bhensarl

Recently I noticed a sort of rattling noise while the air conditioner is running. It only happens when the A/C button is depressed, and only happens periodically. I stopped the car and checked inside the engine compartment, and sure enough I only hear the rattling noise while the A/C compressor is being turned by the belt. Any suggestions on how to fix this or what might be wrong? Thanks.
Brian

springer3

Check:

Compressor mounting bolts
Belt tension
Freon charge

It all these check out, it is probably the compressor starting to go. How is the cooling performance? I would bear with the noise until cooling performance drops off. There is some risk of contaminating your system if you run until the compressor starts to grind up its internals, but cleaning up the system is no big deal.

bhensarl

Is there any way to check the freon charge without special tools? The records say it just got freon in february. Cooling is good, and being just up the road in Atlanta I'm sure you know how hot it's been around here lately. I'm not freezing on 95 degree days, but I'm not sweating either. I really hope it's not the compressor starting to go though. I don't know how I'd face hot and humid Florida in August...
Brian

springer3

Feel the compressor suction line after you have been out driving around. It should feel cold. If it is warm, you are low on gas. Be careful not to grab the discharge line. It is extremely hot. I am not sure low charge will cause noises, but over charge might (system is straining at too high a head pressure. If that is the case, your suction line will feel very cold.

bhensarl

Hmm, things to look into. So my options seem to be these:
1. Leave it be since it works just fine and the noise is more "slightly annoying" than it is really a cause for worry. Or...
2. Attempt to relieve a bit of the overcharge somehow. I'm fairly certain that it has a good charge. After a good drive in hot, humid weather the hoses show a fair amount of condensation which would lead me to believe that the suction line is in fact cold. I'll check this next time I go for a drive. If I do decide to attempt to "bleed" the system a bit, is this something I can do myself? Thanks for all your help, Springer.
Brian

springer3

The quick answer is yes. The schraeder valve in the AC line is very similar to the valve in the tire stem. Depress the pin, and get out of the way. There is a lot of pressure. Problems:

1 You don't know that is the problem. Try cleaning the condensor and see if that lowers the head pressure (every couple of years the condensor needs cleaning, and most shops don't bother).

2 It is illegal to vent R12 to the atmosphere. I don't think you alone will grow the hole in the ozone layer, but you could be made an example of if the EPA learned about what you did.

3 You will lose an unknown amount of oil. If you let the car sit overnight first, the oil will be at the low points and you won't lose as much.

4 You could vent too much and reduce system performance.

5 If the schraeder valve gets damaged or contaminated, you could have a system leak.

Now that I think about it, there is a good way to tell if the system is overcharged without use of gages. Clean the condensor first. There are commercial cleaners that do a great job. Engine degreaser followed by a water jet works well. A pressure washer is useful if you can avoid damaging the delicate fins.

Your AC system is at optimal performance if it uses 100% of the condendor's heat rejection capacity. Hot vapor freon enters the condensor, and by the time the freon exits, it should be near near 100% liquid freon, hense the name, condensor. You want a 2-phase mixture of gas an liquid in all passes of the condnesor, with the early passes nearly 100% vapor, and the last pass nearly 100% liquid. Any 2 phase mixture exists at constant temperature for any given pressure (water always boils at 100 C at atmosphereic pressure, for example. If you have too much freon, you "go solid" before the last pass, and the later passes don't do any "condensing". They will cool the liquid, and that helps cooling performance a little. However, that is at the expense of a seriously overloaded compressor.

This is easy to detect an overcharged system simply by checking the temperature at the condensor inlet and exit pipes for the condensor. If the exit pipe is significantly cooler than the inlet pipe, it means that the gas has been liquid and cooling off longer that it should.

It takes a pretty fancy thermometer to measure the temperature on the surface of a pipe. If you want a coarse method, your hand can tell. Just time how long you can touch the pipe. The inlet will be less than a "one hot". That means you can't even count to "one" before you can't bear the pain. If the system is way over charged, you will be able to touch the exit line and hold for a count of one or more.

Good luck. Let us know what you find.

bhensarl

Wow! So much info. All of which would probably been really helpful, except...
I was driving along today with the A/C on when all of a sudden a banshee of a squeal came from my engine compartment. This squeal was followed immediately by the smell of burning rubber and smoke from my vents. I clicked off the A/C, pulled over and shut off the engine. Popped open the engine hatch and everything looked fairly normal save a bit of smoke that escaped just after I opened the hatch. So I'm thinking the compressor just bit the big one. What do you think? If that's the case, how difficult is it to replace the compressor? Do you have to drain the whole system to do it? Florida in August will be no fun without A/C.
Brian

bhensarl

P.S. If I press the A/C button, it sounds like a belt is slipping and I can smell the burning rubber. Obviously, I've stopped pressing that button.
Brian

bhensarl

Okay, I had a friend press the A/C button while the engine was running and I was watching the compressor. The belt was running the compressor while the A/C switch was off, and when he pressed the button the belt driven portion of the compressor assembly (is this the clutch?) stopped rotating. So I'm stumped. Can you shed any light on what's going on here, springer? I really appreciate all your help.
Brian

DaveK

The belt will run the compressor all the time. But with the a/c off, you should see the "face" of the compressor pulley is not rotating. When you engage the a/c you should hear a click (especially if you are standing by the engine bay) and the clutch will engage - and the "face" of the pulley will start turning. With the a/c on the clutch will cycle on and off periodically to stop things freezing up.

I've added my attempt at art to describe what I mean.

From your description - it sounds like when you switch on the a/c, your clutch engages and the compressor pulley stops rotating (and hence the belt slips on the pulley)? If this is the case - then, although I know far less about a/c than Springer - I'd conclude your compressor is dead, and that engaging the clutch locks the pulley.

springer3

I had mine eat the belt not too long ago. You were luckier than be, because I got a dent in my deck lid from the flying belt.

How tight is the belt? Can you rotate the center portion of the clutch by hand? It takes a lot of torque to turn the compressor. If the belt is even slightly too loose, it will slip rather than turn the compressor. If you can't budge the compressor by hand, the internals are likely locked up.

Replacing the compressor does require removing the freon. The 964 is the easiest compressor to remove of any car I have seen. There are four bolts on top, and a tensioner mechanism on the right bottom. All take a 13 mm socket, IRRC. Your car may be a little different from my 92.

bhensarl

Thanks for the diagram, Dave. You're spot on. The pulley rotates all the time, but the face of the pulley doesn't.
So it looks like it's time to replace the compressor. I've found one off a 92 964 that was totalled with 22k miles. Would it be okay to get that one, or should I look elsewhere? I'd really rather source one myself and take it in to the shop so that I can save a few hundred bucks.
Brian

springer3

A low mileage used compressor sounds too good to be true. Salvage yards are rough business, and it is common to "fluff up" value by lying about age or mileage of used parts. The parts jerk typically has no clue how sensitive an AC compressor is to dirt and moisture. Even if the 22 K is correct, you need to be sure the unit is not abused or damaged before installing it.

I woud avoid a junk yard AC compressor unless I:

1. See the compressor still on the donor car.
2. Verify mileage.
3. Verify the wreck did not put a huge shock load on the compressor.
4. Verify the system has full gas and oil charge.
5. Watch the unit being pulled to ensure that no moisture or contamination enters the compressor.
6. Install clean protective caps immediately after hoses are removed.

A bad compressor will ruin your new receiver dryer and clog your new expansion valve. An worn compressor will not last very long, and will not develop enough pressure to make cold air. There is nothing quite like 45 degree dry air blowing on your face on a hot humid afternoon. There is also nothing quite like sweltering in traffic, and wondering if it would not be more comfortable to shut off the AC and open the windows. Investing a few extra $$$ to get the job done right is money well spent, IMHO.

bhensarl

Thanks for all your help, guys, especially you, Springer. I've found a remanufactured compressor for not much more than the piece that was pulled off the car, and I think that would be a much better deal as far as peace of mind is concerned. I wouldn't be able to verify anything about the salvage compressor until it was here on my doorstep. Once again, I really appreciate all your help!

Brian