Weird A/C Pressure readings

[lyttek]

Trying to figure this one out...

At static, engine off, both high and low sides of the A/C read about 90 psi.

Start the engine, kick the A/C to max, and pressures are 30 psi on low and 120-125 psi on high at idle. Running the RPM up a little does little to nothing on high side, low will drop to 25psi at which point the compressor kicks off. Once the low side comes up a bit, then it cycles on/off as normal. I've added a little freon to keep from cycling quite as much, pressures don't change, just reduced the cycling a little.

Now, after about 10 minutes of this, the compressor kicks off and stays off... and both low and high sides are static at 80psi.

How is that possible??? Static pressure lower on both sides compared to when first starting the tests?? I wasn't watching when the high side dropped, so no idea yet exactly what happens. If I turn the A/C off for a bit, the pressures stay the same, but the compressor will start again. This cycle repeats... cooling stops, turn the a/c off a few mins, then it will cool again.

It's decently cold when it's working... I don't have a thermometer yet to check the air temp.

Other than the low and high side pressure switches, what else would control the a/c clutch?

When the first 'shut off' cycle appeared, I shorted the low-side pressure switch and the compressor did *not* kick on... can high-side switch be tested the same, or ??

 

[Denisefwd93]

Trying to figure this one out...

At static, engine off, both high and low sides of the A/C read about 90 psi.

Start the engine, kick the A/C to max, and pressures are 30 psi on low and 120-125 psi on high at idle. Running the RPM up a little does little to nothing on high side, low will drop to 25psi at which point the compressor kicks off. Once the low side comes up a bit, then it cycles on/off as normal. I've added a little freon to keep from cycling quite as much, pressures don't change, just reduced the cycling a little.

Now, after about 10 minutes of this, the compressor kicks off and stays off... and both low and high sides are static at 80psi.

How is that possible??? Static pressure lower on both sides compared to when first starting the tests?? I wasn't watching when the high side dropped, so no idea yet exactly what happens. If I turn the A/C off for a bit, the pressures stay the same, but the compressor will start again. This cycle repeats... cooling stops, turn the a/c off a few mins, then it will cool again.

It's decently cold when it's working... I don't have a thermometer yet to check the air temp.

Other than the low and high side pressure switches, what else would control the a/c clutch?

When the first 'shut off' cycle appeared, I shorted the low-side pressure switch and the compressor did *not* kick on... can high-side switch be tested the same, or ??

not trying to hurt your feelings or be a smart-ass, you can't figure it out, because you don't have experience in this area, would be my guess.

Static as you call it, Is Equalization of high and low side. It doesn't always happen in Automotive Systems.

The equlized (static) refrigerant pressure should be equal to the pressure related to the ambient temperature. That can be found on a temperature pressure chart or the scale on your gauges.

Also, the high pressure called head pressure and (some of the guys may want to verify this) the compressor probably has an "unloader/internal bypass/ head pressure control" in another words it controls the high pressure so it doesn't go off the scale and blow hoses or worse.

R134a does not react quickly. and no the high pressure will not make a large jump when you add refrigerant. This is why it is always so difficult to charge Automotive Systems buy gauge, instead of weight; the proper way

To actually know what's doing, you need to know how to read outdoor temperature and pressure of the refrigerant in this case, 134a.

Your are cycling on the low pressure control that is why it is going on and off and as you add refrigerant, the cycles become less frequent. However unless it's really warm outside it will probably always cycle on low pressure to prevent freeze up.

Even with an accurate thermometer, it is hard to check air temperature at the condenser coil because it's in front of a radiator, if you could check it, the ballpark temperature of the air going across the condenser should be around (not exact) 15 to 20 degrees higher than the "ambient"

On the cooling side, the temperature should be 15 to 20 degrees lower then the air going in "ambient"

(Just a side note, in refrigeration, the temperature difference is around 10 degrees)

All you have to do for now, charge the system so that the big hose going back to the compressor feels cold and sweating.

There is a trick to getting the air out of the hoses when you connect to a system if you haven't purged the air from your hoses, guess where it went?

To learn more, make YouTube your friend on this particular subject it can be very involved or very simple depending on your experience.

Hope this helps, good luck!

 

[Denisefwd93]

I actually don't know, but depending on age or newness of your vehicle it may have electronic controls on the air conditioning in addition to mechanical pressure controls, things like: cabin temp sensor, time delay, high RPMs.

 

[lyttek]

I actually don't know, but depending on age or newness of your vehicle it may have electronic controls on the air conditioning in addition to mechanical pressure controls, things like: cabin temp sensor, time delay, high RPMs.

That's what I'm wondering, additional stuff. Googling so far has turned up a lot of the 'normal' responses, but nothing that's helped on this one so far. Of course, the key to proper searching is the correct terms to get what you need, lol.

At this point, I don't think it's under or over charged, as it cools really well when it's not refusing to work. Forgot to mention that it's a 2000 Ranger with the 3.0 Flex engine, if that makes much difference.

The fact that I have to turn the a/c off for a bit before it starts working again makes me think there's something electrical going on. When driving around today, I never turned it off, and once it decided to stop working, it didn't start again until after I killed the system awhile. Something is deciding to reset after a certain point.

 

[lyttek]

Also, the high pressure called head pressure and (some of the guys may want to verify this) the compressor probably has an "unloader/internal bypass/ head pressure control" in another words it controls the high pressure so it doesn't go off the scale and blow hoses or worse.

That's what I was wondering... I would assume that high(er) pressures would be seen on gauges before such a protection measure was activated, unless it's something internal to the compressor before the high pressure is actually seen on the port... if that makes sense.

 

[Denisefwd93]

No I would not suggest you assume anything like that, especially if there are electronic controls.

Even if you had a blockage in the orifice tube you would not get a high condensing pressure because the refrigerant just keeps compressing.

That's what I was wondering... I would assume that high(er) pressures would be seen on gauges before such a protection measure was activated, unless it's something internal to the compressor before the high pressure is actually seen on the port... if that makes sense.

 

[Denisefwd93]

There's a remote possibility you have a bad fan clutch on your radiator fan!

 

[lyttek]

Not experiencing any overheating... and it was replaced less than a year ago... but I'll verify it

 

[Rearanger]

Maybe I can show how I set up my system for analysis.

I use a high flow electric fan in front of the rad to simulate running air flow. I open the passenger side door (window down) and put the vent flow on recirculation, this simulates running air temps more closely. Install a digital probe thermometer into the upper outlet vent and adjust air flow out upper vents. Note ambient temperature. Compare ambient temp to outlet air temp. A good running system should produce about 45-55 F at outlet. The best way to measure evaporator outlet temp is as close to the outlet side as possible, but there is usually no access that close. The air temp will rise about 5*F from outlet temp to upper vent temp.

You'll need to purge air from the AC gauge hoses before hooking them up. With a warmed up engine observe the cycling pressures and temperaturs and when the AC clutch is off and on. Take notes.

From my FSM the AC cycling switch is open (AC Clutch Off) when low side reaches 21-23 psi. The switch is closed (AC Clutch On) when low side pressure reaches 42 psi - depending on gauge accuracy. The system cycles to prevent
the evaporator from freezing up.

Hope this helps.

 

[lyttek]

That's pretty much exactly what's happening... it's working as described up until the moment it doesn't.

Airflow doesn't seem to matter. Problem appears whether sitting at idle, or at interstate speeds.

Are there any electronic widgets that control the system? I'm really beginning to wonder at this point if it's the control switch...

When driving it yesterday for an hour on the interstate, when I left I set it on max a/c. It worked fine up until I switched from 'max' to 'normal' and within a few moments, the problem appeared... cooling stopped, and I could tell that the compressor was not kicking in.

Switching back to max does *not* automatically fix it and make it run again, so it's not that obvious of a problem... I still have to turn it off (or at least to the 'vent' position) for a bit, and then it will start cooling again.

 

[Rearanger]

Maybe give your year and someone can tell you what AC controls you have.

My AC cycling switch sends a signal to the PCM. Do you have auto climate control?

I'm not sure what happens when you use MAX setting. It may just put the vent system into recirculation so that the evaporator intake is not on outside air. That reduces the load on the system.

I'd get a digital thermometer for the outlet vent and see how the system cycles. You can watch the air temp go up and down as the AC cycles.

 

[Denisefwd93]

"normal" sends outside air across the evaporator and would slightly increase low side pressure, but would also increase the absorbation rate and translate into high pressure on the condenser side, that could possibly be enough to kick it off on high pressure no way to know without remote gauges and thermometers. Another way would be to let some refrigerant out of the system but I didn't tell you that!

 

[lyttek]

it's a 2000, no auto climate control (that I'm aware of).

Driving today I didn't adjust anything... when it kicked off, I drove another 15-20 mins and the compressor never kicked back on. It wasn't until I turned the control to 'vent' for about 5-10 mins and then back to a/c before it would cool again.

 

[Rearanger]

From what I can see on my 2003 system there is only a low pressure cycling switch which turns the clutch on/off. The high pressure valve is just a safety blow-off valve, I doubt that's your problem.

My manual says the cycling switch can be replaced without discharging the system.

I can't say this is the cause without all the system readings, but it may be a cheap try. I can't say whether an AC clutch relay may be failing. You can try to jump the clutch to see if you get cold air.

Some systems have a temp sensor at the evaporator, but apparently this system does not.

 

[Denisefwd93]

When it does this weird on off staying off thing have you noticed, if the airflow from the blower seems to disappear? if so, that could be the evaporator coil icing up so badly that air can't go through it, this would make it seem like no air conditioning until the frost/ ice melts and allows the air to flow across evaporator coil again thereby allowing low pressure side to rise and close the LP switch.

Is there any chance there's a filter, leaves or rat's nest in the blower compartment in front (upstream) of the evaporator?

Another indicator, would be the cold air much colder than it should be, and it should only be 15 to 20 degrees cooler than the ambient.

it's a 2000, no auto climate control (that I'm aware of).

Driving today I didn't adjust anything... when it kicked off, I drove another 15-20 mins and the compressor never kicked back on. It wasn't until I turned the control to 'vent' for about 5-10 mins and then back to a/c before it would cool again.