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A/C Failures And How To Prevent Them

new 08/12/2004

SHO Taurus A/C system overview:
As promised here is a fairly comprehensive overview of the 1996-1999 SHO Taurus A/C system. I did quite a bit of development work with Ford CCD through Sanden who supplied the compressor. I regret that my knowledge of the system is purely on the mechanical components and not the software/controls side- that was a different department. I do have now about 20 years of experience with research and development on climate control systems at the OEM and tier one level. I will divide this into sections by component.

The Sanden TR scroll is of an extremely robust design, it is far in excess of the requirements for the SHO engine. Two things drove the selection of this compressor- durability and packaging. The Ford standard compressor at the time, the FS/FX series would not take the rapid rpm changes that the 3.4 engine could deliver. In addition it was not feasible to package that compressor in the space allotted in the engine bay. Ford was quite impressed with the scroll design and has licensed it for manufacture in the U.S. There are several aspects of construction we need to be aware of, it like every other component in the system-leaks. The casting is porous and has been impregnated with a zinc epoxy amalgam called imprex; this seals most of the pores of the metal, limiting leaks. It does have a Teflon shaft seal which also leaks, the front housing in shimmed and installed with a large O ring which leaks. There is a Pressure Relief Valve in the rear of the compressor, which will vent if the discharge pressure exceeds 520 psi. It does leak slightly as delivered but if tripped and reset will never seal well again. There is a High Temperature cut out glued in the back of the compressor housing near the discharge valve, this is quite an expensive component and needs to be functional, if wired out your compressor will fail due to overheating if you have a low charge. The scroll compressor does not function well at idle rpm, it needs speed to operate, hence the small diameter clutch and its tendency to overheat at extended high rpm if the charge level is low. The clutch is either a metal to metal or a metal to friction material type. It will need have its gap adjusted every several years for optimum performance, the thickness of a business card will do nicely, too much gap and the clutch will slip, too little and it will not disengage, heat up and destroy the shaft seal.

Hoses leak-Period. SAE J2064 states that A/C hoses may leak upwards of 2 kilos of refrigerant per meter length per year. While they are a bit better than that and getting better by the day they still leak. They are crimped to tubes, these joints leak. The assemblies are put together with O rings, mostly with Ford spring lock connectors, they really leak in the long term due to constant rotation on the long axis.

System Oil:
Our cars came with a Glycol base oil in them called PAG (Poly-alkylene-glycol) it dissolves readily in the refrigerant we use and has borderline lubricating properties. The real good oil is POE (Polyolyester), it is also quite expensive, somewhere around 20X the price. The PAG is quite sufficient for the SHO as the scroll is all ball and roller bearings without any metal to metal friction surfaces. If you are repairing a system switch oils.

The SHO uses a DuPont refrigerant called R-134a, its trade name is Suva. It is not Freon! When you recharge you system specify to use new refrigerant to refill. Suva is a Fluorine based refrigerant that does break down over time and temperature, something like a 20% change in chemical properties over 3 years.

A vehicle using R-134a does not need desiccant. Period. Older cars that used R-12 had problems with water combining with refrigerant and producing harmful acids. R-134a cars do not, I have personally run 3 fleets of cars in development testing without desiccant and 10-30 grams if injected water to gauge the results. 5 years and roughly 15 million miles later we have had no issues in 3 climates. OEMs recognize this, any accumulator you by from service parts will not have desiccant in it (cost savings). Vehicles are still shipped equipped with desiccant from the factory but the amount has dropped, 250 grams used to be standard, they are now down to 50 grams, expect this to drop to zero in the next couple of years.

Servicing the system:
The number one thing to do is to recharge the system every two to three years, my car will lose 15% of its charge in 2 years. A properly charged system will run a very long time barring any component failure. Remove the old refrigerant, vacuum the system completely, this gets rid of any non-condensable gasses that will raise pressures and lower performance. Replace with new brand-name refrigerant, it does not pay to scrimp. Check the system joints, look for oily spots, they are leaks, replace components when necessary before they fail. The compressor is quite expensive, it does not need to fail.

I want to take a moment and personally thank Todd, whose credentials in this area are unquestionable. It is input like this that makes us a unique community.

Todd, Many thanks for your article. I hope we will see more in the future. Most importantly,  the Broom Stick method, perfected by Eric!!!

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