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Discussion Starter #1
Hi guys,

I have searched the forums, and didn't find an answer to this specific issue, so here goes.

My niece has a 95 Passat VR6 at college several hours from here. She has had the car for about 1 year, 270,000 miles, I assume original AC system, no issues with AC, blew very cold, so I had never checked with gauges, etc.

This spring, she heard a loud bang, hissing for several minutes and the AC was kaputt. It was the AC pressure switch F129 that had failed. The plastic portion had separated completely from the metal part.

A kid down at school who is "handy" with cars, unfortunately, tried to be kind and ran by NAPA, got (the correct) replacement switch for F129, replaced it and "charged up" the system with 134A cans of refrigerant. She thought he put 3 cans in, The AC worked well for a couple of weeks, then failed.

I came into the picture when she got home from college later this summer.

When I put it on gauges, low side 70-80 psi, high side wouldn't get above 55-60 psi. Clutch engaging properly.

I figured compressor had failed, maybe had junk in it from drier, moisture contamination, etc., since the kid did't use a vacuum pump before charging. I also assume the system was open for some time (about a week) before he replaced the F129 switch, so the drier was toast.

What I did:

Replaced compressor (see below), replaced the receiver/drier accumulator, replaced the expansion valve. (After flushing the evaporator, lines and condenser with AC flush and compressed air.)

System prepped with vacuum pump for 1 hour. Drew close to 30 in Hg. System held this vacuum for 1 1/2 hours, so I think no leaks are there.

Charged with 134A 1100gm I charged the system through the low side.

So there was still the same dynamic going on. High low pressure still about 75 psi, low high side no more than 60 psi.

Flaps/air flow to the evaporator seems to be in proper working order.

The compressor I found for this car was from a salvage yard, but was a rebuilt Four Seasons unit in original box. They said it had been on the shelf "quite a while" and figured it had be retrieved from the trunk of a car brought into the yard and long since gone. It had all documentation with it and was in the original box, the factory plates to seal the hose connections were there, and actually I got a little hiss when I removed this plate to hook up the hoses, so I figured I had gotten a decent deal (the unit was $40!). I figured what the heck, give it a shot for $40. I retrospect, I think I may just need to bite the bullet, and get a proper compressor, but before I order one, I wanted to make sure I was not missing something.

Some questions:

1) the condenser seemed to flush well, although I know the small passages on some the the newer condensors do not clean well. Is it possible the compressor is operating properly, but the condenser is so restricted that the problem may lie here? As I say, I flushed and recovered the flush liquid--there did not seem to be much if any debris in the flush. I don't want to buy another compressor to find I need a condenser and the rebuilt unit I found was ok. I'm leaning toward the rebuilt compressor being faulty and "too good to be true" at $40. ( I should know better) Any thoughts/easy ways to check this rebuilt compressor unit/vs the condenser?

2) if he unhooked the clamps for the hoses when replacing the F129 switch and over-tightened these, could that provide enough restriction for this issue? Where the clamps secure the metal part of the hoses near the expansion valve and fender, the hoses are not perfectly round if you look at them closely, but they are not "squashed" flat, just a very very little out of round, or maybe I am just imagining this. When the system was running, though, no appreciable temperature drop across these areas, so I don't think this is an issue, but I mention it for consideration.

3) out of curiosity, I pulled the head off the original compressor and noted no junk/pieces of drier/damage to the reed valves

4) since I didn't have a baseline gauge reading on this car, will these F129 switches fail in this way under normal pressures? Could it have been the system was originally overcharged or over pressuring, causing the F129 switch to blow? I would assume the safety relief valve on the compressor head would have vented before this though, which it did not.

As I say, I may just proceed with a new compressor, but I wanted to be certain I wasn't missing something here

Any thoughts here. Your help is greatly appreciated!

4,817 Posts
So there was still the same dynamic going on. High low [side] pressure still about 75 psi, low high side no more than 60 psi.
So the A/C system exhibited the same problems as it did before the old compressor was replaced with the deal-of-a-lifetime off-the-shelf (OTS) compressor, and dryer, and expansion valve? (i.e. system seems to operate properly but no cool air from vents)

If so, there may have been some debris that wasn't flushed completely out of the low pressure line between condenser and expansion valve; the EV may be clogged again.

Regarding the F129 pressure switch: It does not require flow in the line to function (thus no orifice to clog). F129 should still be OK, but you can use a DVM (multimeter) to check it. (Normal [below ~90?psi] pressure: Switch closed=continuity; High pressure: Switch open=no continuity)

Regarding that OTS compressor: If it runs OK and doesn't sound like a bucket of nuts and bolts when running, I'd leave it in place (or ultimately have a shop tell me it's bad before dropping $$$ on a new one for a ~22 yr old vw).

Recover the R-134, then remove the expansion valve for inspection and possible replacement.

Good Luck :beer:

8 Posts
I went through the same problem last summer. The car would cool for about two minutes then stop. You mention particles in the system. It turned out to be a clogged Drier. You can tell by touching the cooling lines or using gauges. Pressures will be high and not low enough on the other side.
A simple replacement part after saving the R-134,a re-charge/oil and it works like new again.
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