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The scroll compressor cascade failure...

bohica2xo on Thu July 12, 2012 2:12 PM User is offline

Year: 2007
Make: Saturn
Model: Vue
Engine Size: 3.5L
Refrigerant Type: R134a
Ambient Temp: 110
Pressure Low: 90
Pressure High: 180

It is hot here. People expect less from their MVAC when it is this hot, but...

This is my wife's car. I don't drive it much, and it sits in the cooled garage. So we went to Kingman AZ last weekend. A/C seemed fine on the highway plenty of cooling. But after a hot soak, in traffic it was exhibiting some really crappy performance. As soon as you stopped it would blow ambient. Not good. Wife says "been that way for a couple of weeks"

I ran home & recovered it. 2 ounces low. Not bad for 5 years an nearly 70k miles. Charge proper charge. 90 psi low side at idle. Spin the engine to 2500 rpm. Boom. Good cooling, 30 psi low side, 250 on the high side. Basically the scroll is not pumping at low speeds. Crap.

But "cascade failure"? Yes. The compressor was dying. It had not tossed metal everywhere yet. The problem is the complicated system that runs the fans on modern vehicles. Because the compressor could no longer make 200 psi (even on a 110f day) at idle, the fans slow down. So the condensor just keeps getting hotter. Really hot. So when you drive away from that light, the refrigerant that comes out of the condensor is really hot at first. The fans spool up, forward motion cools the condensor & things sort of work. None of this is any good for the system. The result of continuing to run the system would be a firewall forward repair.

Around here many shops have plug & play service machines run by untrained personnel. The average owner would take a car in this condition to one of those places, and get a recharged system. excuses would be made, the FSM diagnostic steps would not be done. The owner would think it was just "too hot" and continue to run the system.

I did the FSM diagnostic flow chart. It led to the answer "Replace Compressor", along with inspecting the lines for any sign of debris.

Now for the good news. The wife is a paperwork wizard. I told her it was a compressor, and not an easy job. She pulls out the file for her car. Extended Warranty. Covers the compressor. Seems that when Saturn was selling cars with 5/50 warranties they had a decent third party extended warranty - which she had read with the care of a trained research specialist...

Many of us here have a time or two called a dealership the "stealership". More good news. Monday morning at 0700 I was handing them the keys. Explained that I had used the FSM flow chart from Mitchell (buy a subscription it is worth it!) and the service writer started writing like he was paid by the word. When dealing with a vehicle, let them have the time they need. I told him to keep it overnight if they needed to...

I knew that they have to drop the engine cradle to get the compressor out. Easier with a cold engine. What I had not noticed was the three bad motor mounts. They did. Restored my faith in dealer mechanics.

What could have been an extensive A/C repair was avoided by prompt attention to the declining system. Granted the vehicle design made even a compressor replacement difficult, but it needed to be done. If you encounter a scroll type compressor that does not pump at idle speeds, change it out ASAP.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

ice-n-tropics on Thu July 12, 2012 3:36 PM User is offline

Yote slayer,
I was too slow to draw a bead on a yote a few days ago. I think they are after my beavers.
Hope your not too disappointed after the free compressor change, at idle and LV ambient.
My experience w/Saturns was w/ the OEM controlled capacity 5 vane rotary (CCVR). At idle, all the vanes did not have enough centrifugal force to overcome side plate friction to stay in contact w/ the outer housing. This allowed internal vane tip gas bypass and low refrigerant mass flow. Slightly above idle, I could hear the vanes start to contact/slap the outer housing and cooling began.
The Saturn vane rotary is abnormally small displacement and doesn't cool satisfactory at idle above 105 ambient at idle (to suit my needs).
Mazda 3 w/ vane rotary is the same no idle cooling as Saturn.
All these 'CONTROLLED" capacity vane (or Ford controlled capacity scroll CCS) compressors have internal re-expansion chambers that spoil the isentropic efficiency that is characteristically inherent in the fixed capacity versions. The CCVR has suction restriction to reduce capacity and cycling. The CCS has internal mid point hot gas bypass to reduce capacity and cycling.

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

bohica2xo on Thu July 12, 2012 8:09 PM User is offline


Too bad the 'yote got away - we all like our beaver!

The 3.5L Vue is unique in that it has a fixed scroll, an S105 The performance is in spec, so the condensor did not eat enough junk to lose capacity. They did replace the dryer. We will hammer it this week to be sure.

Been a long time since I got anything out of a warranty besides an invoice. And good service from the dealer too. Should have bought a lottery ticket.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

JJM on Fri July 13, 2012 12:56 AM User is offline

Good to know, my '05 Caddy has a scroll also and I'm at 78K, but well out of warranty. The scrolls don't seem to cool as well at idle anyway, but the Caddy is not that bad.

Excellent overall analysis of the failure cycle.

Motor mounts... on the Northstar they're normal maintenance items. Granted, I use the Northstar, but I've probably changed motor mounts as often as I have wiper blades.


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