AC Gurus.

[BGKYK5]

This isn't my truck, but you guys know your stuff.

My POS DD is a beat down '95 Neon. The AC hasn't worked for years. I can't bear to spend money fixing up a car that is only worth $50 on a good day. But it keeps running and it is paid for so it will be my DD 'till it croaks or I win the lottery.

But I can't bear the thought of another August in KY without AC. I recharged it with R134A last year hoping I would get lucky and it would magically start working. The pressure guage on my R134A recharge can says it is still at 45-50 PSI on the low side so it isn't leaking (much).

The clutch does come on and the compressor does cycle on and off, but the air comming out of the vents isn't even a little bit cool. When the compressor comes on the pressure drops to zero and then it comes right back up to 45-50 PSI when the compressor cycles off. It seems like the compressor is on for about 20 seconds and then off for about 20 seconds.

Is all that normal? What could be wrong and more importantly - is there a cheap way to fix it?

 

[readymix]

You check pressure with the system cycling.

 

[BGKYK5]

I did. Maybe I didn't explain it clearly. The pressure on the low side was 45-50 PSI with the compressor off and dropped to zero when the compressor cycled on.

 

[USSkoval]

That's simply not enough pressure and that's why the compressor is cycling. Add more 134a WITHOUT stop-leak and go from there. If you have trouble getting the system to take more charge, try adding it with the AC running.

 

[bigblock72]

The A/C system is low, add some more R134 and it will get cooler. Make sure the cooling fans are coming on with the compressor, check the high side pressure to make sure it's staying under 250 - 300 psi. If the high side pressure gets much over 350 psi it will kill the compressor in short order. If you overcharge the system the high side pressure will go out of sight, if the cooling fans don't come on the high side will go 400 psi and kill the compressor. R134 systems aren't forgiving to charge level so it's worth investing in a set of gauges if you don't already have some.

 

[BGKYK5]

The rad fan does come on with the compressor. The el-cheapo R134A recharge kit said not to add more refrigerent once the low side pressure got to 50 PSI, and yes I was adding it with the AC on high. I'll check the high side pressure and add more if it is under 250 PSI. Thanks guys.

 

[wasted wages]

If there is a restriction in the system DON'T add more refrigerant.

You really need a set of gauges to tell what is happening on the high side as well.

When the compressor cycles off, does the pressure rise quickly or does it take a while ??

If it rises quickly,,add a little gas

If it takes a while to rise there may be a restriction and adding more gas is a bad thing..

 

[BGKYK5]

It rises quickly. I'll check the high side pressure before adding any more refrigerant.

 

[BGKYK5]

Well, I broke down and got a HF manifold set so I could check the high side pressure. Here are the results with the compressor cycling on and off.

Comp on
Low = 0 PSI
High = 100 PSI

Comp off
Low = 50 PSI
High = 50 PSI

I have three theories, all of which are probably wrong.

1. Compressor is bad
2. Obstruction in the line between the compressor and the high side pressure port
3. Pressure switch is bad, turning off the compressor too soon.

Am I close?

 

[bigblock72]

Nope it's still low on refrigerant. Keep adding until it gets cold and pressures are good.

Low side pressure should read 25-35
High side should be between 150-250 depending on temperature.

 

[rjfguitar]

I also agree that you don't have enough freon. How much does the system hold? There should be a sticker under the hood somewhere telling you info on the A/C. Those standard cans are only 12oz, if the system hold close to 48oz then one can won't hardly make a difference and you'll just be left with lousy high side pressure and the pump kicking on and off like crazy.

 

[BGKYK5]

Nope it's still low on refrigerant. Keep adding until it gets cold and pressures are good.

Low side pressure should read 25-35
High side should be between 150-250 depending on temperature.

It seems counter intuitive to add refrigerant to lower the low side pressure, but you guys are the experts. I'll buy another can of R134A.

Or does than mean the low side pressure should drop to 20-35 PSI when the compressor is on and be higher with the comp off? It is 50 with the comp off and 0 PSI with the comp on now.

 

[lectric80]

You are looking for a low side pressure of between 25 and 35 while the compressor is running. With the pressure still dropping to 0 when it is running, the compressor is being shut off before the high side can build up enough.

 

[BGKYK5]

Ok, got it. I will only worry about pressures when the compressor is on. I'll add refrigerant until I get to 25-35 PSI on the low side or 250-300 PSI on the high side (with the compressor on). Thanks guys!

 

[wasted wages]

250-300 PSI on the high side (with the compressor on).

Whoa dude !!!!!

that is the MAXIMUM it should be!!!! not a target range.....
the lower the better.. what your looking for is about 15 - 20 deg above the outdoor temp converted to pressure.

The correct way is to read the high side gauge, convert that pressure to temperature using a P/T chart.

http://www.glacierbay.com/ptchart134a.asp

Subtract the outdoor temp from this reading. your looking for about a 15-20 degrees difference.

At 300 psi w/ R-134 you would be at about 160 degrees condensing.
Subtract 20 degrees subcooling you end up with 140 degrees.

So unless it's 140 degrees where you live,,something would be wrong.


Now if your reading is about 155 psi that would = 114 degrees condensing.
subtract 20 degrees subcooling and you end up with 94 degrees.

so,, on a 94 degree day you would expect your head pressure to be about 155 PSI.



If your not sure of what your doing you may either :

a. end up hurting yourself
b. destroy a bunch of expensive parts in your a/c system.

 

[BGKYK5]

Got it. I was planning to see whether I hit the low side target or high side (not) target first and stop there. Now I'll stop at the low target or the high target from the chart. Thanks.

If your not sure of what your doing you may either :

a. end up hurting yourself
b. destroy a bunch of expensive parts in your a/c system.

I'm never "sure" about anything

But I'll try not to hurt myself. If something else in the AC breaks it won't be any more non-functional than it is now. If I can't fix it quickly and cheaply I'll live without.

 

[BGKYK5]

First off, you guys are awesome!

The AC is blowing nice and cold now, for the fist time in years! I charged it to about 25 PSI on the low side and 190 on the high side with the compressor running. It blows nice and cold.........But the compressor never shuts off. I charged it a little higher than Wasted Wages' chart in hopes that it would trip a high pressure switch and turn the compressor off. No luck. Any ideas?

Here is my theory. The AC stopped working a few years ago. Then about a year and a half ago, the Neon was in a little fender bender that broke the radiator. I replaced it with a junk yard radiator with the condenser still attached. I'm guessing there was a pretty good leak in the old condenser and the "new" one is OK (it has held my initial half-assed charge since then). Now that it is "properly" charged it seems to work pretty good, except for the compressor not shutting off.

The car is almost literally worthless, so I didn't want to spend any money on the AC. I figured something expensive was broken and it was a lost cause. $50 for a HF manifold set and $7 in R134A and it seems to be pretty close to working right. I should have done this a year and a half ago!

 

[wasted wages]

well,,,here's the thing....the gas went somewhere....
a/c systems don't "consume" refrigerant.....

And that means you have a leak....and depending on the size of the leak the charge may last 1 day or a year...By law I can't tell you to continue charging the system when it runs low as this is a violation of EPA laws...

So my advice to you is to take it somewhere and have them find the leak.

or if the car is a beater....well,,,you know what to do.....

 

[Blue85]

Wait a minute. The system was vacuumed down with a pump after you replaced the condensor, right? Any air in the system messes everything up.

As for the compressor staying on, that might be OK. If it is hot outside and the car is getting a lot of sun, the system might just be maxed out. You could try it at night or with the blower turned down. The compressor will only turn off when the temperature of the evaporator gets low enough. The less load, the sooner this will happen. You can measure the temperatures with your gauges because temp = pressure. The low side is near freezing when the compressor cycles off.

Of course, it is also more likely to cycle off when you are driving. Compressor flow at idle is sometimes pretty sad and airflow with the vehicle sitting is worst-case.

 

[bigblock72]

The A/C system in your neon is different from a K5. The only time the compressor will cycle in the Neon is if the evaporator freezes up or (on some) if the high side pressure goes way high. A K5 uses a fixed orifice (neon uses an expansion valve) and a low pressure switch to keep the pressures where they need to be. The neon's expansion valve keeps the pressure and refrigerant where it needs to be instead of cycling the compressor. This is a simplified explanation of course but gives you an idea of what's going on. Bottom line is if it's blowing cold and the pressures are close then leave it alone.