AC Short Cycling - Driveway Fix

[JHZR2]

Hi,

My 98 Chevy S-10 ZR2 doesnt get a lot of use. Started it today, turned on the A/C (80F and pretty humid), and found it short cycling, though plenty cold.

Ive never touched the A/C, never added any refrigerant, etc. I thought I noticed the engagement being shorter and shorter, but it was not any big concern. I suppose I should have been a bit more proactive, but dont drive the truck a ton.

So... Ive seen a variety of recommendations on how to add refrigerant. I have 30# of R134a and US made gauges that are good and just a few years old. My thing is, I dont have a big emissions station fan to blow air at the truck, Ill be doing this in my driveway.

So what is the best approach for checking pressures and adding? Im thinking this:

1) Get a 16" house fan and blow it at the condenser.
2) Park the truck, full hot after driving in the driveway.
3) Set A/C to full blast, max setting (recirculate)
4) Set engine RPMs to 1500
5) Verify pressures and add as needed according to ambient temperature.

Is this sound? Should I crack the window or not use recirculate to add to the heat load? Should I have someone sit in the vehicle when doing this to make the heat load inside more accurate?

Thanks for any advisement.

 

[mechanicx]

I think it would be best to to evacuate it and charge by weight, but I'm thinking you don't have a vacuum pump and don't want to release refrigerant.

Usually you have the A/C in outside air mode, fan max, windows closed to put full load on the system. Check it at around 1500-2000 RPM after the A/C has ran for about 5 mins to stablize temperature. You don't need an external fan just the engine fan and the hood up, and not in direct sunlight. I would shoot for ~34 psi on the lowside depending on outside temperature and humidity. Be sure to open the valve on your manifold guages to purge air out of them before connecting them to the system, but don't open the high side valve for safety.

 

[JHZR2]

At some point I may evacuate (at a shop), probably replace the R/D for good measure and then refill by mass after vacuuming to <500 micron. All depends upon what I find. But it is a fun exercise to do to see what I find out on my system regardless... Just want to do it with as correct and optimal a set of conditions as I can...

 

[JHZR2]

Another question, a bit OT. Once I used my gauges to help someone do a static pressure test (to try to verify if based upon ambient, if he actually had liquid refrigerant in there or not), and then we looked at the pressure with the AC running. What does it mean if one side is normal and the other is high, like the low side is 20 and the high side is 225, or the low side is 35 and the high side is 350?

As I recall, theirs was like the later example I stated. Since it was off, I pointed them to the AC man... And never really investigated further.

 

[mechanicx]

If the high side seems high and the low side normal, there could be air in the system or it's slightly overcharged, or maybe some restriction on the high side (debris at the expansion valve/orfice tube). high side pressure can go pretty high on 134a systems with the vehicle sitting depending on temperatures though.

 

[Dakota1820]

usually when the high side is really high it means a blockage. but ive seen some vehicles with 35 at the low side and 350 at the high side and it was normal under certain conditions. usually though a high high side and a low low side is a blockage. but a normal low and a high high side is odd. maybe a slight blockage? or compressor is working really hard? im not sure.

i think your plan is pretty good. the fan isn't necessary but if you have access to it i would. you can use a hose to a spray the condenser. i would roll windows down turn it on max a/c and after its run for a few minutes and cooled down do the test and see where the pressures are. you probably only need a little freon. i wouldn't evacuate the whole system over a little freon. but that is me. is it wrong to evacuate it? No but i dont think it is necessary. charge it by pressures and you'll be fine. charging by weight isn't exactly accurate either. different gauges different hose lengths play a role in it. I am not saying its wrong to do it that way because it isn't i am just giving some food for thought.

 

[TFB1]

Hot day, blower on high, add just enough till it stops cycling, done...

Has worked for me 100% of the time for small leaks...

 

[Kestas]

I read pressures just to know I'm in the ballpark. I usually charge until vent temperatures stop dropping. At that point you're filling the accumulator.

Some techs are so good they charge further until the accumulator is full, and charge no more so they won't slug the compressor with liquid. Filling the accumulator safely overfills the system to provide a cushion to for leakage before it becomes a problem.

Ditto to spraying water on the condenser. It'll reduce the system load and head pressures, which again is why I don't rely on pressure readings.

 

[mechtech2]

Recirculate has LESS load. [Cooler air in to begin with.]So use normal outside air through the evaporator.
Windows open, AC fan on high. Dial or lever set to the coolest temp.
Any fan in front is better than none.

I'd beg, borrow or appropriate a digital infrared thermometer.
Then read the inlet and outlet of the evaporator after things stabilize.
They should be within 5 deg F. Outlet hotter is undercharged.
Inlet hotter is overcharged.

Simple, isn't it?

BTW, a static pressure test IS useful. But the car and system have to be dead cold - at ambient temps.Generally, this is used to test for undercharge. The pressure should be about equal to the ambient temp in F. Online charts abound.

 

[wtd]

The low and high pressures for your vehicle at 80*F is 26 and 299.

 

[JHZR2]

After driving home today, set up the fan and ran the AC with windows open, etc.

Put my gauges on and got this:

Photobucket kills the pics, but it was 24 and 170. It was 88F outside. So it was a bit low.

Have to wonder If this is my issue:

I was under the impression that oily residue indicates a leak.

Put some charge into it anyway. Chart said 45-55 low and 250 or so high side at 90F.

Charged it to this:

48 low and 230 high.

Didn't want to add more.

No more short cycling and colder too.

 

[Chris142]

Ya it looks like your low pressure cycling switch has sprung a leak. You can safely remove it w/o losing all your R134a. Theres a shrader valve under the switch. Replace the O-Ring seal while your there too.

 

[mechanicx]

What Chris said. Your low side of 45 psi sounds high to me like it might be overcharged but if you say the vent temps are colder, that's what matters. I guess at 90 degrees that may be right for this application. Was the engine running at ~1500 rpm for several minutes at those readings?

 

[JHZR2]

I was using this chart as reference.

Glad to hear that I can replace that sensor. Do I need to swap the sensor or is it possibly just the o-ring?

I didnt have a helper so had a tough time keeping RPMs up. I did put a fan up front to assist in shedding heat.

What difference should I assume given that I did this idling at 550? I figured worst case Id pay someone to evacuate anyway if need be...

 

[ltslimjim]

^You want the system you're vehicle has...it's stated low/high side pressures. Those seem to be 'general' guidelines.

However, I may be wrong. Isn't the spec noted on a sticker somewhere under the hood?

 

[mechanicx]

Yes it could be the o-ring. That chart is very generally and would be too high for some applications. At higher RPM you might see slightly lower low-side and higher high side pressure than at idle.

 

[JHZR2]

So how would one tell if it is too high for some? The chart is playing a cycle based upon the thermodynamic characteristics of phase change of a chemical. How much can they vary? What would cause them to differ?

There is no specific pressure listed on my truck. While I can appreciate the exactness of filling by mass, where does one determine the correct operating pressures for a specific system design at certain ambient temperatures?

 

[ltslimjim]

The system's information is 'possibly' in the FSM?

 

[jjcom]

Sounds like you did it right to me. I've worked on a lot of AC systems. If it's in the 80s-90s out, I normally shoot for about 45 psi on the low side, and 250 or so on the high side so I'd say you nailed it. The pressures switches on those are known for leaking like that...no big deal to change. Just got to your local advance/napa pick up a new one. Also an area to check out is the AC compressor itself. The GM units are prone to leaking compressors.

 

[mechanicx]

Lowest evaporator temperatures and coldest vent temperature that's above freezing point would be 27 psi for R134a. This system being a cycling clutch works over a range, cycling on at around 45 psi an cycling off around 25 psi.

It wouldn't be practical to charge the A/C to 28 psi at 90 F because it would cycle too often at cooler ambient temperatures. On the other hand, if you charged to 50 psi at 80 degrees, the vent temperatures wouldn't be as cool and on hotter, humid days the high side could get too high.

What the lowside pressure should be depends on ambient temperature and humidity, engine speed, compressor condition etc, but usually around 88 F 2000rpm it should be around 35 psi max.