'Tis the season for DIY'ers to be recharging their own AC systems with the several versions of R134 Canister-Trigger Dispensers found at many retailers. I find the instructions for using them to be lacking in detailed information.

I know that if your compressor engages & disengages frequently when the AC is turned on, it is often due to inadequate refrigerant in the system. I have successfully used a Canister-Trigger Dispenser to charge the system and observed the compressor-cycling return to a normal frequency as the temperature of the cabin air gets colder.

My questions involve how to make sense of the readings of the gauge on the Trigger Dispenser. When the engine is not running, I connect the dispenser to the low pressure port and it jumps into the red zone even though I know the charge is low and the compressor is cycling frequently. When I start the engine the gauge drops to a low reading and it seems to take more than a canister to get the gauge to start rising. toward the green. When I shut off the engine, the gauge jumps sky-high. The instructions fail to make it clear what the gauge readings mean when the engine is off or on, what the implications are of over-charging the system, and how do I know when I'm overcharging the system? TD

 

Here's some info on it. The main reading is based on your low pressure gauge with the engine running.

R134a Pressure Gauge Readings

 

By the way a simple search on the net and youtube will give you all the info you need in understanding and charging a typical AC system. Let the net be your friend

 

The pressure should always be sky high when the engine is off. That is good. However, overcharging is a concern, since you don't know high side pressure.


A word about cycling: Most cycling is to keep the evaporator pressure up. This is just freeze protection, and it's a good thing. If the system is empty, then of course there's no danger of freezing, but the designers figured that's your problem. they designed to work really well when it actually does have refrigerant in it.


I am a licensed a/c technician and also a chemical engineer. So I have thoughts on this, but they all come down to guesswork on the 850's, which don't have a high side port.


So generally I try to get it cooling. If it's cooling then there are certain reasonable things that have to be going on. I would not really charge one beyond the point where the compressor is running all the time at idle in really hot weather. I certainly would not add all the refrigerant than the car is supposed to hold, unless I was sure it was pretty much empty. You can always do that; empty the system then charge back what the full specified charge.


Keep in mind that when you speed the engine off, and get more wind through the condenser, it's going to cycle. Also if it's 80 degrees out, it'll cycle. That is good.

 

I meant to say "speed the engine up" while driving in the last line, but fat-fingered it. Can't edit for some reason.

 

You should just get an AC manifold. $20-25 on eBay.

 

Thanks for all the info. I feel more confident that I'm proceeding properly. I do have a set of good gauges on an AC manifold but I believe, like "firebirdparts" mentioned about 850s in his post, there is no high pressure port on a 940 either, so I can't know what that reading is. Things are cool for now (pun intended)! TD

 

Just added an full can of R134 and air is certainly cooler but not cold. Low pressure gauge is only at 25psi and with ambient temp @ 80 today, I should be fully charged at about 40 psi. Should I add another can?

 

Sorry but your explanation leaves me in doubt of your skills. Professional AC shops work strictly with a gauge set and pressures vs outside temps. On these older systems a cycling system is not a normal function unless you have a problem with the system. In these older Volvo's when converted to 134a or the later ones with 132 should have center vent temps of around 40* on a 80-90* day.(see the chart https://www.google.com/search?q=ac+s...zyA_loLjm0M%3A. There is a low pressure switch on the receiver/dryer and a high pressure switch on the high press. line. As I mentioned there are many articles on the net on auto. AC systems. I'm not a AC professional but have worked on mine and others for the last 40 years and feel comfortable with them. I am a retired Mechanical Engineer.

 

If you're doubting my AC skills, you are spot on, but I am eager to learn. The first thing I need to learn is where to find the high pressure port on a 1993 Volvo 940 turbo so I can hook up my AC Manifold gauge set. TD

 

If you read the post you will see I did not doubt your skills. As for the high pressure port on a 940 I don't have the exact location but I would look on the back of the compressor . There have been other posts that say Volvo did not put a high side service port and if that is your situation then you would charge the system using the low pressure port. Check the chart I posted in my previous post and match the pressure with the tempature. Also make sur the engine is running at about 1500-2000 rpm and that you have a fan blowing in front of the rad/condenser.

 

I'll check the back of the compressor. I assume it would look similar to the low pressure port? If I can't find it, I'll try the method you suggest. TD

 

Any idea why a system that's fully charged and works well most of the time would temporarily shut down while on the freeway. Then come back on when it was switched off and back on?

 

1st , which system do you have . There is the standard AC system and the climate control type system. Also by working well what do you mean. If you put an AC temperature gauge in the center vent what temperatures are you getting. Under normal driving on a 80-90* day you should see 40-45* at that vent. If not you need to check the freon in the system using a set of AC gauges to determine what the level of the charge is. Based on those readings it will determine what your next move is.

 

I have the standard system, no climate control. And I get low 40's at the vents on an 80 degree day. Like I said, it works great, except once in a while it will shut off, never in town, only on the freeway. But I turn it off and turn it back on and it works again? Pressure switch?

 

Have you put a set of gauges on the system and if so what is the low pressure reading at 1500 rpm . If it is in spec then you need to start checking the various sensors and switches. The high pressure switch won't be intermittent , it's either good or bad. If your system is to cold the evaporator can freeze up and this would shut down the compressor. If you turn it off and back on (do you know if the compressor goes off and on) then you might have an electrical problem with the switch . Another area to check.

 

I did use gauges when I recharged - but no high side port or reading. Can't recall the low side reading - it was months ago. Hard to tell if the compressor's running at 70 mph. I like your idea of the evaporator freezing - forgot they could do that. Next time I won't turn it off and back on and see if it comes on by itself, after it defrosts?