Been working on my 2000 Bravada, just getting it ready for the stepson to start driving.

The AC has been working fine for the past few months, but the compressor started cycling on and off, every few seconds. Still blew fairly cold air, so maybe I shouldn't have messed with it, but I did.

Hooked up the gauges initially, and the low pressure side read really high (in the red for the gauges) while the high pressure side read low.

Started the Bravada, and as the compressor cycled on, the low pressure side would drop to below the "green", high pressure side remained constant low.

I've only had the truck a few months now, don't know its history, so figured I'd start from scratch with it. Had the refrigerant removed, then brought it home, hooked up the system vacuum and vacuumed it out for an hour or so. It seemed to hold vacuum pretty well.

Looks like the system should hold about 2 1/4 12oz cans of r134. Started filling it up with the r134 I had on hand (new cans). As soon as I started putting some r134 into the system, the low pressure seemed to go up pretty high, while the high pressure side stayed pretty low. Not even 1/2 a can into it, it started doing the same thing as before (though, I don't have any cold air now).

Compressor cycles on and off, and the low pressure side spikes accordingly (into the red on the gauges). High pressure side still reads low on the gauges.

I'm a bit concerned about dumping more r134 into it, even though I know it isn't anywhere near filled yet, since the low pressure is spiking into the red on the gauge. Really don't want to just replace the compressor and all that, but I'm not sure what to try.

AC systems have never been something I've enjoyed working on, just not my cup of tea. I really don't want to take it anywhere either though, anyone have any advice on what to check and/or replace?

Thanks in advance!

 

In short, it sounds like it's functioning normally, but still undercharged.

Whenever the clutch kicks on, the low pressure should drop and the high pressure should rise. When it kicks out, they will equalize (unless it kicks back in before that happens). When you're adding refrigerant, the low side will rise because of the pressure from the can.

Put the called-for amount in it, and see how it works. As long as your high side stays reasonable (<300PSI or so, depending on ambient temperature and other factors), you won't hurt anything. The low side should be around 35-40PSI under normal conditions. All numbers are estimates. YMMV depending on conditions.

I'm NOT trying to insult you by saying so, but it sounds like you don't have much understanding of how air conditioning works.

 

Thanks. My concern is the low pressure side is going up to 80-90 psi when the compressor turns off (or not running at all), and I understand the low pressure should be in the 40 PSI range (as you stated). Does that mean there's a blockage, clogged accumulator or is it normal for it to spike that high (low pressure side) until it is full?

 

That's totally normal. When the compressor is not running, the high side and low side will be exactly the same. The exact number depends entirely on temperature.

 

Copy, thanks for the info.

 

The high pressure and low pressure sides of the refrigerant system are separated by the orifice tube near the accumulator. When the system is at rest the pressures equalize through the orifice tube at around 90 psi for 85F ambient. When the compressor starts the high side start going up and the low side starts dropping. This will continue until the pressures stabilize unless the high side goes over the high pressure switch value or the low side goes under the low pressure switch value. Either switch value being violated causes the compressor to shut off and the pressures start equalizing again. Then rinse and repeat. The proper way to charge the system is to dump as much charge in as the system will accept (the system pressure equalizes with the can pressure), then turn on the system and dump the rest of the plate charge weight in the system through the low side port. Then check the operating values against the specs (low side pressure, high side pressure, vent temp, ambient temp, humidity). If you do not have a scale or calibrated charge bottle to weigh in the charge then you have to charge to the target operating specs and use experience to get it close. Under or over charging drops performance and stresses the components. Cycling is normal under certain operating conditions of low evaporator load. If you have any follow up questions please quote the actual pressure values, I don't know what red means on your gauges.


George

 

Also, you don't leak test with a vacuum you test with pressure using dry nitrogen. Lastly, to insure a trouble free system, especially after any serious replacement work you vacuum to under 500 microns and make sure it can hold that vacuum after you blank off the pump . The problem with using a charging gauge set for getting and holding to a proper vacuum is that the dial shows roughly 27" hg for anything from an acceptable vacuum to a vacuum showing water content still left in the system or a vacuum with an unacceptable rise, its not sensitive enough to do the job right. In the absence of a garage full of HVAC equipment then you can vacuum for a couple of hours and hope for the best.


George