AC high pressure side practically zero
 

I apologize if this had been covered in previous threads but I couldn't find anything describing my AC problem.

I have an 83 SC that has an R134a conversion kit that was installed by a PO. I have owned the car for about a year and since my ownership the AC has not worked. It blows hot air, temperature depends on outside air temp. It is definitely not blowing cold air.

The compressor appears to work, I can see the clutch engage and disengage depending on settings. All AC equipment appears (by visual inspection) to be in order. At least all the components are mounted in the car and blowers all work.

The other day I purchased an AC manifold gauge set. Today I hooked it up for pressure readings. With the car running at about 1800 rpm's and the AC setting on high I get 35lbs on the low side and just about zero on the high side. The high side pressure does go up to about 10 lbs when I shut the system off.

To me it sounds like a clogged system, but where do I start? Any suggestions on where to start looking (i.e., Expansion valve, Evaporator, etc)?

Thanks for your help!

I imagine that your low pressure problem is caused by little to no refrigerant in the system . . .

Your system is mt. When you shut the car down both hi and low side equalize if you don't know what you are doing have it charged at a shop and have them check for a leak Freon does not just go away
Should you attempt safety glasses are a must
Ernie 81 sc

Ronnie, its the high side that has a bad reading. I think 35 lbs on the low side isn't too bad.

emac, if there was a leak, wouldn't all the refrigerant leak out in a years time? There wouldn't be 35 lbs on the low side.

I hate to throw thousands away on an AC repair when I can do most of it myself at a much less expense. I just don't want to start replacing everything. I am sure these readings must point to something in the system. A starting point.

Why would the pressure read almost zero on the high side and at the bottom of the good range on the low side?

Are you sure the gauge was seated all the way and the adapter was screwed all the way on?

There is no pressure cutout on my 88

Yes, it was seated all the down and screwed down to open the valve. I did get a low reading until I turned on the AC. At that point it went to zero.

Do you have the two readings reversed? Like the high side reads 35 and the low side 0?

That's a good thought, but no.

I have a Harbor Freight AC manifold gauge set with red and blue lines. Also with the R134a conversion kit I can't hook them up wrong.

The readings are weird, I will try again.

lsf911
You stated that you had the car for a year and the A/C never worked, so you don't really know how long the system has been low on Freon. I have seen a system that had a rock pierce the condenser and needless to say the freon lasted seconds, on the other hand a very slow leak could in fact take a year. As I stated in my post once the system is off both sides balance out so if you are getting around 10 lbs on the high side I would guess you have about 15 on the low. They sell dye that could be introduced into the system and this would be a starting point for finding the leak. As you stated everything appears to be normal. The bottom line is with the readings you have you will not cool, I stand by a nearly M/T system

Thanks Ernie. That does make sense to me. I will try the dye.

You will put the dye into the suction side (low) side.
Im sure you will find the issue.

Assuming you are not turning the thermostat down when the compressor cycles off it does that normally only when the evaporator has been chilled in accordance with the setting.

~70 PSI for both high and low is the approximate norm with the system off and having time to equalize. 35 PSI is the approximate norm for the low side with the compressor functioning or having just cycled off.

I would mistrust the guages.

PO installed a pressure switch?

Low charge and the compressor functioning, pumping the low side should ALWAYS be lower than the high side....wrong color caps on the fittings....it happens.

you still have schreader valves?

i have had the part in the hose that pushes the valve down go bad. i bought a bag of rubber seals and the center part at the parts store.

if not, you have the hoses connected wrong. i cant think of any way you could have 35psi on the low side and 0psi on the hi side. unless something in the compressor could do it, but then i still have my doubts.

does the compressor have the service valves that you can close the ports?

Do a search on Jim Simms and AC. Do you have a 134a receiver/drier? The adsorbent on a R12 isn't compatable and will foul and block the system, same with the oil in the system and the expansion valve. I'm no expert but you can't just change the refrigerant. BTW, I'm no expert, my weak A/C temps tell me that.

Sense you have gages. You aught to drain the refrigerant in it and refill with the correct amount of refrigerant or somewhere close and start from there. Then you might have good a idea of what is working correctly and what is not. Is the compressor clutch cycling allot or is it running all of the time.

AC system check is a series of process checks.
Pull a vacuum on the system and you will find out if it will or will not achieve vacuum watching your gauge manifold while the pump is running. If vacuum is not achieved, you have a big leak.
If vacuum is achieved. shut off the gauges and turn off the vacuum pump and watch for a few hours for any pressure increase. Loss of vacuum and pressure increase is indicative of a leakage.
If still inconclusive, pressurize system with nitrogen to 150 psi and watch for loss of pressure over a few hours.
If it checks out leak tight, then you can start checking compressor operation by putting a refrigerant charge and watching low and high side pressures. If high side pressure is still zero or low, compressor discharge valve is suspect. Replace compressor.
Once compressor is replaced, you may find your AC is working but may also find it does not have enough cooling capacity and the frustration sets in and you start thinking about upgrades.

Step one is to look at the pressure in the system.

If you get an impossible reading it is time to evaluate.

Okay, thanks to all who responded.

I performed the pressure readings again. I think the high side gauge is not functioning properly out of the box.

I used the low side gauge and took measurements on both low and high with the results being 35 lbs on each side with the system off. When I turn the AC on the high side drops while the low side remains constant.

To answer a couple of questions from replies, I did use the thermostat to cycle the compressor on and off. It's cold here now (well not today) so the garage has been around 50 degrees. When I turned the thermostat from low to high the compressor kicks on and off.

I do not know if the PO replaced the receiver/dryer to an R134a. The only thing I know for sure that he replaced is the compressor and fittings for R134a.

I am thinking what Ernie said is true. Low refrigerant with a leak somewhere...possible other things too.

35lbs is your highest reading on the "high side" then it drops from there when you turn the compressor on? I'm asking because that would be so low that it would likely not register on a high side gauge.

I would also add that if your high side pressure is 35lbs max then you need to stop using the compressor until you can get a proper charge run in.