Questions: Recharging 2002 XLT AC

[sfbayjay]

2002 XLT 4.0L V6

BACKGROUND:

AC stopped working over the winter (worked last fall, but not very cold). Compressor not engaging when AC active at dash. Checked electrical connections at compressor. No voltage at compressor electrical connector when AC switch on. Disconnected AC cycling switch (aka "low pressure switch"), jumped the connector, turned on AC at dash, and compressor immediately kicked on. This suggests either low refrigerant charge or faulty AC cycling switch.

Rented manifold gauge set from AutoZone, along with a vacuum pump.

Connected gauges on high and low sides. Pressures read virtually zero on both gages (made sure to check connectors and ensure screw-down valves on high- and low-side connectors were screwed down). Suggests compressor not starting because system pressure below minimum.

Suspecting leak, decided to try and pull a vacuum on the system. Connected vacuum pump to yellow manifold hose, turned on vac pump, then opened both high- and low-side valves on gage set. Let pump run for 40 minutes. Gage on low-side hit a minimum of -25" Hg after about 20 mins and stayed there. High side gage pegged at minimum (neg. pressure).

Closed both valves on gauge set. Turned off vacuum pump. Let system sit for 1 hour. No change at either manifold gauge - same readings (-25" Hg on low side, and "pegged" at low end on high side).

This suggests no leak (where did refrigerant go?) or VERY small leak. System hasn't been serviced/recharged in about 10 years, and wasn't blowing too cold last year when clutch was still engaging.

QUESTIONS (any help appreciated!):

1. At this point, can I assume system is "empty" of refrigerant (since system pressure measured at 0 on manifold gauges), or might there still be some small amount of refrigerant in the system? I know how much R134a is supposed to be in the system when it's "full" (56 oz, since I have aux climate control), but am I starting from empty?

3. If there may be some residual refrigerant in the system, should I just gradually refill until I hit "correct" pressures on the high and low sides (as opposed to just adding 56 oz)? Are the "generic" charts that show high- and low-side pressures at different ambient temperatures suitable for doing this, or is there a specific temp/pressure chart for Ford and/or Explorers?

4. Since I clearly have a small leak somewhere, any concerns re. using a refrigerant that is pre-mixed with a dye and/or with some "stop leak" stuff?

5. I never opened the system, and I haven't replaced any components, so I'm not really worried about adding makeup oil, right?

Thanks for any help - trying to do this as "right" as possible on a budget and considering truck is getting old...

 

[SoHK_alumni]

From your description, I'd just about put money that one of the service valves is leaking -- when connected to your gauge set, the connectors will hide the leak.

So, I think at a minimum, I'd replace both schrader valves, and maybe the accumulator/drier (good practice!)

But in order:

1: Yes, you've pulled a vacuum on the system, there won't be any refrigerant left in the system.

3: No residual refrigerant be in the system but adding refrigerant gradually is usually the best way. A good ball park target for pressures is 35 - 38 psi on the low, 195 to 205 on the high But, since you're starting from "0", try to get as close to the factory spec by weight listed on the under hood label.

4: I'd avoid the "stop leak" at all costs, particularly if you don't replace the accumulator/drier -- the moisture trapped in the drier will react with the stop leak potentially causing a blockage. UV dye on the other hand is a good idea even on a gas tight system. It will leak one day, having the dye in there before the leak starts will assist with finding the leak when it does happen.

5: Correct, you've not replaced any components so you've not removed any oil from the system.

 

[sfbayjay]

Thanks so much for your detailed reply! A few quick follow up questions if you have time

From your description, I'd just about put money that one of the service valves is leaking -- when connected to your gauge set, the connectors will hide the leak.

This makes complete sense - I'll have a look at the valves. Is it just replacing a standard Schraeder stem (like on a bike tire)?

So, I think at a minimum, I'd replace both schrader valves, and maybe the accumulator/drier (good practice!)

How critical to replace accumulator/dryer? Looks like a pain in the butt...

1: Yes, you've pulled a vacuum on the system, there won't be any refrigerant left in the system.

I wasn't sure if some of the refrigerant might still be in there in liquid form - probably stupid? I guess it all goes over to gas, and any tiny bit of liquid that might have been leftover would have been converted and sucked out when I put the vacuum on (whoops?). But the system pressure was zero when I first connected gauges (w/o car or AC running), so I think it was empty...

5: Correct, you've not replaced any components so you've not removed any oil from the system.

I am a little confused about this - everything I've read says to add "makeup" oil for any component you remove. But then I heard (somewhere) that the oil circulates with the refrigerant? So if the refrigerant is all gone, is some of the oil gone with it?

 

[sfbayjay]

Oh yeah - and one other thing - should I be worried that the vacuum gauge on the low-side hit a minimum pressure of -25 inches of Hg? Some youtube vids I've watched show the gauge pegged at -30.

At first I thought this was a sure sign of a leak, but then when I let the system stand and it held negative pressure (as described above), I kind of figured maybe the rented AutoZone pump was just too weak to pull the vacuum all the way down?

 

[SoHK_alumni]

Any auto parts store should have the valves -- some are exactly the same as tire valves, others are quite a bit different (and need different tools).

Replacing the accumulator/drier is certainly "best practice" since a vacuum won't extract moisture from the desiccant without additional heat but, if you get a deep enough vacuum and any residual moisture does not overwhelm the desiccant, you should be OK (but NOT best practice)

You are correct: The refrigerant carries oil around with it and can carry some out at a leak. In fact, looking for the oily spot is the 1st thing to look for when looking for a leak.

Since your system appears vacuum tight, it suggests the leak is very, very small and the oil loss would be minimal.

Lastly, gauges are mechanical devices (or use electro-mechanical sensors) and are usually not really high precision devices. Often you can adjust the reading on the gauge with a small screw driver -- but unless it's been recently been calibrated and certified, it's a hit or miss on how accurate they read.