The problem is you were given an initial estimate for repair that was way too low. When the air conditioning system has to be opened for any reason, the refrigerant is supposed to be sucked out first with a recovery machine. At the completion of the repairs the system must be pumped into a very strong vacuum to boil any moisture out, then new refrigerant is pumped in. General Motors cars are famous for being designed to go together very quickly, (inexpensively), on the assembly line with little regard to future serviceability. No other brand would require the AC system to be disassembled to do other repairs, but on GMs that is very common. It's a big reason they're so expensive to repair.
A leaking coolant reservoir is pretty easy to diagnose with a simple pressure test. If the leak is too small to find that way, a small bottle of dark purple dye can be added, then they would search later with a black light and the dye will show up as a bright yellow stain that can be followed back to the source. In this case they must have been confident in their diagnosis. The proof is if there is no coolant leak now.
What they should have done is told you in advance that the AC system would have to be discharged. The problem is too many people assume they're trying to sell you unneeded work, and that leads to the mistrust people automatically have for the industry. Also, with such an estimate, some mechanics are afraid you'll go somewhere else for a second opinion and have the work done there, so they give you a low estimate to start with, then once the work is in progress, they tell you about the other stuff. That's not the way I like to do business but if the additional work or procedures really are needed, there was no attempt to defraud.
Some shops account for every possibility they could run into, like rusty and broken fittings and fasteners, and present an appropriate estimate. Most of the time they don't run into all of those potential problems, then they can surprise you with a bill lower than you expected. That too leads to the assumption you were ripped off because they didn't do all the work you were expecting. Either way, we lose. Mechanics are held to much higher standards than doctors. If we plan for surprises that will add cost to the repair and you go somewhere else for a second opinion, that second shop will of course be lower priced. You're mad at the first shop. If we DON'T plan for those surprises and some show up, we have to charge more, and you're still mad at us.
You didn't say which engine you have, and without seeing the car we can't tell which type of fittings are used on your AC system hoses, but many manufacturers have gone to "quick-connect" fittings that snap together on the assembly line. That saves on labor cost but the car owner ends up with a less than perfect seal. Age and under-hood heat degrades the rubber seal in each of those connectors until a little normal vibration, or pushing on the hoses causes some refrigerant to leak out. The system starts with quite a bit more refrigerant than is needed for proper operation because the engineers know it will leak out over time. There is a magic point at which the "low-pressure cutoff" switch will stop the system from working. That is done to avoid the possibility of the compressor drawing in outside air with its humidity. Moisture mixed with refrigerant creates an acid that is deadly to the metal components in the air conditioning system. There will still be refrigerant under pressure in the system when that switch trips, and just before that happens the system can appear to be working fine.
Even if the mechanic didn't have to take anything apart on the air conditioning system, simply pushing on a hose could have allowed some refrigerant to escape. It could have been so little that it went unnoticed, but if the system was already leaked down just to the point of tripping the low-pressure cutoff switch, that momentary leak could have been enough to do it. The clue would be there is still pressure in the system.
Be aware refrigerant is extremely dangerous to work with. It can cause frostbite and blindness. Still, some people will poke a fitting to see if there's still refrigerant in the system. The proper way is to connect a set of pressure gauges. If there is any pressure still there, the mechanic did not disconnect anything related to the air conditioning, and you just reached the point where natural leakage stopped the system from working. That would not be the mechanic's fault and he didn't do anything wrong. We all fear this type of problem, and it happens too often. Things happen while the car is in our possession. If the problem occurred an hour earlier or a day later, you would not blame the mechanic, but if it occurs while it's in the shop, it has to be his fault.
What I would do is go back to the same shop, explain that the AC system was working before the service, and ask if any hose had to be disconnected to do the repair on the cooling system. Have them connect a pair of gauges to the system to see if there's any pressure in it. Once the low-pressure cutoff switch stops the system from working, there will still be pressure in it for a long time unless there is a pretty bad leak. If there is no pressure, you'll know something had to be disconnected. Pumping the system into a vacuum to remove the moisture, ("evacuating" the system), and recharging it with refrigerant would have been part of the job in that case and they should have included it in the repair estimate and bill. You should have been charged for that additional service, but now that it's a day later, many shops would take their lumps and recharge it at no cost to you. They would suffer the loss of your confidence, and doing this for free would be an attempt to restore their reputation. The problem here is thanks to restrictive government regulations, refrigerant costs about eight times what it needs to and shops can't afford to give that away. They might offer to do the work at no charge if you pay for the refrigerant.
By the way, related to that, there is no accurate way to know how much refrigerant is in the system unless there is no pressure at all. That means they have to suck out whatever is in there, then pump in a carefully measured amount. Unlike your home refrigerator, cars can tolerate quite a bit extra refrigerant, which is good because, also unlike your home fridge, cars use rubber hoses which are porous, and quick-connect fittings that are prone to leaking. The system IS going to leak resulting in failing to turn on, but the goal is to make that take as long as possible. The natural leakage occurs just due to the nature of the parts used in the system. That has little to do with mileage or how much you use the AC system. It's mainly due to age, and by now, a '99 model would have typically needed to be recharged at least once already. (If this was a '99 Ford product, it would have been recharged a half dozen times by now).
Back to that pressure test with the gauges; if there IS still significant pressure in the system, say 10 - 50 pounds, there is no way any hose could have been disconnected because the mechanic would have been frozen by the escaping refrigerant, and 100 percent of it would have boiled out within a few seconds. If there's some pressure still in there, the mechanic didn't do anything wrong and it's just a miserable coincidence it quit working now.
When the system is low enough on charge just to the point the low-pressure cutoff switch trips, it may work again for a few minutes on a hot day. That's because the refrigerant will expand, increasing the pressure and turning on the switch. That won't last and is not an indication the problem has resolved itself.
Wednesday, October 2nd, 2013 AT 12:44 PM