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Dead compressor or wrong recharging procedure?

U-96 on Tue June 24, 2008 9:53 AM User is offline

Year: 1995
Make: VW
Model: Golf Mk 3
Engine Size: 2.0L
Refrigerant Type: R134a
Ambient Temp: 85
Pressure Low: 80
Pressure High: 100
Country of Origin: United States

I just replaced TXV on my car, vacuumed the system and then started charging it. With engine and A/C shut the system sucked it about 150-200 grams of refrigerant (out of required 800 grams), the high / low reading was about 75 PSI. Then I closed the high line and started the engine and the A/C. I ran engine to 2000 RPM but the pressure didn't go higher than 100 psi on high end / lower than 75 on low end and the system won't suck in any more of refrigerant. The compressor is running (clutch engaged) and not making any suspicious noises...

Now does all of this means the compressor is dead and need replacement? Or could it be that it just does not have enough refrigerant in the system to get started? (There are some info indicates that it may happens with this Sanden SD7V16 compressor...)

I would hate to spend $500 and a weekend on replacing the compressor and then just find out the compressor was fine and I missed some step during charging the system...

Thanks!

Edited: Tue June 24, 2008 at 9:55 AM by U-96

GM Tech on Tue June 24, 2008 9:59 AM User is offline

I've seen this happen before- Mostly on Saturns rotary vane compressors-- you have to get all the charge in before it will start pumping right--I did it by gently warming my 30 lb tank- with a tank heater blanket- the charge then all goes in -- in about 90 seconds- then start the car and all is fine...

You can warm your cans carefully- in hot water bath- will help a lot.....

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

U-96 on Tue June 24, 2008 10:08 AM User is offline

Thanks for the advise, will try.
I just wonder how much pressure the "over the counter" refrigerant can from the Autozone can take before blowing up?...

GM Tech on Tue June 24, 2008 10:39 AM User is offline

I wouldn't heat them over abouty 130 degf--still touchable to the human hand-- gotta figure those trucks that haul them get pretty warm inside-- your PT chart will give you exact pressure- but you still have to know burst pressure of the cans-- which is usually a safety factor of 3x....

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

bearing01 on Tue June 24, 2008 10:43 AM User is offline

A hot water bath will try to keep the little 12oz can at around water temperature (100'F ?) to keep the can pressure higher than the suction pressure. This won't come even close to blowing up the can. You never apply heat in the form of a flame to any refrigerant bottle. The pressure in the can is completely dependent on the temperature of the liquid refrigerant in the can, which is typically around 70psi at 70'F. The problem is that as you pull refrigerant out of the can the pressure in the can starts to drop. When it drops, more of the liquid in the can has to vaporize (boil) due to pressure drop. In order to boil it has to draw heat from its surroundings (can feels cold) but it primarily gets this heat from itself. As the liquid refrigerant's temperature falls (because it gave up heat to boil/vaporize refrigerant) its pressure falls and this reduces the push behind the refrigerant in the can into the system. You need the can pressure to be higher than the compressor suction (low side) pressure and to ensure this you put the refrigerant can in a bath of warm/hot water to help replenish the heat back to the can's liquid. This keeps the temperature up and therefore the pressure up so the refrigerant can will push its contents into the AC system.

The SD7V16 doesn't need a full charge to to start pulling the low side down below your 70'F / 70psi can pressure. If there's enough refrigerant in the system to keep the compressor clutch activated then if you goose the throttle you should be able to make the compressor suction pressure oscillate and dip down around 40psi or so to draw refrigerant in.

The SD7V16 should be able to take a charge, unless the RCV refrigerant control valve bellows has sprung a leak.

Edited: Tue June 24, 2008 at 10:47 AM by bearing01

mk378 on Tue June 24, 2008 10:52 AM User is offline

Charging with the engine off, warm up the can and turn it upside down to get liquid out. Then you don't have to supply heat to boil the entire contents of the can, just enough to keep the pressure up as the liquid goes out. As long as the can is warmer than the rest of the car, the liquid refrigerant will all move into the car.

Was it doing the same thing before you replaced the TXV? It is looking like a problem with the control valve in the variable compressor, but get a full charge in first.

U-96 on Tue June 24, 2008 10:54 AM User is offline

Quote
Originally posted by: bearing01
The SD7V16 should be able to take a charge, unless the RCV refrigerant control valve bellows has sprung a leak.Does it mean a leak into the outside environment or a leak back to the low pressure side of the system? I did not detect any leaks to the outside while repairing / vacuuming the system...
Is there any way to tell for sure whether the compressor (this particular model) is dead?...

mk378 on Tue June 24, 2008 11:23 AM User is offline

This compressor senses the low side pressure internally and when it reaches about 25 psi, an internal bellows-driven control valve reduces the displacement of the compressor so it will not pump down lower. Typical failure has the displacement is stuck at approximately zero regardless of low side pressure. The unit will spin but not compress. It does not leak to the outside.

U-96 on Tue June 24, 2008 11:38 AM User is offline

Quote
Originally posted by: mk378
This compressor senses the low side pressure internally and when it reaches about 25 psi, an internal bellows-driven control valve reduces the displacement of the compressor so it will not pump down lower. Typical failure has the displacement is stuck at approximately zero regardless of low side pressure. The unit will spin but not compress. It does not leak to the outside.Is there any way to fix this problem, other than replacing the compressor?

Also, when this problem occurs does it mean the compressor won't compress at all? Mine seems to do something (80 psi on low side, 100 at high side at engine 2000 RPM), but just nowhere near enough...

bearing01 on Tue June 24, 2008 12:05 PM User is offline

When the RCV fails the compressor will still compress a little. There is a starter spring on the wobble plate that offsets the plate to a small angle and this causes the compressor to have a little displacement. As a result, if the RCV is dead, you may still get like 60 or 70 psi on the low side and maybe like 110 psi or something on the high side.

The RCV is replacable. If you search this site for the SD7V16 you will find links where you can buy the RCV. From the threads it appears you can replace the RCV but depending on the RCV you get, if it's not exactly like the original one, the system doesn't work well. I don't know, it's worth a shot if the RCV is much cheaper. I considered this on my car but got a new SD7V16 from www.ackits.com for something like $350, which to me, was worth it.

U-96 on Tue June 24, 2008 2:26 PM User is offline

bearing01, I think you are right, it's just not worth it to hassle with repairing the old compressor.
Now, the ACKIT.COM shows two new compressors for my Golf:
SD7V16 1100 PV6 12V 4EAR
SD7V16 1167/1102 PV7 4.62in

I wonder what the difference between them and whether they are interchangeable... Tried to ask the ACKIT.COM but haven't heard anything from them since yesterday.

bearing01 on Tue June 24, 2008 3:36 PM User is offline

Take a look under your hood. My SD7V16 for 2.0L ABA engine, 1997 VW Cabrio looks like the 4EAR one at $329. That's the one I bought from ACkits.

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