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Old 04-29-2010, 08:40 PM
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Join Date: Jan 2003
Location: Dallas
Posts: 776
AC is pulling vacuum on low side

AC just quit cooling suddenly.
Compressor is running, but not pumping freon thru sight glass.
Checked pressures: high side is 100, low side is vacuum, about 20 inches.
Shut it off, and low side very slowly returned to 100, after 30 minutes or so.

Assume clogged expansion valve?

Car is '91 300SE with all original system. Worked great yesterday.

Did a search on replacing expansion valve - not a pretty sight!

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Old 04-29-2010, 08:43 PM
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Join Date: Apr 2010
Location: Sterling, VA.
Posts: 286
i'd expect the high side to be higher than that, sounds like your low on freon.
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Old 04-30-2010, 07:13 AM
Posts: n/a
You are most likely have a refrigerant leak. In this case, if it is indeed low on freon, you should NOT simply charge it and forget it. When a system gets to a point where it is drawing vacuum and the leak happens to be on the low side, you are drawing in contaminated outside air. Leak should be diagnosed, refrigerant recovered, leak fixed, r/d replaced and system THOROUGHLY evacuated before recharging.
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Old 04-30-2010, 09:39 AM
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Join Date: Sep 2004
Location: DFW / Collin County Texas
Posts: 1,882
I just had this happen to me on another non-MB car. It can be deceiving. My problem was caused by a stuck TXV. After replacing the TXV and R/D, I discovered a massive leak from an O-ring on the high side. I suspect what happened was:

1. TXV got stuck
2. high side pressure got too high
3. O-ring on high side became compromised
4. refrigerant leaked, causing what appeared to be a simple low charge condition

I checked my pressures when it first started acting up, and it was about 300 on the high side and vacuum on the low side (R12). When I checked again later in the afternoon it was about 100 on the high side and vacuum on the low side.

I recommend checking for leaks, particularly on the high side. After charging, you will know pretty quickly whether or not your TXV is stuck.
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Old 04-30-2010, 01:25 PM
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Join Date: Jan 2003
Location: Dallas
Posts: 776
Thanks guys.
But, the plot thickens....... kinda long, I did a lot of stuff today.

Today I kicked it on and it cooled great. All seemed normal!!!!
(Yes, Old Pearl has healed herself before).
Below are the test conditions and results I saw.

- Engine speed 1400, AC system on full cold, fan Hi, doors open.
- Outside air temp 85. Just the belt-driven fan pulling air across the
condenser (electric fans did not come on, but they do work when needed).
- Allowed it to stabilize for about 5 minutes.

- sight glass bubbling away as it's always done since new.
- vent temp 55 degrees.
- hi side pressure 220-230 (maybe a bit high?)
low side pressure 22-25
- low side return line nice and cool
- when I turned it off and left the gauges connected, the low side pressure moved back up VERY slowly. After 30 minutes the high side was back to a normal 100psi, but the low side was up to only 65 psi.

The operating parameters seem pretty close for a normal system. The high side might be a bit high for 85 ambient, but probably due to the less-than-stellar air movement of just the belt-driven fan (see below for more theory).

Possible conclusions:
- Freon level is okay. Cools fine now.
- elevated hi side might indicate a partial (sometimes total) hi-side
restriction, such as in the expansion valve.
- Slow hi-low side pressure equalization after turn-off might indicate
restriction in expansion valve.


Other trivia:
- late last summer, and earlier this spring, the compressor would randomly
just stop running for several minutes, then kick back in and cycle just
fine. I never got to actually troubleshoot it, cause it would heal just about
the time I got started looking. I assumed a pressure switch or a bad conection (please not the MAS). I cleaned all the connections and unplugged/plugged the MAS, and it had not misbehaved lately. When the "restriction" problem happened to the wife, I thought it was just the compressor off again, but it was running (but not cooling) this time.
COULD THIS BE INTER-RELATED to the current "restriction" issue?
I don't see how, unless the hi pressure was going waaay up, enough to trip the hi pressure switch (450 psi?).

What I'll do:
- Right now I'm leaning to the restricted expansion valve theory.
I think I'll drive it and see if it fails again. Then run the tests again.
If it does act up, barring new evidence;
-I'll get the freon recovered, see how much it had in the system, and conclude if I did have a leak.
- Replace the valve and drier, and maybe a flush-out, depending on what I find in the valve. Replace accessible o-rings.
- Vacuum test for leaks, pump her down for a couple hours, and shoot in my last R12 cache, along with the proper qty of the correct oil.

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Old 04-30-2010, 03:13 PM
Posts: n/a
Since you apparantly don't have a leak, an R/D might not be necessary unless it's just been ages since last replaced. I do now believe that you did indeed have a restriction.

If you can find a shop with a recovery/recycle machine set up for R12, you might be better off to pay them to recover, replace the TXV and recharge with your refrigerant. That way you keep your reserve in hand.

Since you're in Dallas McCains probably has an R12 machine and they are reliable and reasonable in their pricing. That said I haven't dealt with them in several years, but if Mark is still there then I expect that they are still golden. In fact if Mark is still there, call him and tell him what you've found. He is a very helpful and approachable guy and one of the few people around that I'm comfortable with when it comes to leaving my car with someone.

One of my machines is set up for R12, but it has a bad controller board. I'm not sure if I'll ever get it fixed.
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Old 04-30-2010, 10:23 PM
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Join Date: Jan 2003
Location: Dallas
Posts: 776
Yeah, I was down at MaCains a few years ago to check what I thought was a freon leak at the lower condenser connection. They moved awhile back from near the Mazda house, to Floyd Cr, behind my 2nd home-TI. Had them sniff it and they found nada. Turned out to be a slight front crank seal leak, throwing a small bit of oily stuff on the AC pipe. They're still really nice folks.
But, I'd prefer to do the legwork myself if I can, that's what retirement for a gearhead is all about.
Drove it around today, vent temp hit 44 degrees. Guess I'll let it go until it clogs again, and then I'll feel pretty sure it's a restriction, and go for it.

Thanks for the inputs,
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Old 05-01-2010, 12:09 PM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,514
Very strange problem you have there. The only thing I can think of that would cause readings of 100/0 on a fully charged system is a restriction on the high side before the high side service port. So somewhere in the condesor or one of the high side hoses. A stuck TXV would cause high side readings to be high and low side to go to 0.

If it does it again, try feeling the high side pipes/hoses. If there is a restriction the hose will be cold after the restriction, rather than hot.
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Old 05-19-2010, 12:15 PM
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Join Date: Jan 2003
Location: Dallas
Posts: 776
Just to wrap up; installed the new TVX valve and drier (whew) and shot in a couple ounces of oil and 45 ounces of freon.

As the guy falling off the Empire State Bldg said "So far so good".
Temps and pressures are good, and the sight glass is actually full for the first time since I bought the car in '91. When I shut it off, the hi & low sides equalize pressures a lot sooner than before.

A couple of observations:
- TXV install was all it's said to be - avoid this if you can.
- No evidence of any crud in the old TXV however, looked clean as new.
- Cut open the old drier, absolutely dry inside, no evidence of any oil. The dessicant looked to be crumbling a bit, but no particles found in the line all the way to the TXV.
- We pulled vacuum on the new install, shut the pump down and let it sit for 10 hours - no loss of vacuum at all, maybe the system is pretty tight.

So, I guess my conclusion is still a faulty TXV valve.

Thanks for your inputs.

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