all day long. Recirculation on, full cold setting. After driving the car for about 10 minutes it was all fine.
Rogerbox gear selector in 3 and at 2000 rpm, temp difference between the passenger side footwell was exactly as described by Dr. Bob: 40°F. It was never this cold in my car before, great!
Bottom line: Dr. Bob is absolutely right, converted ac systems that have been filled by weight with small cans are often low on refrigerant! The sight glass method in combination gauges is the way to go, don't rely on refrigerant weight only.
After all the usual problems have been checked or repaired, get your ac properly charged and it will perform very well!
Thank you Dr. Bob for your detailed explanations about how to properly charge and check the HVAC!
Below a picture of my 90° elbow valve mounted on the passenger side heat exchanger return line. It is accessible without removing anything, 10 seconds and there is not hot coolant flowing into the heat exchanger ever, great for those very hot summer days. Gives you a nice head start after driving and parking the car in the summer heat!
Location of the valve.
Close up. Valve is made out of stainless steel.
Passenger side view from front.
Easily accessible.
Last edited by Schocki; 07-01-2021 at 03:11 PM.
The following users liked this post:
PorKen (07-02-2021)
07-01-2021, 09:04 PM
#10
Rennlist Member
Hello guys,
Its been like 2 years since I last logged in. But I’m back with my black 87. Got it running again and this west Texas heat is doing a number on me while i drive. Cold air seems to be hotter at the weather gets closer to 100. What hacks are there to get my AC running cold when its that warm? AC is fine when its like 70-80 but then once the heat gets going it seems to exponentially go warmer inside the car. If its like 75-80 i can have AC blow 35-40 at 0 setting but once its closer to 100 outside i cant get it lower than 70. I just recently got a pusher fan from 928Motorsports but will install in the next week or so. Ive reached out to Greg Brown several times over emails about his higher capacity motor for fans and relay but hasn’t responded back. Next week I’ll be loosing at temps from 102-105. What tricks or products can some one recommend?
Thanks
Ricardo Vega
1987 928 S4 Black/Plum
125,XXX Miles.
Its been like 2 years since I last logged in. But I’m back with my black 87. Got it running again and this west Texas heat is doing a number on me while i drive. Cold air seems to be hotter at the weather gets closer to 100. What hacks are there to get my AC running cold when its that warm? AC is fine when its like 70-80 but then once the heat gets going it seems to exponentially go warmer inside the car. If its like 75-80 i can have AC blow 35-40 at 0 setting but once its closer to 100 outside i cant get it lower than 70. I just recently got a pusher fan from 928Motorsports but will install in the next week or so. Ive reached out to Greg Brown several times over emails about his higher capacity motor for fans and relay but hasn’t responded back. Next week I’ll be loosing at temps from 102-105. What tricks or products can some one recommend?
Thanks
Ricardo Vega
1987 928 S4 Black/Plum
125,XXX Miles.
A 2nd fan isnt required, its just hardware in the way of airflow.
Are you running the rear AC as well? If so, the OE condonser cant really operate both units, and cool well. Greg Brown can upgrade you to TWO condensers, but you'll be spending big money. But at 105d in Midwest humidity, I can run 25d vent temps, front and rear.
And depending how your recirc flap is working, if its NOT, then if its 100d out, 70d air from the vent is perfect. Cant get much colder than 25-30d ambient through the evaporator.