A/C issue

[toyak]

A/C has a charge, the plumbing to the fire wall is appropriately cold and the return lines warm. However, the A/C only functions for about a minute. Sometimes if I turn off the A/C and wait minutes while I am driving and then turn it back on it will work for about a minute before failing again. The A/C system has the current refrigerant R134A and PAG oil installed but no parts changed. I am also trying to locate an a/c clutch switch but the internet has not provided the answer. I can find one for most chrysler products from the 70's but nothing specific for the C bodies 74-78.

 

[CBODY67]

Plymoputh Valiants used a cycling switch for the RV-2 compressor from the factory. To use such you will need to remove the EPR (evaporator pressure regulator) valve at the larger inlet line joint to the rh side of the rear of the compressor.

I suspect you could replace the current low pressure switch on the receiver/drier with a cycling switch, if desired. Hopefully, that cycling switch is calibrated for R-134a use.

You mentioned "fail", does that mean the compresor stops running? The compressor clutch surfaces are so degraded/eroded that compressor clutch slippage sends sparks out from it as the belt tries to turn the compressor? Was all of the old oil removed from the system when you changed the gas in the system? What gas charge level is now in the system?

Why is it felt that a cycling compressor might solve the performance issue?

Just some thoughts,
CBODY67

 

[MONC440]

You mentioned "fail", does that mean the compresor stops running? The compressor clutch surfaces are so degraded/eroded that compressor clutch slippage sends sparks out from it as the belt tries to turn the compressor? Was all of the old oil removed from the system when you changed the gas in the system? What gas charge level is now in the system?

Yes please provide more info. What system do you have, manual A/C or Auto? Do you have a set of gauges for r134a? If the compressor is turning off after 1 minute it could be a low pressure issue, the switch or it is actually going too low in pressure and the switch is fine. If that is the case it should eventually come back on after some time when the pressure equalizes in the system. My first step when diagnosing A/C is reading the gauges, if the pressures are within range when the compressor is on then you know it is something else. If the pressure is too low on the gauge you could have a restriction in the high side. The high side line going into the evaporator should not be really cold until after the TXV. The TXV is the part the turns the liguid refrigerant to a vapor and causes the cooling effect. If the line before the TXV is ice cold you could have a restriction.

 

[toyak]

It is the manual a/c system. When I bought the car in January the a/c did not work and I assumed it was lacking Freon. I took the car to an acquaintance’s shop and they leaked tested and it held the gas. The oil was supposedly drained and changed to PAg and 134a added. The system was tested and supposedly worked w/o issue (in a garage that is environmentally controlled) but not tested in the 85 degree weather outside. When I picked the car up after closing, the a/c was not working. The car went back the next day they evacuated the 134, pressure tested the system and it was w/o issue. The refrigerant was placed back in at the appropriate amount (about 2/3rd’s of the amount of R12 and supposedly at the correct pressure). It was then noted that the compressor clutch switch was cycling on and off constantly most of the time. When the shops device was plugged in instead of the clutch switch the compressor ran w/o issue but still no cold air into the passenger compartment. In retrospect I am thinking the cold air I first feel from the vents is just the cooler air from the car sitting overnight and the outside air and interior of the car being hotter. I was pretty sure it was the hvac actuator doors failing (are they accessible w/o removing the dash and can they be manipulated?)

 

[CBODY67]

There should be NO cycling switch in your system if the vehicle had to be converted to R-134a. There IS a low pressure switch on the receiver-drier, usually mounted near the condenser on the grille side of the core support. I know the '72 C-bodies had the pressure switch as our '72 Newport Royal had it, OEM.

Other than the oil in the oil pan/sump of the RV2 compressor, there can ALSO be oil accumulated in the bottom of the condenser. Might need to evac, remove and flush the condenser, and put in a new receiver-drier (if that was not already done, due to vehicle age issues).

C-bodies used the EPR valve to keep the evap core from freezing up (too cold) and have good cooling performance. As in 40*F vent output temps. It modulated gas flow to do this.

The A-body cars used a RV2 compressor and a cycling switch to modulate evap core temps. As they did not have an EPR valve in their system to do that.

Can you please verify the model year of your car?

Just some thoughts,
CBODY67

 

[toyak]

It is a 1976 Chry New Yorker. If the low pressure switch (the switch I am calling clutch switch for the compressor) fails can that then cause the compressor to switch on/off every few seconds? Could the old a/c oil not being fully drain cause the cycling issue? Why, when they garage plugged in their cabling to the low pressure switch and the compressor seem to work normal was their still no cold air being delivers to the cabin but the plumbing to the firewall was nice and cold?

Thanks for taking the time to discuss the issues because this is far from my realm
Of experience.

 

[CBODY67]

Thanks for the additional information and comments.

I am NOT sure why they would be messing with the low pressure switch at all? Unless they were shorting across it to get the compressor clutch to engage to charge the system and check performance. That switch should be the last switch in the circuit to the compressor clutch, to protect the compressor from running with no gas in the system.

That switch is either "on" or "off". It should not be cycling if there was something like 12+psi of gas pressure in the system. Unless it had failed and needs to be replaced. To replace it, the system would need to be evacuated, the switch changed, and re-charged to R-134a specs.

As to any of the R-12 spec oil in the bottom of the condenser, it might be necessary to determine if the oil they added was "universal in nature", that will mix and work well with the new R-134a gas. If they added normal R-134a oil, then it can allegedly congeal with the new gas and old oil residues and can cause issues. Not sure if that is not what might be going on.

Originally, the tupe of oil used in a R-134a system depended upon where the vehicle was manufactured. THREE different R-134a oils, in the earlier years of its use. As I suspected at the time, "chemistry" has yielded a more-universal oil that is mostly compatible with all gases, from what I have seen. Even some synthetic versions.

In general, changing the low pressure switch would be the easiest thing to do. THEN to also look for other electrical switches (added after the vehicle left the assembly plant, by others before you got the car) in the metal tubing under the hood, for the a/c system.

Presuming your car is a normal a/c car and does not have the "Auto Temp II" notation on the a/c control panel.

Please keep us posted on what you find out,
CBODY67

 

[toyak]

Definitely Not an auto temp 2. They were shorting the low pressure switch to verify performance. There did not appear to have any after market mods past the low pressure switch. The oil used is the latest generation and is supposed to be universally compatible.

What I do not understand is when the low pressure switch was “shorted” out and the compressor was running normally and the delivery metal tubing going to the fire wall was nice and cold but still no cold air out of the dash vents unless something is up with the door actuators or perhaps cabling?

 

[CBODY67]

Again, thanks for the additional information and comments.

Might need to check the heater water valve and how it works. Vacuum or mechanical cable. Need to ensure it is fully closed in the "A/C" functions. Otherwise, the vac actuators work the various doors to direct air through the hvac box via the pushbutton switch on the instrument panel.

There is also an expansion valve visible near the firewall area of the underhood a/c lines. Might need to verify that it is working as designed? Also need to verify that a good vacuum signal is getting to the hvac system units that need it. Then make sure that all of the functions work as needed (i.e., changing vent outlets and such).

You might look in the Chrysler MasterTech series of videos at www.mymopar.com (might need to manually input that address) regarding a/c and hvac units. The basic systems have not changed much over the years, going back to 1966 or so. Heater water valves have changed from cable to vacuum and back, but mostly little else (on the manual systems). There was a blip in the earlier 1970s when they ran the compressor all of the time, even in "OFF" position. Paired that with a super-low fan speed, too. Which resulted in an additional compressor switch in the cable mechanism for the water valve. I know our '72 Chrysler is that way. Ways to get around that situation. I suspect that your car is well past them doing that on them.

Thanks for your time and additional information,
CBODY67

 

[toyak]

Again, thanks for the additional information and comments.

Might need to check the heater water valve and how it works. Vacuum or mechanical cable. Need to ensure it is fully closed in the "A/C" functions. Otherwise, the vac actuators work the various doors to direct air through the hvac box via the pushbutton switch on the instrument panel.

There is also an expansion valve visible near the firewall area of the underhood a/c lines. Might need to verify that it is working as designed? Also need to verify that a good vacuum signal is getting to the hvac system units that need it. Then make sure that all of the functions work as needed (i.e., changing vent outlets and such).

You might look in the Chrysler MasterTech series of videos at www.mymopar.com (might need to manually input that address) regarding a/c and hvac units. The basic systems have not changed much over the years, going back to 1966 or so. Heater water valves have changed from cable to vacuum and back, but mostly little else (on the manual systems). There was a blip in the earlier 1970s when they ran the compressor all of the time, even in "OFF" position. Paired that with a super-low fan speed, too. Which resulted in an additional compressor switch in the cable mechanism for the water valve. I know our '72 Chrysler is that way. Ways to get around that situation. I suspect that your car is well past them doing that on them.

Thanks for your time and additional information,
CBODY67

Thank you for all the help Cody67

 

[MONC440]

MyMopar on youtube has some A/C videos. They are old dealer tech training. I watched all of the 69 and 70 videos when I was working on converting my system from auto temp 1 to manual A/C. Very helpful.
Below is just one if you look thru their channel they have a bunch.

1974, Volume 74-2 1974 Air Conditioning Air Flow​

​

 

[toyak]

Thanks for the video and link monc440!

 

[toyak]

Problem solved and A/C works. After doing a lot of reading and not clearly seeing a specific reason for the symptoms I decided to at least replace the a/c compressor low pressure cut off switch and drier before tackling behind the dash. When I was shopping for the switch Classic Industries (car parts) web site noted two important things; one being that the switch is actually low pressure and high pressure cut off switch and more importantly when moving to R134 you must use a cutoff switch that is designed for the new refrigerant or the A/C will not work! I did not see this specific piece of information noted anywhere else.

 

[darth_linux]

So your cut-off switch was the original one designed for R12? What part number did you replace it with?

 

[toyak]

Yes, the cut of switch was original and meant for R12. This is the new switch and also works with the c body.

 

[68yourker]

R134A has higher working pressures. There are a few ways to get around too high of a pressure. Several were requirement of doing the conversion. One was to use 80% system charge. Next was to ensure continuous airflow across the condenser, so most would add an electric fan-more air=lower pressure and better performance, or replace the fan clutch with a HD unit. And the third was to add a switch in the high side to shut off the compressor if the pressure climbs too high in situations that the air flow is not correct across the condenser to prevent damaging the compressor. Some diagnosis can be done just using your hands (caution of moving parts, and the high side parts could be VERY hot). From the compressor to the condenser will be the hottest. All the condenser should feel pretty warm as well-the hotter the day the hotter it will be. From the condenser to the firewall (expansion valve) should also be warm (never cold), and this includes the dryer. From the expansion valve back to the compressor should feel cold as this is the low pressure (temp) side of the system. If it is cold between the dryer and the expansion valve, that indicates a restriction in the dryer (Dryer has descant, a filter and acts like a storage unit for refrigerant). The "site glass" could have bubbles in it with 134A, so don't use it to determine system charge. If all the tubes and hoses feel correct but the air is still warm inside the car, then it could be the blend air door (temperature select lever) in the dash, or as was pointed out previously, the water shut off valve. Just sharing system information, that hopefully will help others.