Automotive Air Conditioning Information Forum

Ahhhhhhhhhh the saga continues.

Removed R/D and installed another R/D as planned. Vacuumed down for an hour and installed 24 oz of 134a. Vent temps the same, and the short cycling low pressure issue remains.

LP at 48, clutch cycles on, pressure begins to drop to 17-18 then the clutch cycles off. HP side peaks at 150.

Ambient is 71, 1500 RPM on the engine.

To recap. This issue has followed 2 expansion valves, a new evaporator, and 2 R/D’s

The new R/D has a sight glass window. When the compressor kicks on the glass fills with fluid, when it kicks off, foaming and bubbles, and the fluid level lowers.

'LP at 48, clutch cycles on, pressure begins to drop to 17-18 then the clutch cycles off. HP side peaks at 150.'

BUT what was the vent temp??? That is important.

And why is
"LP at 48, clutch cycles on, pressure begins to drop to 17-18 then the clutch cycles off. HP side peaks at 150."
this bad news? Seems perfectly acceptable for 71F.
Step back and wait for warmer weather like 80F

Mark86 wrote: ↑Sat Feb 25, 2023 7:59 pm The new R/D has a sight glass window. When the compressor kicks on the glass fills with fluid, when it kicks off, foaming and bubbles, and the fluid level lowers.

Although a sight glass isn't normally used for gauging the amount-of-charge in an R-134a system (only the weight of the charge is), the behavior you describe suggests that the system is optimally charged.

tbirdtbird wrote: ↑Sat Feb 25, 2023 11:05 pm 'LP at 48, clutch cycles on, pressure begins to drop to 17-18 then the clutch cycles off. HP side peaks at 150.'

BUT what was the vent temp??? That is important.

And why is
"LP at 48, clutch cycles on, pressure begins to drop to 17-18 then the clutch cycles off. HP side peaks at 150."
this bad news? Seems perfectly acceptable for 71F.
Step back and wait for warmer weather like 80F

Sorry left that out. Vent temp was 50*F Evap temp 39*.

At 2500 rpm, clutch cycling every 9 seconds, with a vent temp of 56.

JohnHere wrote: ↑Sun Feb 26, 2023 12:02 am

Mark86 wrote: ↑Sat Feb 25, 2023 7:59 pm The new R/D has a sight glass window. When the compressor kicks on the glass fills with fluid, when it kicks off, foaming and bubbles, and the fluid level lowers.

Although a sight glass isn't normally used for gauging the amount-of-charge in an R-134a system (only the weight of the charge is), the behavior you describe suggests that the system is optimally charged.

Thanks for the notes. I weighed in exactly 24 oz as a gas. Im hesitant to pull that TXV valve as I feel I’m throwing parts at this point.

"Vent temp was 50*F Evap temp 39*.
This suggests you have reheating...meaning that heat from your heater circuit is in some fashion mixing into the the vent air. Damper door not closing properly, foam seals shot, etc.

The behavior of everything you have described , again, seems normal to both John and myself.
You seem to be getting discouraged.
Sir back, and wait for warmer weather. Surely you have other projects you can work on for a month and come back to this one. I would walk away myself and not look back for a month.

As far as the charge goes, it seems you are right on. If we work on a customer's car in cooler weather (less than 70F), and the charge is known, we will install that, but not test it out until we see 80F on the dial. If the charge is unknown, we will not charge it at all until we see 80F, although we may install just enough 134 to get it out of a vacuum (we record the weight). We do not like leaving a system in vacuum...you are asking for trouble

Thanks for the continued interest.

The heater core is downstream of the AC evap housing. There is a small rectangular door, held in place with clips for the access to a thermistor that sits on the exit side of the evaporator. I am using that access door to take evap exit air temps, with a probe stuck in the the evap coil fins. So I’m confident that there is no heater core bleed air effecting the evap.

The behavior is abnormal, and no clutch can handle the cycle frequency at current state.

If I went in my Buick (thank to the forum) at these temperatures the vent temp would stabilize at 37* and you could hand meat in there. AT IDLE.

Tested the old exp valve in ice water and could detect no movement at all under a 16x scope.

"I am using that access door to take evap exit air temps,"
That is not the correct place to take the reading due to the influence of the heater core.
Let's try the old standby set of readings:

Ambient
Vent temp at core not at that exit door
Hi
Lo
Engine at 1800 RPM

And if you want to improve the cabin temp, try to figure out a way around the re-heating that you have. Can you install a manual shutoff in the heater lines?
Be advised that some cars do not allow you to do this for some asinine reason

The heater core is 18” away from the evap, seals are in like new condition and the blend door is functioning as intended. Verified when I had the evap housing out of the dash. I could see the blend door, the seals, and the cable actuator as well as if we’re on my bench. The access plate is on the evap housing, so it allows access to the evap air PRIOR to that air which passes the heater box/blend door.

My temp probe in my Fluke VOM is physically stuck in the evap coils. It’s giving me the best reading I can obtain of evaporator core temperature, which is 39*. And that fluctuates up to 47* as the compressor cycles.

FAN——-EVAPORATOR———Access hatch————————————BLEND DOOR——————————-HEATER CORE—————-VENT

We may be confusing each other.
You stated:
""Vent temp was 50*F Evap temp 39*."
How do you explain the addition of 11F to the evap temp by the time it exits the vent....should only be at the most another 5F warmer.