AC compressor replacement

Tiny
CORNERPIECE
  • MEMBER
  • 1992 MAZDA 929
  • 6 CYL
  • RWD
  • AUTOMATIC
  • 160,000 MILES
I am getting ready to replace the AC compressor in my car. Does anybody know the correct procedure to remove and replace the AC compressor?
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Friday, March 3rd, 2017 AT 4:54 PM

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Tiny
CARADIODOC
  • EXPERT
First the system must have the refrigerant recovered with special equipment. Be aware that refrigerant is extremely dangerous to work with. It can cause frostbite and blindness. Professionals wear gloves and safety glasses, and most also wear a face shield.

Your best bet is to get a copy of the manufacturer's service manual and read through the pages of instructions. You can also buy an online subscription that covers the same information.

Once the new compressor is installed, the system must be pumped into a vacuum for at least half an hour. While the hoses are open, air will get in, along with the humidity in it. Water in the system combines with refrigerant to form an acid that will corrode metal parts. A droplet of water can also freeze at the orifice and stop the flow of refrigerant. That will cause a lack of cooling until that ice melts. In a vacuum, that water boils at seventy seven degrees and can be drawn out.

The last step is to pump in the exact amount of refrigerant called for. If you try to do that with the little cans, there is no way to get the correct amount. Unlike home refrigerators, automotive systems can tolerate a little over-charge without causing damage to the compressor, but with too much refrigerant, it will change from a liquid to a vapor beyond the evaporator in the dash. You want that change to take place right in the middle of the evaporator. That is where it becomes real cold, and that is where the incoming air blows over it.
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Friday, March 3rd, 2017 AT 5:21 PM
Tiny
CORNERPIECE
  • MEMBER
I have already purchased the art twelve kit that comes with the hose, and three small cans. Is it possible to purchase a larger cans or is there a way to make the three small cans work for me?
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Friday, March 3rd, 2017 AT 5:28 PM
Tiny
CORNERPIECE
  • MEMBER
R12, not art 12. Sorry
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Friday, March 3rd, 2017 AT 5:28 PM
Tiny
CARADIODOC
  • EXPERT
You need to have a way to pump the system into a vacuum before the refrigerant goes in. I bought a compressed air-powered "pump" from Harbor Freight Tools many years ago, but it cannot get even close to a perfect vacuum. Now I take my projects to a friend's body shop and use his charging station.

I have the luxury that all my vehicles are Chrysler products, and they all have a sight glass in the receiver drier to show when the system is fully-charged. (That is supposed to be replaced too any time the system is opened for repairs. It has a desiccant in it to absorb a few drops of water). I can just keep adding refrigerant until the vapor bubbles disappear in the sight glass. For all other manufacturers, you have to go by the amount called for on the label under the hood, and program that into the charging station.

You might consider converting to R-134. This is the time to do it. The oil in the compressor has to be changed to a different formulation that is compatible with R-134. The modification used to be real involved and impractical, but it has been simplified to just changing the refrigerant and oil, and adding a pair of port adapters.

A 30-pound cylinder of R-12 used to cost well over $800.00. That was mostly due to taxes, and in most states you can only buy those if you have a license to work on AC systems.

There is a minimum pressure that must be in the system for the compressor to turn on. If that pressure cannot be reached, you'll need to find the low-pressure cutoff switch and temporarily bypass it. That will make the compressor run and draw the low side down so more refrigerant will leave the can. Usually you can get the first can in, but by then there is enough pressure in the low side that no more will flow in from the second can. A trick to overcome that is to place the can in a pot of hot water. Always keep the cans upright so only vapor goes into the system. If you hold it upside-down, liquid will go in. It is being pulled in the low side near the compressor, and they cannot tolerate pumping a liquid. At a minimum, the compressor will lock up for a little while. More commonly the valves will be damaged.
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Friday, March 3rd, 2017 AT 5:49 PM
Tiny
CORNERPIECE
  • MEMBER
I think I am going to do the conversion from R12 to R124a. Thank you for the recommendation, I will let you know when I receive the necessary components and enable to do the repair.
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Friday, March 3rd, 2017 AT 6:15 PM
Tiny
CARADIODOC
  • EXPERT
The story used to be you needed a different compressor because R-134 is less efficient. It was thought you need double the high-side pressures. Different hoses were needed to handle those higher pressures. Also, refrigerant will seep out through the rubber hoses because rubber is porous. R-134 molecules are much smaller than R-12 molecules, so they wanted us to use hoses with nylon liners. You still had to worry if the evaporator and condenser could handle those higher pressures.

By the time I started working at a dealership, the conversion had been simplified to replacing the oil, refrigerant, and fittings. Most of the adapter fittings had barbs to lock them in place. That was to prevent someone from removing them, then using R-12 equipment to recover the refrigerant. That would contaminate the recovery machine, and potentially all the cars that machine was used on after that.

The flow of refrigerant going into the evaporator is controlled by some type of adjustable orifice. As it turns out, with the less-efficient R-134, it just vaporizes faster, then more liquid is let in. The flow just increases to handle the cooling load. Those higher pressures are not needed. You may still have to add some refrigerant periodically, but the small cans are not nearly as expensive as R-12.

Keep me posted on your progress.
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Friday, March 3rd, 2017 AT 6:35 PM
Tiny
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Sounds great. I will keep you posted.
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Friday, March 3rd, 2017 AT 6:38 PM

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