Automotive Air Conditioning Information Forum

Ran it in the driveway after letting it sit in the sun all day. 78* in the shade. Clutch cycling on/off every 25 seconds. Vent temp 50*f

Engine 1500 rpm.
LP cycling between 20 - 45
HP cycling between 110-175 (needle bouncing +/- 10 psi)
Vent temp 50*f

I feel something is still wrong. The cycling and the Lp side going so low.

This is my first project with an exp valve system. My understanding is the HP side supplies the valve inlet, the sensing bulb throttles the flow through the orifice, excess is bypassed to the return/suction side.

It presents as a flow issue to me. Lack of return flow.

Mark86 wrote: ↑Sun Feb 12, 2023 12:13 pm I charge with a 30lb cylinder, and a refrigerant scale. I have a vacuum pump, which for my temp/altitude pulled 27.5”. Vacuumed for an hour.

Sounds like you're evacuating and charging it correctly. The second thin line coming off the TXV is either an equalizer tube or an oil return line and not a refrigerant line per se. A maximum evacuation reading of 27.5 in.Hg. on your Manifold Gauge Set indicates that you're located at about 2,500-3,000' elevation. If not, did you verify the low-side MGS reading using a micron gauge? Sometimes, a MGS can be off quite a bit.

Remember that you want to record pressures while the compressor is running, not when it cycles off.

The compressor is cycling at around 25 seconds, most likely to prevent the evaporator from icing-up and blocking airflow because of the still relatively mild ambient temperature of 78°F. That's probably just normal operation.

The 50°F center vent temp is not normal. Have you checked for a re-heating condition—that is, heated air mixing into the refrigerated air? There are a few reasons for that to occur: Hot water flowing from the engine to the heater core that doesn't shut off due to a mis-adjusted cable to the heater control valve or a faulty valve (if it has one), deteriorated seals on the blend door(s), or a failed blend-door actuator.

At 78°F ambient with the compressor running at your stated RPM, the LP should be ~30 PSI and the HP ~175 PSI.

Thanks John.

The second thin tube coming off the TXV is a return oil line and not a refrigerant line.

- That makes sense.

A maximum evacuation reading of 27.5 in.Hg. on your Manifold Gauge Set indicates that you're located at about 2,500-3,000' elevation. If not, did you verify the low-side MGS reading using a micron gauge? Sometimes, a MGS can be off quite a bit.

- No, I don’t have a micron gauge. I’m around 2200’ msl.

Remember that you want to record pressures while the compressor is running, not when it cycles off.

- Yes. The needle never stops. When the comp cycles on, the LP needle begins to decrease until reaching around 20, then the comp cycles off. (Low pressure switch I presume)

The compressor is cycling at around 25 seconds, most likely to prevent the evaporator from icing-up and blocking airflow because of the still relatively mild ambient temperature of 78°F. That's probably just normal operation.

- I opened the little access hatch to the exit side of the Evap and center exit air was 39.

The 50°F center vent temp is not normal. Have you checked for a re-heating condition—that is, heated air mixing into the refrigerated air? There are a few reasons for that to occur: Hot water flowing from the engine to the heater core that doesn't shut off due to a mis-adjusted cable to the heater control valve or a faulty valve (if it has one), deteriorated seals on the blend door(s), or a failed blend-door actuator.

- Yes. In removing the evap housing, I was able to inspect up close the blend door. All seals are intact and its movement (cable driven) is complete stop to stop.

At 78°F ambient with the compressor running at your stated RPM, the LP should be ~30 PSI and the HP ~175 PSI.

- I walked in 4 more Oz of 134a and it made no difference on the low side, but raised the high side a tad. It was doing this exact same thing with the previous exp valve and evap.

When I removed the old evap and TXV the TXV valve body had a molded flexible rubber encapsulating it, and the sensing bulb had foam tape and and some other gooey adhesive along with the metal C clip. I reused the molded rubber piece on the TXV valve body, and used adhesive backed 1/8” neoprene foam with the C clip on the sensing bulb.

I’ve seen a mix of insulated and non insulated sensing bulbs in various applications. I’m assuming the molded condom on the TXV valve body was from the factory. Looks like it had been removed and reused. The insulation on the sensing bulb looked like HVAC style insulation. If the Sensing bulb was not suppose to be insulated, would that cause the valve to sense too cool, restrict flow?

"I’ve seen a mix of insulated and non insulated sensing bulbs in various applications. I’m assuming the molded condom on the TXV valve body was from the factory. Looks like it had been removed and reused. The insulation on the sensing bulb looked like HVAC style insulation. If the Sensing bulb was not suppose to be insulated, would that cause the valve to sense too cool, restrict flow?"

The sensing bulb MUST be insulated. Those installs you have seen without insulation are incorrect.
The insulation can be of any of the various types available out there, but there must be something.

You said you had a vacuum pump....does it run off an electric motor or compressed air?

Still insufficient ambient for testing

I have had customers want me to recharge or check the charge in the winter, and my response is, "come back in the spring"
Same for residential HVAC, which I also do. I will change out a system in Dec or January, and release the charge that came with the condensing unit into the system, but I advise that fine tuning cannot be done until it gets warmer

This 78* in the shade, is this the temp inside the car or the outdoor temp. The outdoor or ambient temp needs to be at least 80, because that is the temp the condenser sees.

For MVAC I like the ambient to be at least 80 and I prefer to have the car in full sun, thus mimicking hot mid-summer conditions found in July and Aug when you really need the AC

Not sure here why the pressure is on to dial in the system in the middle of February.
Where do you live?

Mark86 wrote: ↑Sun Feb 12, 2023 10:59 pm When I removed the old evap and TXV the TXV valve body had a molded flexible rubber encapsulating it, and the sensing bulb had foam tape and and some other gooey adhesive along with the metal C clip. I reused the molded rubber piece on the TXV valve body, and used adhesive backed 1/8” neoprene foam with the C clip on the sensing bulb.
I’ve seen a mix of insulated and non insulated sensing bulbs in various applications. I’m assuming the molded condom on the TXV valve body was from the factory. Looks like it had been removed and reused. The insulation on the sensing bulb looked like HVAC style insulation. If the Sensing bulb was not suppose to be insulated, would that cause the valve to sense too cool, restrict flow?

I don't think the rubber sleeve on the body of the TXV does much of anything. However, the sensing bulb must be mechanically secured to the evaporator tube with the C-clip, and it must be insulated, as already said. I use Prestite to insulate it. Tim at ACKITS.com, this site's sponsor, stocks it.

I have a 1/2 HP electric vacuum pump. I live in Phoenix AZ and after sitting in the sun yesterday for 6 hours the interior of the car was probably 95-100 degrees. I’ve seen sensing bulbs on AC units and refrigerators be un-insulated, just a discussion point and observation.

I insulated with aircraft neoprene sticky back foam.

My Buick, which thanks to you guys here blew 38*f air out the ducts when tested yesterday in the driveway, at idle. My Wife’s Traverse will do the same, along with my Silverado,

Diving to 20 psi on the low side, even when +5oz overcharged along with short cycling tells me there is a problem. Same issue with old and new TXV valves, and Evap. Could the compressor be the problem?

Thanks again for the help and suggestions.
Mark

I believe the compressor in your vehicle is the scroll type, which isn't renowned for reliability and longevity. Was the compressor that you installed a rebuilt, new aftermarket, or new OEM? I would replace a scroll compressor with a new OEM unit, if possible. The compressor could cause the low-side pressure to drop on system start-up if it was shedding debris internally—metallic or plastic shards—which most likely would become trapped in the condenser on the high side of the system. Is the compressor making any unusual sounds when it's running?

A couple of other reasons why the low-side pressure would drop when the compressor engages are a partially stuck-shut TXV, or another type of restriction in the LP side.

You mentioned in your first post that you replaced the receiver/dryer. Is it an OEM unit? A so-called universal aftermarket R/D could have had the wrong type of desiccant in it for use with R-134a, and perhaps the desiccant bag ruptured, spreading some of the desiccant beads downstream. A hose that's beginning to disintegrate internally could also cause a restriction. As for the TXV, this wouldn't be the first time I've heard about a "new" TXV malfunctioning. Causes include loss of gas in the sensing bulb.

Thanks John


I believe the compressor in your vehicle is the scroll type, which isn't renowned for reliability and longevity. Was the compressor that you installed a rebuilt, new aftermarket, or new OEM?

- Brand new compressor, not rebuilt.

Is the compressor making any unusual sounds when it's running?
- No noises, very smooth.

A couple of other reasons why the low-side pressure would drop when the compressor engages are a partially stuck-shut TXV, or another type of restriction in the LP side.
- The LP consists of about 2’ of hose, the rest is AL pipe, and I flushed, blew air through it, which moved freely.

You mentioned in your first post that you replaced the receiver/dryer. Is it an OEM unit?
- No, not OEM, but identical in every way, which doesn’t mean it is internally. It’s not a generic fit all.

As for the TXV, this wouldn't be the first time I've heard about a "new" TXV malfunctioning. Causes include loss of gas in the sensing bulb.
- good point. I’ve been burned by “new” parts more often than I like. I have the old TXV and may bench test it for an idea if it was working or not.

To recap here are the series of events.
Installed new Comp, Cond, R/D, Flushed all lines, blew down with N, Installed 8oz, PAG 46, Vacuumed down, Charged with 24oz 134a.

Had low side issue, discharged, capped open ends, R and R Evap and TXV. Leak tested, Vacuumed, and recharged.

"after sitting in the sun yesterday for 6 hours the interior of the car was probably 95-100 degrees"

And the ambient was......??

tbirdtbird wrote: ↑Mon Feb 13, 2023 4:43 pm "after sitting in the sun yesterday for 6 hours the interior of the car was probably 95-100 degrees"

And the ambient was......??

78*f in the shade, probably 90* in the sun. Phoenix AZ.