My car is a 1987 coupe, L98. Typical problem....trying to charge A/C system, but compressor keeps triping on low pressure. Cycles off, almost instantly, after comming on, therefore, it won't draw the refrigerant from the can.

I know you have to disconnect one of the switches, but I forget which one. There are 3 switches, 2 on hp line and 1 on the low line at the discharge of the evaporator. No help in service manual. Is there anything else I need to know???

All help appreciated!

 

you have to disconnect the low pressure switch and put a jumper wire in there.

 

The other thing you can do is get a warm bowl of water and set the can of refridgerant in there.

 

You should not have to jump the low pressure switch. With the compressor disengaged, once it has taken all the refrigerant it will take, it should come on. The problem is that the pressure cycling switch reaches a point where it thinks it needs to cycle the system.

You should not need to jumper the pressure cycling switch connector, but if all else fails, jump it long enough for it to take a little more refrigerant.

Are you SURE that you don't have a significant leak? If you did, the behavior would be as you originally described.

Good luck,

 

A lot has to do with the outside air temp (and if it's 60 or below, you can pretty much forget it), but assuming you pulled a vacuum, that should have sucked in the first can. For the second, disconnect the low pressure switch at the Evaporator and put a paper clip across the harness connectors. Disconnect the fan switch on the high pressure line so that the main fan is running. Start the engine and block the throttle so that it's running at 1200 to 1500 rpms. Turn the a/c on max and the system should pull in the 2nd can. If you're using R134, stop there and check your high side pressure. I wouldn't go more than 2.2 to 2.5 times the temp of the air across the condensor. When charged, hook everything back up but leave the a/c on max and disconnect the blower motor. The lack of air flow should drop the low side and it should cycle off at 22.5 psi for R134 or 25 psi for R12. If not at these numbers, turn the screw between the low pressure switch terminals counterclockwise to lower the cutoff threshold; clockwise to raise it. Next, leave you gages hooked up and make sure the main fan comes on when the high side reaches 210 to 230 psi and that it cycles off at 170 to 190 psi - Note you probably won't be able to do this unless it's at least 70 to 80 degrees at the condensor, so if you're below these temps, wait for an 80 degree day to finish checking it out.

 

Thank you gentelmen, very much! Sorry for the lack of info on my system. I have no leaks. I held a steady vacuum for 2 days. The low pressure switch is a new 134a retrofit switch, supposedly, set up for the conversion. Condenser, compressor, hoses, accumulator/dryer, oriface and switches all new. in fact, only part being reused is evaporator.

I appreciate and will try all advice, and again, THANKS!

 

I think suncr is on to it. With a low ambient, it will be difficult to get it charged. Since you're in North Texas, you shouldn't have to wait to long for a 75 degree day.

The way that the pressure cycling switch works, it cuts out the compressor at a very low pressure, about 25PSI or so, then when the pressure gets above forty something, it kicks back on. If you just keep giving it refrigerant it should eventually take it. If you are using individual cans, you can put the can down in a bucket of luke warm water to boil it off. If you are using a large cannister it will be more difficult to heat enough to boil off refrigerant. They make heaters that wrap around the cannisters but that would be too expensive for a DIYer.

Good luck,

 

System sounds tight - I'm not a fan of heating up a can in a pot of water. At a 100 degrees, that gas is going to have a pressure of about 120 psi. Who knows the design limits on these things (and with my luck, I'd probably blow something up). Some just put it under their arm, but I've never seen a shop with a warmer.

 

I've probably put several hundred cans in LUKE WARM water over the years with no problems at all. The pressure of refrigerant is eighty something pounds regardless of how much liquid is in the container, as long as there IS liquid in the container. It does not raise the pressure, it only boils the liquid into gas so that it will come out. Any pressure change is very minimal.

Have a great day,

 

Well, either Dupont or my pressure/temp charts are wrong - but what works for you, works for you. At 80 degrees, you have a pressure of about 85 psi. Bring it up to body temp and you go somewhere around 100. Better to keep the condensor lower than the can temp which is why you put a fan in front of it (doesn't work very well on a Vette) or simply keep the fan running (disconnecting the fan switch up until '89 does the trick).

 

Yes, you're right, temp will, of course, increase the pressure, but it has never been a problem.

My point, which I did not state very well, was that the pressure in the can is the same whether there is 13 ounces of liquid in the can or .1 ounces of liquid in the can. That is, of course, given the same temp for each.

Regardless, I have done this lots of time with total success and the cooler the ambient the more necessary it becomes. Also it is FAR preferable to turning the can upside down and slugging the compressor with liquid.

Have a great day,

 

liquid charge the high side

 

NO! Do NOT charge on the high side! People have been killed doing that! You can blow up the can in your hand charging on the high side!

Best of luck,

 

That's DOUBLE DIDO!!! Don't Charge from the High Side.


In fact, I don't even think the high side fitting will fit on the charge line fitting. If I remember correctly, on my 84 Camaro w/ R-12, the High side fitting was a different size than the Low side fitting. But regardless, just make sure you ALWAYS charge from the Low side!!!

 

Yes, in the mid seventies, most carmakers put a smaller size fitting on the high side in an effort to curb charging on the high side. It helps prevent it for some people who only use a suicide hose, but if you're using gauges, as you should, you can just as easily open the high side valve to connect the can to the high side.

It is okay to charge into the high side after pulling a vacuum and BEFORE starting the car. When I evacuate, I open both sides of the manifold for evacuating, then after the vacuum is drawn, I close both valves, disconnect the vac source and connect the refrigerant supply, purge the supply hose and then open BOTH sides to allow as much refrigerant into the vacuum as possible. THEN I close both valves, start it up and then start charging into the LOW SIDE ONLY.

I try not to be too nitpicky, but when it comes to safety issues I intentionally make a pest of myself to get peoples attention. I may insult someone along the way because they are already quite aware, but that is better than seeing someone get hurt.

Have a great day,