I left the vacuum pump running on the A/C for about 5 hours, then disconnected it and went to bed. In the morning, the system was still holding vacuum, so I hooked up the refrigerant (r12) and set to charging. It was about 75 F outside, but the pressure on the high side would only rise to about 100 PSI. I put approximately 37 oz of refrigerant in, but the pressure would not go higher. Low side pressure was approximately 30 PSI. The A/C blows cool air, but not cold, and I checked the air mixing flaps to be sure they weren't causing problems, and I ziptied my heater valve closed because I have yet to trace down that vacuum problem. Reading up on the A/C system tells me that I should expect to see about 2.2 times the pressure as the outside temperature, or for hot/humid days, temperature plus 100 as a rule of thumb. I was expecting to see at least 150 PSI, but I came in at only 2/3 of that and as I said my A/C only blows cool air (definitely better than nothing, but I don't believe it's right yet). Anyone have this experience or know what the problem might be. I tried wetting down the condenser and the pressure dropped a bit but still when the water evaporated the pressure didn't go above 100 PSI or so. I also took it out for a drive on the highway and it was a little cooler running, but not cold then, either. Thanks in advance

 

The 37 ounces is correct if you have no rear air.

At 75 deg ambient, the usual head pressure is 165-200 psig.

One possibility is that there is a restriction in the high side. Cautiously feel the high side lines and condensor, and see if you feel a cool or cold spot. Be careful - all of that side should be hot - about 140 deg F.

A cool or cold spot shows a restriction.

 

What is the history on the compressor? Has it been checked?

 

Probable issue is with the compressor, although there's a very slight possibility it could be a restricted suction-side issue with the expansion valve. Did you replace the expansion valve during your system refurb? It's about $35 or so last time I looked, cheap enough to merit replacement whenever any major system work is done. Takes less than 10 mins to replace when the system is open.

The expansion valve throttles refrigerant flow through the evaporator. Goal is to allow enough liquid mass through to carry a lot of heat away, yet keep the presure in the evaporator low enough to make sure that liquid mass flashes to vapor inside the evaporator. If there are obstructions to flow in that valve, suction pressure would drop while high-side pressure would go up some, but not much in the end since there would be no heat added except heat of compression. Ultimately it would go down when the fans are able to carry that smaller amount of heat away.

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Take a look in the sight glass in the top of the drier, and see if you can see liquid flowing there. The refrigerant should be almost clear, but you'll be able to see traces of oil flowing with the refrigerant as thin yellow streamers moving past. If you see a lot of bubbles, there's a problem. With no evidence (in pressures) that there's heat being added to the system, and considering that you've added a full charge, it should normally be completely liquid. I don't have the Mollinier diagram for R12 handy so don't know if it should ve vapor at the low high-side pressure though.

 

Four things will fix it, a good solid flush, a new compressor, new expansion valve and a new drier and of course a complete charge and oil.

 

New expansion valve and drier were put in, as well as all new o-rings. I found some r-12 in my grandfathers garage, so I have enough for at least three full charges. Therefore I decided that I'd keep with r-12, but the system is set up for the conversion now. All I'll have to do is swap out the charging ports and it's converted.

The compressor was not checked, and truthfully I was afraid that it would be a weak compressor. I've heard that compressor failure doesn't always happen at the main seal, but instead internal pieces can wear down resulting in reduced suction pressure. The only thing I didn't do was a full flush, and although I know I should have done it, I hoped that I could get away with it since I wasn't swapping r-12 for 134. If I do have to flush, is there any procedure out there which details this process? The stuff I've read indicates that you don't flush the expansion valve, the reciever/drier, or the compressor. Does this mean I'd have to disconnect all the lines and run ethanol through them individually?


Thanks for the help!

 

Even though you've locked the heater valve closed, the flap inside the valve is notorious for leaking through. So, you may be passing hot water through the heater coils and keeping the air from coming out really cold. The quick fix is to block off the heater hose, or kink it while you're checking the AC. The correct fix is to replace the valve with one of the solenoid style valves that stays completely shut.

Your local radiator shop will have this tool handy.

http://www.lislecorp.com/tool_detail.cfm?detail=268

 

I will suggest one more time a free, quick, easy check for a restriction in the high side, which appears to be the most likely cause for your symptoms.

Run the system for several minutes, preferably with the engine at a steady 1500 - 1600 RPM. Carefully feel of the entire high side of the system for any cool spots. A cool spot indicates a restriction. Cover all of the lines, all of the condensor, etc.

 

Doc, the original symptoms were described as low cooling capacity .and. not enough high-side pressure. Failed heater valve might cause one, but not both.



To the OP, what symptoms were you chasing when you decided to clean and recharge the system?

Compressors do wear out, but they are truly durable IF the oil charge is maintained and they aren't run empty, or for long periods with low charge. Poor cooling fan operation will contribute to short compressor life thanks to high head pressures and heating. In general, compressors are a source of debris in the system a lot more than they are victims of it.

There are several options for compressor replacement. Used are available. The compressor itself can sometimes be found with different manifold plates for other vehiicles, so you get to swap on your connector plates and maybe your clutch to get one to work. Charlie Griffiths, a sponsor here, sells full kits that include a more modern compressor, plus all the brackets and hoses need to install in your car.

Flushing the condenser and evaporator can be done as a DIY project but most DIYers end up with hokey flush methods that threaten to contaminate the system with even more stuff. Generally, crud that starts with a grenaded compressor makes it through the condenser but is stopped by the drier and for sure by the expansion valve. So removing and cleaning the condenser out of the car is OK, new drier is OK, and flush the hoses and pipes to and from the expansion valve. That leaves the evaporator, which requires a circulating flush rather than a fill-and-purge flush to be truly effective. The design of the evaporator has parallel circulation paths from header ends, so while a simple one-time fill-and-purge will get oil film out, it won't move debris that's in the end headers at the bottom. That really requires a high-volume high-velocity flush method.

 

Ok

 

The air conditioning never worked since I bought the car. When I got it, I took it to a very well-recommended mechanic and had him look it over. He said there was a leak in the A/C system (since it wouldnt hold vacuum), but he didn't bother to trace it down, as per my instructions. I just wanted to get an overall review of systems. I can live without A/C, so I didn't think any more of it until recently when my driver's side window started going intermittant on me. I decided that fixing the A/C might be a decent solution, so I hooked up our air compressor (that we use for painting cars, it's got a very effective water trap in the line, and I immersed in a bath of dry ice and denatured alcohol for insurance). I found the leak in the tubing that leads from the condenser into the reciever/drier where the pressure and temperature switches are located. I put the new part in and the system held pressure, so I put it under vacuum for about 5 hours. I had previously drained and flushed the compressor with fresh polyol ester oil and then refilled it. Since I didn't flush the system, I figured there would be enough oil left in the system and compressor to keep the system running, so I went ahead and charged it up. I got up to about 100 PSI as I said before, and the pressure wouldn't increase past that even though I know I put the correct charge in. For the record, concern over not being able to flush properly was the main motivator for not flushing at all. No local A/C shop will touch the car unless it's converted to 134, and although I can do that, since I had r12 lying around I figured I could do it myself and keep the old system for quite some time, plus if I didn't switch over I might get away without flushing the system.

When I drained the compressor, there was a small amount of fine black particulate matter that came out with the old oil, but there wasn't a great deal and I ran probably 16 ounces of fresh oil through it before I topped it off to make sure I got as much out as I could. The oil came out clear at the end, so I think the compressor is clean and properly filled with oil. I was a little concerned with the vacuum removing some oil from the system, but since the compressor had slightly more than 4 ounces of oil in it, I figured that it would be okay. Before I mounted the compressor, I turned it over by hand a few times to keep from lugging the compressor upon turn-on.

I think that Wally has a point. If the constriction is in the right place, it could cause low pressure readings at the gauges but consequently higher pressures before the obstruction, so I will do a spot-check to check for this. Also I will replace the heater valve as a matter of course, and hopefully that will help things.

 

What would the expected air temp be for a normally functioning A/C system?

 

Depends...

A properly functioning system, ambient below 90ºF, adequate airflow in the condenser. engine above ~~1500 RPM, interior fan on 2 or less, car not hot soaked? Temp should cycle around 38ºF due to freeze switch function.

 

Thomas--

Look carefully for that obstruction in the condenser area. It's about the only place it could be. It has to be between the compressor discharge and the test port on the drier for your symptoms.

 

I think that a really good 928 A/C system can have output air at 35 deg F at highway speed.

Most reasonably good systems are probably between 40 deg and 50 deg at highway speed.

50 deg or above, and you will be pretty warm in 90+ deg ambient.

It isn't unusual for the rear air to be colder than the front air, since it is always recirculated air, with no heater, mixing flaps, blending flap, hot input air, etc., etc. involved.