A/C compressor cycling...no high side pressure

[SyberTiger]

• 2002 Ford Explorer 4.6L
• 150K miles
• A/C has never been serviced
• Central Florida - yes, it's still hot here and we run our A/C pretty much 11 months out of the year
• At full blast the a/c system is blowing slight cool air...not cold

I've got the manifold gauges set up tonight with the current ambient air around 70 degrees. The compressor is cycling as follows:

Low side pressure begins at 15 psi, it takes 7 seconds to get up to 40 psi then pressure starts to drop when the compressor stops and it takes about 4 seconds for it to drop back down to 15 psi and the process starts over with the compressor kicking on again.

High side pressure is zero. I don't know what to make of it. Is the high side manifold gauge shot? I would think if the pressure was just too low because there isn't enough refrigerant then the low pressure sensor would cut off the compressor altogether...but maybe there's just enough refrigerant for the system to run but I'm confused why the high pressure side would be zero psi...you'd think there'd be some pressure.

Any thoughts on what the issue is before I attempt to add some refrigerant?

 

[SyberTiger]

I'm going to have to get back to y'all with this. It appears that the POS manifold gauge set rental from O'Reillys was so abused that there's something not right about it. When I fiddled with the high pressure hose connection to the gauge the reading came up like it had been pinched closed. I'm going to buy a set and try over again. Right now I'm thinking I have a slow leak in the condenser as evidence by some goop with a sheen located in the area which is likely PAG oil.

 

[plasticseng]

You should see the low side drop when the compressor kicks on. When the compressor is off, the two sides start to equalize in pressure, so the low goes up and the high goes down. Once the compressor kicks on, you should see the low go down and high go up.

 

[SyberTiger]

Okay...now that I have brand new a/c manifold gauges these are the reading I got.

Ambient temperature 70 degrees and 45% humidity. The sight glass was clear the entire time before and after recharge.

Before adding more refrigerant the compressor cycle time was about 5 to 7 seconds back-n-forth all day long:

• High pressure side was 100 to 120 PSI
• Low pressure side was 46 to 17 PSI

I added enough refrigerant (about 10oz) to get the compressor to stop the rapid cycling and the reading were:

• High pressure side was 150 to 255 PSI
• Low pressure side was 35 to 24 PSI

I didn't measure the time period at this point but it took a very long time for the compressor to shut down.

What would you expect the pressure reading to be and other than a slow leak I have what other issues are likely?

 

[lincolnshibuya]

the pressure should stabilize otherwise you're still low.

 

[SyberTiger]

the pressure should stabilize otherwise you're still low.

Thanks for the chart.

So, you are saying that the low and high pressure points should more or less stabilize at a fixed pressure point rather that moving between the lower and upper ranges I posted?

When I get back from this trip to Atlanta I'm going to get some dye in the system to find out where the leak is but for now my guess is the condenser. Hopefully, what I put in there will not leak out until I get back to Orlando in a few days because it's still getting to about 82 degrees in the daytime. I'll add a little more refrigerant to see if it stablizes to more fixed pressure points.

 

[shucker1]

As long as the compressor is running the high and low side pressures should not vary much.

Any time I charge a system I have the windows open, the blower on High. The inside can never cool off and the Freon has a chance to "Boil Off" in the evaporator. This keeps the compressor running. Add a little Freon at a time and watch your low side come up to keep from overcharging.

A thermometer stuck in the dash helps a lot too. You can see what your outlet temp is as you add Freon.

I usually shoot for about 35-40 PSI unless it's really hot outside.

Once you think you have enough Freon in the system, you can actually close the windows and turn the blower on a lower setting. Within minutes the low side pressure should begin to drop to a point where the compressor begins to cycle.

 

[plasticseng]

This is what I had after a complete system rebuild & weighing the specified charge in. This is a front system only. Vent temps are high 30's with the blower on low, mid 40's on high.

 

[Number4]

Forget testing the system and worrying about your pressures. If it's low at all, you have a leak. Adding refrigerant is like throwing money out the window as you drive down the road.

Buy a can with dye in it. Two if you like. Run that for a period and rent the flashlight and goggles (or buy them) necessary to see the dye. It'll be a neon color. Find your leak, fix your leak and recharge the system properly.

 

[lincolnshibuya]

Thanks for the chart.

So, you are saying that the low and high pressure points should more or less stabilize at a fixed pressure point rather that moving between the lower and upper ranges I posted?

When I get back from this trip to Atlanta I'm going to get some dye in the system to find out where the leak is but for now my guess is the condenser. Hopefully, what I put in there will not leak out until I get back to Orlando in a few days because it's still getting to about 82 degrees in the daytime. I'll add a little more refrigerant to see if it stablizes to more fixed pressure points.

it should stay at a fixed point and you also have to check the pressure when the rpm is at 1500 or 2000 rpm (cruising rpm)

r134a cans are very cheap online (ebay), they are like $4 a can or $5-$6 in walmart.

 

[SyberTiger]

Forget testing the system and worrying about your pressures. If it's low at all, you have a leak. Adding refrigerant is like throwing money out the window as you drive down the road.

Well...I'd rather not drive 500 miles home to 84 degree temps in Orlando....I'll address the leak when I get home and have a chance but for now, if it will hold enough charge, I'm gonna put in something for comfort.

I was in Walmart yesterday...all they had was the $14/can stuff that had StopLeak or whatever gunk I don't want in my system so I went to BigLots for the $7/can without the gunk. I'll need to check out eBay for the lower cost stuff.

 

[shucker1]

Watch out for that stop leak stuff.

It leads to something call "Black Death" in your A\C. It gets in there and clogs the O Tube.

Now might be a good time to replace the Dryer Can while your at it.

Remember if you open the system you are going to need to pull a good vacuum for about 30 minutes to get all of the moisture out.

Freon and water vapor make Flouric Acid. It eats the system from the inside.

I know about the A\C situation. Down here in south Louisiana it's 75 outside today....

 

[SyberTiger]

I added one more ounce then it stabalized and seemed to be blowing cold. We'll see how long it holds. Thanks for the help

 

[imp]

@SyberTiger "to 84 degree temps in Orlando...."

R.H. about, what, 85%? "Feels like" 92` when actually 84`.

My location, 84` with R.H. 8%, "Feels like" 74`. Out here, they actually don't call it "heat index", but rather "comfort index". The Dew Point often is Zero degrees, or less! Cold beer can never "sweats" up yer tablecloth! imp

 

[TechGuru]

Well...I'd rather not drive 500 miles home to 84 degree temps in Orlando....I'll address the leak when I get home and have a chance but for now, if it will hold enough charge, I'm gonna put in something for comfort.

I was in Walmart yesterday...all they had was the $14/can stuff that had StopLeak or whatever gunk I don't want in my system so I went to BigLots for the $7/can without the gunk. I'll need to check out eBay for the lower cost stuff.

Just get a case of DuPont SUVA

http://www.ebay.com/itm/Dupont-Suva...-Refrigerant-Freon-12-In-A-Case-/281255151634