No pressure change in A/C system

Used '91 Explorer. Previous owner put a lot of sealer/dye in the A/C. Worked fine for 2 years. Last used 3 months ago. Tried it this month and no cold air. Looked under hood, compressor not engaging. Put
pressure gauge on low side. 18 PSI (system not running). Due to two sealer/Dye stickers on the unit, I automaticly assumed the system wasn't working due to low freon. Well time to change over. Had system drained, flushed and a long, low vacuum (by commerical machine). Held vacuum over night fine. 24 OZ of 134a (40 PSI, system not running). And system still not running. (What?) Power thru the low pressure senor. Power to compressor. (Don't tell me bad clutch?) Wacked the compressor and clutch grabbed. Turn A/C off and clutch releases. Turn on and wack the compressor and clutch grabs (air gap grew during the winter?) While compressor is running, I decided to wait while scratching my head. Thats when I noticed that the A/C was still blowing hot air. With the clutch turning the compressor, the system is still reading 40 PSI on the low side and 40 PSI on the high side. System running, and the freon isn't moving? Has anyone had this problem? Compressor shaft some how not moving? Some internal part sealed shut from the sealer? Any knowledge would be aprecated before I start knocking my head against it.
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On Wed, 15 Apr 2009 20:27:12 -0700 (PDT), origdirtyoldman

It sounds like a bad compressor valve. Was this a retrofit from R-12?
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Sounds like your compressor is completely shot. Are you sure your system only takes 24 oz of R134? Because, unless it was less than 50 F outside, the static pressure of the non-running system should be much higher, about the same PSI as ambient degrees F, e.g., 60 PSI @ 60 F. You could remove the compressor belt, apply battery voltage to the clutch terminal and try to spin the compressor by hand. It should be difficult or impossible if the compressor is working. If it spins freely when the clutch is engaged, you need a new compressor.
By the way, as the clutch face wears, the air gap grows to where the magnet is not strong enough to engage it consistently. A little corrosion on the clutch shaft only makes matters worse, which is why it doesn't work well after sitting idle for 3 months.
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It appears that there is more than 1 problem with your AC system.
-Multiple applications of sealer. This is often due to an evaporator leak. Evaps require instrument panel removal ($$$) to replace. These sealers basically don't work and usually damage other parts of the system. People insist on "trying them anyway, just in case" on the false hope that it will save an expensive repair. If one or more applications was a 2-part sealer, the entire system is now trashed. Sealer is a HUGE no-no in an AC system.
-Compressor not building pressure when engaged. Assuming there is any refrigerant to compress, Usually caused by a broken compressor shaft due to continued high head pressure caused by overcharging, sealer contamination, air in the system,condenser blockage, condenser airflow problems.... Can also be caused by a missing orifice tube or the tube being blown into the evaporator. You should notice some slight fluctuation in the gauge readings in this case. No fluctuation means broken compressor shaft.
-Compressor clutch not engaging unless it is "assisted" Clutch air gap too wide. Caused by normal wear, excessive head pressure over an extended time, low current to the clutch coil, weak clutch coil. If the gap is .025" or more, it's too wide. It can be adjusted, but it appears your compressor is also damaged. Adjusting the gap won't fix the compressor.
The only way to know the extent of the damage is to remove the orifice tube and look at the debris. I can guarantee you will find steel chunks, hardened sealer, pieces of burned Teflon ring material, aluminum chunks or some combination of those. There may also be a black gooey substance indicating a "Black Death" compressor failure.
Due to the sealer, age, and obviously poor prior repairs, your only viable option is to replace the compressor, accumulator, condenser and all underhood lines. (Firewall foreward replacement) Less than this is flirting with problems and wasted money/effort down the road.
You also need to have the evaporator pressure tested to check for a leak. This is the #1 reason for sealer use. If it leaks, figure replacement costs into the price. If a 2 part sealer was used, it MUST be replaced.
This is NOT going to be an inexpensive repair. If the vehicle has a rear evaporator unit, the cost will be even higher. Luckily I don't think the Expy ever had a rear unit, just some duct work to the rear seat.
You can DIY the repair and save a good bit, but be wary of "inexpensive" parts, especially the compressor. Your Explorer uses a Nippondenso FS-10 compressor, so if the price is under $150, don't be tempted. Cheaper FS-10 remans are very prone to shaft seal leaks and other issues after about 1 year. (You'll be fixing it again $$$oon) Good remans/new units are about $200-250.
I would estimate the DIY cost of the repair at about $750-800, using decent quality parts. At a shop, it would be in the $1500 range. Many shops won't even touch a vehicle with sealer contamination.
If you decide to tackle it yourself, check out www.autoacforum.com first. Do some reading to get a handle on how to fix it correctly.
Good Luck!
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