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Pressure on low side too high

JohnGriffin on Thu March 11, 2010 8:14 PM User is offline

Year: 1993
Make: Ford
Model: Mustang
Engine Size: 140ci
Refrigerant Type: 134a
Ambient Temp: 70
Pressure Low: 70
Pressure High: 180
Country of Origin: United States

134a conversion.
I installed a new compressor, new hoses, new dryer, new orifice tube (inside the hose) and changed the O-rings. Per recommendation from the compressor rebuilder I added 8oz PAG46 oil. I bought straight refrigerant, no magic adders, no oil, no stop leak.
I flushed a gallon of AC flush/cleaner thru the condenser and the evaporator, used shop air to force the cleaner thru and flush the cleaner out.
I connected the vacuum pump and evacuated the system for about 40 minutes, turned off the pump, waited, confirmed no leak... ran the pump for another 10 minutes and then connected a can of 134a and let the vacuum suck the refrigerant into the system.
I started the engine and turned the AC to "max" - there was enough refrigerant (and pressure) in the system so that I did not have to jumper the switch to get the clutch to engage.
Pressure on the high side went up to 150, pressure on low side went up to 60. At that time I had almost one can of 134a in the system, not enough to get any cool air... but too high pressure to add any more refrigerant....

High pressure on low side was the problem BEFORE I replaced all these parts. The system was converted to 134a about six months ago and I suspected that not all of the R12 freon and oil was removed. I suspected the orifice tube and dryer to be gummed up. Appearently now that I replaced all these parts I still have the same problem.

What can cause the low side pressure to be this high?

iceman2555 on Thu March 11, 2010 9:07 PM User is offlineView users profile

There is simply an insufficient amount of refrigerant within the system to properly determine what and where the problem exist...if it exist at all.
Although not a fan of reman'd compressors, if this was a true reman'd and not a rebuilt, it is highly doubtful that the issue is with the compressor. The compressor is it should. The system needs to fully charged, then and only then do pressures have a relevant meaning. Get the system fully charged!
Since the ambient temp is border line for favorable AC performance....try warming the cans to 'force' the refrigerant into the system. Try placing the cans into a container with warm (not hot)water. This will increase the internal pressures in the cans and force feed the system.
Know that the perceived pressures for this system will have little reference to what one would assume were acceptable pressures for an R12 system. Charge the system until the evap inlet and outlet are the same temp. Place the vehicle is a MAX COOL, HIGH BLOWER, DOORS OPEN, ENGINE @ IDLE. When the two temps are the same or within 3-5 degrees of each other the system is properly charged.
Once the system is charged, check pressures...if a high side pressure is encountered....DO NOT REMOVE REFRIGERANT......increase condenser cooling to reduce the high side pressure problem. Removal of refrigerant will lead to a reduction of compressor longevity due to lack of lubricant migration.
Good luck !!!!

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

JohnGriffin on Thu March 11, 2010 9:32 PM User is offline

I agree I need to get more refrigerant into the system but the pressure on the low side will get to 80 or 90 if I keep trying to add 134a. I've had it up to 90 today and released pressure on the low side because I was afraid it might blow up the 134a can ... when the low side was at 90 the high side was at 200. Still I did not get any cool air. The hose/tube assembly that runs to the evaporator is hot, the dryer is hot also...

The compressor is a rebuilt unit from a AC shop in Texas (ordered online). It seems to work fine but so did the old compressor.

I got the dryer/hose assembly as well as the hose with the orifice tube from AutoZone.

Since replacing these parts pretty much did not improve anything i am beginning to wonder what else could be wrong. There's a pressure switch on top of the dryer - I did not replace it.

mk378 on Fri March 12, 2010 9:00 AM User is offline

Make sure to measure properly, always have both hand wheels on the manifold closed to take pressure readings. While you are charging, the low side gauge will falsely go up because of the pressure in the can and the drop in the hose to the car. There's no risk of bursting a can as long as you only open the low side valve. If the pressure is too high it simply will stop flowing out of the can. Can tappers should also all contain a check valve to reduce the risk of over-pressuring a can.

Does the low side drop when you rev the engine? Low side too high when fully charged usually means the compressor isn't working properly. A clogged orifice tube would make the low side too low-- since there is no flow into the evaporator the compressor will pull it down, even into negative pressure sometimes.

The switch on the accumulator / drier cuts the compressor off when the low side goes below about 25, so it should be staying on in your case, seems to be working.

Edited: Fri March 12, 2010 at 9:03 AM by mk378

JohnGriffin on Fri March 12, 2010 9:40 AM User is offline

First of, thanks for your replies. Although my AC still doesn't work your replies have helped me understand the system a little better. this is actually the first time i'm working on an AC system.

Just to make sure I'm getting good readings I took the manifold hoses off the car, opened and closed the hand wheels, the gauges returned to '0' ... I then reconnected the hose to the low pressure side and again it read high.

The low side pressure does drop when reving the engine.

iceman2555 on Fri March 12, 2010 3:04 PM User is offlineView users profile

Please attach the low and high side hoses to the vehicle. Leave the charging hose disconnected. INSURE THAT THE VALVES ARE CLOSED. Start the engine, allow to reach normal operating temperature. Engage the that the clutch drive plate (front of clutch assembly) is rotating. Post the indicated pressures.
There is a good possibility that air has been introduced into the system. This could account for the higher pressures.
The system must be fully charged prior to any pressure diagnosis. At this point everything is merely a guess. Get the system charged correctly. If you are unable to complete this task, take the vehicle to a recognized known reputable AC shop and have them complete the recharge. But the system must be fully matter what.
Not a fan of 'rebuilt' compressors....the difference between rebuilt and remand is like night and day. However, it does not seem that the compressor is the issue.
This same issue was evident with the previous it seems that there is a definite issue with the system.....back to the issue that the system must be fully excuses....get the system serviced....and them post results.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

Dougflas on Fri March 12, 2010 4:51 PM User is offline

Do not overlook the possibility of refrigerant getting past the orings on the O tube. Make sure the metal tube section the orfice fits in is perfectly round.

Many moons ago I had a Chevy that someone before me serviced. The section for the Orfice tube at the evap was slightly butchered from probably not using an orfice tube remover. The inside of the tubing was deformed so refrigerant passed the o tube. I ended up cutting the section off and using an orfice tube kit. Lost my butt on that job.

Edited: Fri March 12, 2010 at 4:52 PM by Dougflas

JohnGriffin on Fri March 12, 2010 6:43 PM User is offline

From iceman's reply: increase condenser cooling to reduce the high side pressure problem

Bingo.... we have a winner.... the radiator fan didn't work.

I hot wired the fan motor and now it's blowing ice cold air like never before. Pressure on low side is 45, high side 160.

Edited: Fri March 12, 2010 at 6:48 PM by JohnGriffin

HECAT on Sat March 13, 2010 7:28 AM User is offline

Originally posted by: JohnGriffin
The hose/tube assembly that runs to the evaporator is hot, the dryer is hot also...

Got to get rid of the heat. That was my thought when you posted the above symptom. Sorry I did not post that thought.

Glad to hear you are narrowing down the problem.


HECAT: You support the Forum when you consider for your a/c parts.


JohnGriffin on Sat March 13, 2010 10:43 AM User is offline

Narrowing down ??? It's fixed.... with the radiator fan spinning the AC works better than ever.... embarrassing that I didn't noticed the fan not spinning before I replaced the compressor and the dryer... well, at least the system should be good for another 15 years now

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