Help me before I melt
June 6, 2008 7:12 PM   Subscribe

(AutoRepairFilter) It is hot in Texas and the A/C in my 2001 Frontier just gave out. What can a semi mechanically - inclined person do about this?

The cool does not work at all and the heat works but slowly. The fan is running and the green A/C light turns on most of the time, but the performance of the cool is absent regardless to whether the light is on. I realize I will more than likely have to take this thing in and get it worked on. From what I understand it takes specific tools to work on most A/C units not to mention the danger in dealing with the chemicals. My mechanic wants to charge me $45 just to run a diagnostic. Is thier anything I can do to at least get an idea what is wrong with it before I get ripped-off?
posted by Brandon1600 to Travel & Transportation (6 answers total) 1 user marked this as a favorite
It sounds like the system has lost pressurization, so it probably needs to be recharged and before that can happen, the source of the leak needs to be found. Most modern cars use the A/C to condense water out of the air before heating it, even in the winter, so it's not surprising that it has affected the heat as well.

Some easy things to check (I'm not familiar with the Frontier in specific, so these are general in nature):

* While it's rare for the A/C pump to have its own belt these days, it might, and if it does, make sure it's still on and tight. The A/C Compressor will be cylindrical, attached to the front of the motor to one side, and will have at least two insulated lines coming out the back. That said, looking online, it looks like the Frontier's AC Compressor is run from the big multi-belt, so it's unlikely to be the case.

* With the engine running, be sure the compressor is actually turning. If it has seized, you'd almost certainly know about it, as the belt would break or there would be a horrible screeching noise, but you never know.

* Look over the big hoses coming out the back of the compressor to be sure they seem to be intact and attached.

Having said all that, if your system has lost pressure, you really need to have it professionally re-pressurized. The $45 will cover a half-hour of your mechanic's time to put a pressure gauge on the system and perhaps preform a pressure test. A/C systems do tend to slowly lose their refrigerant over time, so you may be able to get away with just re-pressurizing the system...but anticipate one or both hoses or the compressor being bad.

I'd guess $200 to re-pressurize the system, $500 parts + $150 or so labor if it's the compressor, maybe half that if it's the lines.
posted by maxwelton at 7:33 PM on June 6, 2008

Most likely you've got a freon leak, which isn't something you can really test at home-- I suppose you could loosen a hose briefly and see if a noxious, deadly, incredibly bad for your health gas escapes (which will likely also break your locally laws) but I wouldn't recommend doing so.

Other then that, your heat sensor might be shot, if you put your A/C to 'max' it generally bypasses that sensor.

Since it's a modern car, it's likely got a serpentine belt (the same belt that powers the various parts of the engine also goes via the A/C) so it's unlikely the belt's snapped (otherwise your car wouldn't move..)

The clutch could be gone on the A/C compressor-- open the hood and find the A/C compressor, it's generally tucked on the right hand side, you should be able to identify it because the tubes lead off to the radiator on the front of your car. If you turn off your A/C, then turn off your engine you should be able to freely move the wheel, turn the car back on and put the A/C on, then turn the engine off again, if you're still able to freely turn it then it might be damaged.

Check your fuses and see if any are blown...

Personally, for $45 I'd just get it checked out by a pro. Best case you're looking at a little leak and a regas of the system-- you can ask them how much that will be. Worse case you'll need a new compressor (I lost mine a while back, and from looking at it, it was pretty obvious is was dead-- clutch had locked up so it never spun free and burnt out the compressor).

Good luck!
posted by Static Vagabond at 7:46 PM on June 6, 2008

h/t to maxwelton, I should have previewed and saved a calorie.
posted by Static Vagabond at 7:47 PM on June 6, 2008

I spent a summer in Austin without vehicular AC. This isn't a diagnostic tool, but a coping one: Carry a spray bottle with you in the car. As you tool around with the windows down, occasionally spritz your face. Instant cooling.
posted by mudpuppie at 7:52 PM on June 6, 2008 [1 favorite]

You can buy an A/C coolant recharge kit in your local auto-parts store for about $30 (it looks like a big spray can with a hose and a pressure gauge attached). If your A/C system has a leak in it, filling it back up won't be a permanent fix - you'll be cool for awhile, but the ozone layer will suffer.
posted by killdevil at 8:23 PM on June 6, 2008

Best answer: If your system has lost its refrigerant charge, your compressor will not cycle, thanks to a low pressure switch; this is to protect your compressor. Usually, the compressor clutch is prevented from engaging by a control circuit which turns the clutch on and off as needed, normally. When the low pressure switch says there isn't enough refrigerant, the control circuit is simply not ever allowed to energize the compressor clutch; the drive belt will look like it is turning the compressor pulley, but no electrical signal is supplied to engage the compressor clutch, and it won't be passing power through to turn the compressor. Many vehicles also use this signal for the engine idle bump, that kicks up the engine idle a couple hundred RPMs when the air conditioner is engaged; so, if turning on the air conditioner doesn't cause a slight jump in your idle as it usually does, you've got secondary confirmation that you probably have a low pressure condition in the refrigerant loop. You can actually directly measure this in many cars, with a voltmeter, if you find the signal pin in the AC wiring harness.

Any vehicle newer than a 1997 will use R134a, which is less deleterious to the ozone layer than older R12 refrigerant. R134a is not particularly toxic, and you can safely attempt to recharge your own system with one of the $30 kits killdevil mentioned. These come with a can or two of R134a, a cheap gauge to measure your compressor pressure (so you'll know when you've put in the right amount of refrigerant), a tube of ultraviolet dyed oil to put in your system as a leak detection aid, and some fittings with hoses to connect to your compressor's line fittings. Some even have a CD or a DVD to demonstrate the whole recharge process.

Basically, if you can screw the top on a reclosable 2 liter soda bottle successfully, you can recharge your own air conditioning system.

If you have a leak, this won't be a permanent fix, the first time, but it will demonstrate that your problem is indeed loss of coolant, not a bad compressor, and with the aid of a $10 flourescent black light, you can probably find the leak, by spotting the UV dye in the leak detection oil. It mainly just requires a willingness to look over your whole system, carefully, inch by inch, in the dark, with the UV lamp as your only illumination, looking for traces of the UV dye oil, after you've run the air conditioner for half an hour or so. Unless the leak is in the internal evaporator up under your dash, which is tough to see without disassembling the heater core box, it really is just a matter of patiently tracing all the lines in your system, and being particularly observant. Often, a leak will simply be an O-ring in a hose fitting, or a pressure hose, itself, which you can replace yourself. Then, with a second recharge, you've fixed your system.

Another common problem in cars 7 to 10 years old, is that the air conditioning systems contain a sacrificial element called the drier or accumulator, which is a kind of filter with dessicant media, designed to collect any tiny amount of water vapor or dirt that gets introduced into the system, before it blocks the small orfices in the compressor valves and evaporator controls. Eventually, this part will go bad, and need to be replaced, and that can cost from $50 to $200, depending on the part cost and labor. This is something a do-it-yourselfer with the right tools and some patience can do, but a pro will generally pull a vacuum on the system after replacing this part, to verify that there are no negative pressure (cool down) leaks, and to evacuate any moisture in the system before recharging it. For that reason, if you do elect to change the accumulator yourself, I recommend letting a pro do a vacuum test, and recharge your system for you, afterwards.

But even if you don't undertake your own repairs, knowing that your compressor is good, and you're just dealing with a leak, a plugged accumulator, or other minor problem, will put you a long ways ahead in negotiations with your repair shop.
posted by paulsc at 9:56 PM on June 6, 2008 [1 favorite]

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