A quick AC question.

I just got a 99 with AC. The AC of course doesn't work. So I went out with my little recharge kit and gauge all prepared to stick some R134a into it, and found that the pressure is already right up there. What happens when it runs is that it cycles every few seconds, sending the gauge up near the danger zone, then clicks off, etc. There's little if any cooling going on, of course.

If you're used to well water (see non-tech of last week or so) it's just like having a waterlogged pressure tank.

So my question, being ignorant of most AC issues, is what is this a symptom of? Overcharging? And any recommendations for what to do about it, other than "take it somewhere" or the usual Vermont option: "rip it out?"

Could be the pressure switch bad. Cutting off early thinking it's saving the compressor.

Thanks for the response, but the problem isn't as obscure as I thought. It occurred to me to check my recharging valve, and found that the point hadn't seated right in the can the first time. I wiggled it until it sprayed a little, started the engine up again, and it slurped up two cans of refrigerant, started working, and now the gauge says it could use one more can. I just hope it holds it in for a while. I'm willing to bet that the original owner never bothered with maintenance, so there's a hope that it's just gone low after 7 years. There was still enough juice in the system to start the compressor, so it wasn't totally empty.

You probably already kow this, but run a little freon through the hose to purge the air out before you add freon. That little bit of air in the lines can cause problems with the high pressure and adds up if forgotten a few times.

Hopefully it will hold the charge, next time put the dye additive in there too, then use a black light to find any leaks...took two or three cans before enough dye leaked out to show the bad evap by staining out of the condensation hose on the firewall on my 98.

The AC system is hermetically sealed, there really isn't anything you can do for preventative maintenaince on it, other than the belts and to check how well it operates from time to time. I guess you can adjust some of the AC clutches out there.

Over charged AC will cool really well, but then the hi-side pressure goes really high, it will disengage the compressor when pressure hits the hi-pressure switch as a safety feature and then you'll get the AC cycling on and off, but get it cooling very well while its running. Most AC have a pressure relief valve or burst disc so if the pressure hits even higher then the hi-press shut off switch, it will vent the refrigerant before the system bursts.

Rick Anderson said:

The AC system is hermetically sealed, there really isn't anything you can do for preventative maintenaince on it, other than the belts and to check how well it operates from time to time. I guess you can adjust some of the AC clutches out there.

Over charged AC will cool really well, but then the hi-side pressure goes really high, it will disengage the compressor when pressure hits the hi-pressure switch as a safety feature and then you'll get the AC cycling on and off, but get it cooling very well while its running. Most AC have a pressure relief valve or burst disc so if the pressure hits even higher then the hi-press shut off switch, it will vent the refrigerant before the system bursts.

Click to expand...

Thanks. As I said, I know little about working AC, because it's very rare for me to have a vehicle whose AC isn't already hopelessly gone, and my habit has always been just to rip it out. In this case, the low side pressure was cycling up to the red, then shutting off, and I thought this might mean that the level was OK. Once I got my valve properly aligned to the can, and it took refrigerant, the low side pressure gauge became steady (and rather low). But it's cooling, at least for the moment.

Although it is cooling now it probably leak out again. Before it does, use a mixture of dish soap and water in a spray bottle. Spray the fittings etc. Look for indications of oil. There is a leak somewhere.

Matthew Currie said:

Thanks for the response, but the problem isn't as obscure as I thought. It occurred to me to check my recharging valve, and found that the point hadn't seated right in the can the first time. I wiggled it until it sprayed a little, started the engine up again, and it slurped up two cans of refrigerant, started working, and now the gauge says it could use one more can. I just hope it holds it in for a while. I'm willing to bet that the original owner never bothered with maintenance, so there's a hope that it's just gone low after 7 years. There was still enough juice in the system to start the compressor, so it wasn't totally empty.

Click to expand...

I think the capacity is around 28 ounces, so I won't add anymore if those were 12oz cans. Overfilling can damage the compressor. Knowing just the low-side pressure doesn't tell you too much, other than confirming if it's too low for the compressor to even turn on. What you really should look at is how fast the low-side pressure cycles between the low and high points. Really fast means the compressor is sucking the low side down quickly because there isn't enough freon there. The FSM has a ballpark figure saying it should cycle less than 10-times/minute with the A/C full blast.

I'll probably leave it as is for a while and see how it goes. I'm not too worried about a leak, because this is a 7 year old vehicle and I'm guessing it has never had AC service before, so if there was enough refrigerant in it still to run the compressor at all, it's not leaking badly. Probably unwarranted optimism, but this is Vermont. We only need AC about two days a year anyway.