Automotive Air Conditioning Information Forum

Hi Guys,

I'm helping a friend sort out a problematic system.
This is a cobbled together system in an early Mustang.
It uses the original under dash evap, but everything else is custom. Compr. is a Sanden, condenser is new and huge.

When first started at idle, the pressures settle to 25/150 on a 70F day and it blows 44ish out the vents.
As soon as I raise the rpm above idle, the pressures go to 10/250-275 and the vent temp climbs.
If left to idle after being at 1800rpm the high side stays over 200 for quite some time.
It definitely has a low charge, but I suspect any more refrigerant would shoot the high side to the moon.
I forgot to look at the equalization time when shut off.
Feeling the in/outlets by hand, the condenser is doing a good job.

I'm thinking the old TXV is not working properly, that that seem reasonable?

Although there's a bit of a story regarding the guy who made the hoses......
Apparent he did half and returned a week later because he didn't have the correct fittings on hand. During this time he apparently left the drier open...
I know it needs replacement because of this, but could a moisture saturated dryer cause enough restriction to cause these symptoms?
I could not feel a temperature drop across the dryer, but am unsure if that's a valid test.

Can anyone provide any advice as the the possible problem and any further test methods before I open it up to replace the TXV?

Thank you all for your help

wetlook wrote: ↑Tue Apr 30, 2024 8:42 pm This is a cobbled together system in an early Mustang.

Which model year is this Mustang? I presume you've charged it with R-134a. Do you have the original R-12 specs for this car as a baseline?

wetlook wrote: ↑Tue Apr 30, 2024 8:42 pm When first started at idle, the pressures settle to 25/150 on a 70F day and it blows 44ish out the vents. As soon as I raise the rpm above idle, the pressures go to 10/250-275 and the vent temp climbs.

At idle, the pressures both seem low at that ambient temperature, but normal RPM for testing purposes is around 1,800, as you mentioned below. So the pressure readings at idle don't tell us much because the compressor isn't doing much. However, with the engine running above idle (~1,800 RPM), the low pressure is still too low and the high pressure is still too high.

wetlook wrote: ↑Tue Apr 30, 2024 8:42 pm If left to idle after being at 1800rpm the high side stays over 200 for quite some time.
It definitely has a low charge, but I suspect any more refrigerant would shoot the high side to the moon.
I forgot to look at the equalization time when shut off.
Feeling the in/outlets by hand, the condenser is doing a good job.
I'm thinking the old TXV is not working properly, that that seem reasonable?

When shut down, some systems take more time to equalize their pressures than others.

How do you know that it's low on refrigerant? Did you charge it by weight using an accurate refrigerant scale (not the sight glass) equaling the original R-12 specification, or a bit less? How much oil did you put in it? Did you thoroughly flush the evaporator to get rid of all the mineral oil left over from the car's R-12 days?

With a low-side pressure at only 10 PSI, it's possible that the TXV is defective, as you suggest, or that it's partially clogged by debris.

wetlook wrote: ↑Tue Apr 30, 2024 8:42 pm Although there's a bit of a story regarding the guy who made the hoses......
Apparent he did half and returned a week later because he didn't have the correct fittings on hand. During this time he apparently left the drier open...
I know it needs replacement because of this, but could a moisture saturated dryer cause enough restriction to cause these symptoms?

The desiccant could be full of moisture, especially if it's the original R/D. It could also have some kind of debris in there that's partially blocking the internal screen.

wetlook wrote: ↑Tue Apr 30, 2024 8:42 pm I could not feel a temperature drop across the dryer, but am unsure if that's a valid test.

It is a valid test, but you'd have to gauge the temperature on each side using the proper equipment, not necessarily by feel. If the R/D is partially plugged, you would be able to measure a temperature drop between the inlet of the R/D and the outlet.

On the TXV, does the equalization line and/or the sensor line look damaged? If so, definitely replace it, even if you haven't examined it internally yet. And, of course, replace the R/D now, and each time the system is opened to the atmosphere.

Thanks for your reply. I've answered your q's below.

It's a '66, but nothing in the AC system is stock other than the evap. The r/dryer is probably half the size of the original, the condenser is much bigger due to the radiator support panel being cut out to the '67/68 size to use a bigger rad/condenser. Stock compressor was a York, now a Sanden. All hoses are custom made. The stock charge is 20oz R134a, I expect this setup to hold a little less. I put about 12oz in before I saw the H side go higher than it should have while the L was still around 10psi.


Yes, agreed on the p's at idle. I thought the idle pressures were worth mentioning because after a short run at higher RPM, it does not settle back to these same pressures when returning to idle. I thought maybe this may contribute to a TXV diagnosis.

I have made an assumption on the low charge based on the static P vs T and the fact I believe it probably should hold more than the ~12oz I charged.
The TXV looks okay externally. I guess I'm going to have to remove it for a look internally, and will probably just replace it while the system is open. It's possibly 55+yo and could have sat on a shelf for decades at some point.

I really just wanted to know if my thinking was correct and this is where I should be looking.
Thanks again for your input.