I have a 92 s10 with an updated r134 a/c system. Originally the a/c wasn't working but it was all r12 stuff anyway. Everything was converted properly and new parts installed.. everything except the hoses (no leaks detected), the coil next to the radiator, and the coil inside the dash. The a/c guy (friend of a friend of a friend thing who has an a/c business) properly loaded up the appropriate pounds of freon on three separate occasions. Each time the a/c system worked properly (fan motor blowing cold air) for a period of time. Being in Arizona it can get hot here in the desert. Each time, when the day's heat was highest, with the car at idle, a noticable smell came into the car followed shortly by a loud whoosh sound... and what looked like steam (but wasn't as it dissipated immediatly) flowed from under the car for a few seconds. This was usually followed by no more cold air coming from the vents. Apparently somewhere along the system some pressure is building up in high outside temperature with the ac on and the vehicle running in idle, to a point that something.. be it a release valve, leak, whatever, is releasing gas.. and the crazy thing is that the gas is not all released at one time (kinda reminds me of those volcanic steam vents releasing periodically at Yellowstone Park). There is still pressure in the line which might suggest something is releasing then closing; leaks don't generally do that. During this process the compressor does not shut down. Today when it happened I still had residual cold air coming from the blower vents. As soon as the whoosh sound was heard I jumped from the vehicle to pop the hood to find the location.. but it happened too fast. I was just in time to see residual gas in the area around or below the silver accumulator. No gas/oil residue was found on the compressor or at any other joint/fitting location. Any ideas anyone?

 

Only thing I would think of around the accumulator that might vent is the pressure switch. That would be weird, though.
There's a HP safety valve on most compressors. I had that happen once on a denso compressor, like you very hot out. In that case, it was a pellet valve and had to be replaced.
This guy with the AC biz.. is that automotive ac or residential?
Overcharging causes higher head pressures and You should only add up to 85% of R12 capacity. **
Did he know that?

Converting from R12 requires special procedures because of oil differences. He needed to at least drain all the mineral oil he could from the compressor and fill with same amount Ester oil. Optimum is flushing the compressor but a new accum did the job for me on three cars. Theory at the time being that the mineral eventually clumped in the accumulator. Who knows.. it worked.
I used this system: http://www.sadik.net/nissan/howto/ac_r12.htm

Using PAG -without compatibility additives- in it or just leaving the mineral oil could cause condenser blockage.
134a does not carry mineral oil well. That's why it's likely to migrate to and stay in the accumulator I guess.

However ester and PAG are not at all compatible with each other. To make it more interesting, GM compressors want hi-visc PAG oil. Ester fits all.

** And here's a chart that explains why it would happen on a hot day:

 

..for your reply. We did in fact flush according to Hoyle (with the correct oil... hehe). As far as the qualifications of the fellow doing the job... I have no issues with that as his work was checked by another certified guy anyway. Hell, for as often as we've done this now even I can qualify for being a non-certified back-yard-dangerous auto a/c service technician.
Anyway.. thanks for your thoughts.. and I will check the chart you provided. Right off the bat today's temp here in the desert was 115... kinda off your chart.

 

Well, that would explain the fail safe venting. Point being that if he put in more than 85% of the R-12 capacity, that would have caused it.

Or any other fault resutling in marginally higher high side pressures.

Well, well!


Do you have a high pressure cutout switch on your truck?

That was part of the GM retrofit TSB
General Motors Retrofit Procedures

 

There isn't any 'vent' in the system. If you are blowing a charge, it is because a seal somewhere has opened up. They aren't supposed to do that. The function of the high pressure switch is to cut off the compressor to prevent damage to the system, not to blow the charge if the pressures get too high.

I can't understand why you keep getting it filled back up and why these 'mechanics' are doing that without tracing the source of the leak. You are throwing your money away. Find the leak. If it comes out as fast as you say it does, I would suspect the seal in the nose of the compressor. Go over the entire system with a black light (UV light).

 

Well, as I said... this "venting" is not occuring with any predictability. We charge up the system then I can drive using the a/c just fine for days/weeks until such time as the environment/conditions are such as to set it off (obviously high outside temperature resulting in high system pressure.. each time running at idle). This isn't simply a convenient leak that will show up as soon as you charge the color dye into the system. Using a vacuum pump didn't reveal anything prior to charging, no noticable leaks when the system was initially charged with R134 (at the correct pounds). While I recognize that likely 75% of all technical problems brought up by posters on auto repair themed sites have roots with ignorant owner backyard mechanics or cousin Joey from Jersey knowing just enough to make a wet dream wetter (thus wasting everyone's time), let's presume in this case, for the sake of argument, that my guys (and me) did the installation properly... and this is an odd leak situation that only occurs under certain conditions days/weeks later. I appreciate the thinking points everyone has brought up thus far. Our next step will be to use the color dye and wait for the vent to occur again.

 

Did they not put dye into it already? After the first blow they definitely should have... Have you taken a look around with a black (uv) light? I wouldn't put another ounce of refrigerant into the system until you have gone over everything with a light.

I can't imagine how much money you have spent in refrigerant already. That stuff isn't cheap.

 

Look, I know I overloaded that last post... the point I found on a lot of digging is that GM specified adding an overpressure cutout switch on your model.

I'm not being picky here, just pointing out that the factory thought it was necessary and, if you read up, they otherwise went the cheapest route on this.

As to 'designed vent' I have no idea.. just noticed that parts houses carry the replacement part for the HP safety blowoffs.

 

I want to revise my statement a bit. Pressure relief valves are common in automotive HVAC systems, but their set points are sufficiently high enough such that the system will not reach this pressure unless under very severe conditions. The relief valve is really only there to prevent an explosion due to VERY high pressures/temperatures (think car fire).

After a little digging, I have found that the HR-6 & V-6 compressors were both used in '92 and both have a high side pressure switch (HSPS) that would disengage the compressor clutch if the pressure gets too high (~350psi or so). After the compressor clutch disengages, the system pressure would drop almost immediately. This switch would trigger well below the relief pressure which should be in the 400-450psi range (425psi for the Harrison A-6 compressor). The A-6 compressor was also used in '92 and was available with a HSPS, a superheat switch, as well as without a switch... All of the old R12 systems I have messed with had a HSPS which would prevent this kind of problem.

You must have the A-6 compressor that doesn't have a cutoff switch that would turn off the compressor before triggering the high pressure relief. Still, the pressure must be climbing pretty high to open the pressure relief unless the relief valve is weak.

Before doing any more recharges, I would check and/or replace the relief valve. You can check the relief valve operation by charging the system with nitrogen up to ~425psi at which point the valve should open. If it opens at a lower pressure, the valve is defective. The valve is located off the back side of the compressor below & to the left of the lines when looking at the back of the compressor towards the front of the vehicle. It is threaded into the back of the compressor housing.

Also, you should make sure that the condenser & radiator are both clean with no bent fins, the fan shroud is intact, and the fan clutch is operating properly (i.e. locking up when the engine is up in temperature). Inadequate cooling can definitely lead to increased high side pressure.

 

Thanks for that reassuring information, Swartlkk. Makes me feel good we were on that right track with our thought process. We in fact have known about the rear-of-the-compressor pressure cutoff. I did mention in an earlier post that we did replace the compressor. I got it at a Checker-now-O'Reilly's. I guess one can assume that these old compressor cores that are brought back end up coming back from China or Mexico refurbished and looking new.. and sold to guys like me, but I don't imagine that necessarily means the pressure switches have been swapped out. On the other hand... during the previous ventings of gas/oil there was no residue in or around the compressor, the fittings, or that pressure switch. That just further mystifies us. An earlier poster in here (sorry.. nick escapes me) suggested that GM did some mod to the system for my year and added a pressure release somewhere in the system. Not sure where that might be.
When we started this project the hoses were in good shape so we kept them. I did not replace the coil inside the firewall nor the coil on the radiator. We flushed the system after the first to venting events (oddly there was still pressure in the lines, albeit not enough to cool).
Regarding the actual moment of the mystery gas releases.. I was at idle, and the engine temperature guage (no idiot light) was reading normal. From that one might deduce that the fan was working properly hence there was no overheating as a result of a poor fan. The compressor never shut down... thus speculating that any electric cutoff pressure switch is not likely a problem (unless a switch was not switching electric off properly).
I appreciate all the posts as it makes us here re-think the problem through.