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A/C Problem on my '91

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Dear fellow NSX owners,

I was driving my NSX yesterday and decided to turn on the a/c. Since it was around 70 degrees outside, I turned the temperature dial all the way counter-clockwise. I did not get any cold air, just blowing air. When I came home, I checked the engine with the car idling. I noticed the a/c compressor was not engaged. The eye-glass did not register any fluid either (probably since the compressor was not on). I shut-off the engine and tried to turn the compressor by hand, it turned with a little bit of resistance, but I don't think it seized up. I plan on troubleshooting the climate control system this everning. I bought the car 06/04, the a/c worked perfectly with plenty of fluid showing in the eye-glass. I have done searches on a/c problems, but could not find anything simmilar to my issue. The car is original with R-12. Any advise would be greatly appreciated.

John
'91 Red/Ivory
 
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You will need a multi-meter (VOM) to help with the diagnosis. I suppose you could use a test light, but it takes longer and is a lot more complicated.

See if you are getting voltage to the compressor clutch. If yes: clutch is not happy.

If not: There is a black pressure switch on the front right (passenger side, next to the top of the fender). This switch will mechanically turn off any power to the compressor if the pressure is too high or too low. Check the resistance and voltage between the leads.

If no resistance and yes voltage: switch is not happy. The system is low or the switch is bad. You can jump the leads for a few seconds to see if the compressor spins and check the sight glass.

If no voltage (compared to chassis ground) then concentrate on the control system.

You can also run a long jumper wire (with fuse please!) from battery to compressor and see if you can get the compressor to spin.

Do these checks and get back to us.


Drew
/I would not check the climate control first...do this much later.
 
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You should NEVER use a test light to test ANY circuit which is controlled by a solid state control unit. IF you have a wiring diagram, and IF you know that the wire you need to test DEFINATELY comes from a power source other than the control unit i.e. fuse, ign, batt, then a test light could be used.
Some circuits are so sensitive that even an analog VOM can cause damage.
Only a digital (DVOM) should be used to check these circuits. There are hardly any wires on n NSX that don't lead to an ecu (electronic control unit)
somewhere in the car, so the DVOM habit is a good one to get into if you are gonna play with wires.

As Drew pointed out, the a/c won't engage without freon, and this is the most likely source of your problem. I would not jump the compressor to batt power however- the purpose of the pressure switch is to keep oil from circulating out of the compressor. The freon pressure keeps the bulk of the oil in the compressor, and when there is not enough freon to do this (about 9 oz's) then the pressure switch interupts the clutch relay. In a newer NSX, this test would probabally not harm anything in a few seconds, but in an older car I would not run the compressor that way for any time.
You can do this test with the car off and the compressor UNPLUGGED from the harness and just listen for the click sound of the clutch pulling in.
Odds are less than one in a hundred cars in with a/c inop have a bad clutch coil.

The resistance indicates some freon is presant. In an empty system the clutch spins fairly free.

Be sure and check the fuses AND the clutch relay. Use the online manul to identify this relay, and swap it with a known good one from elsewhere in the fusebox. All realys in the front boxes that look identical, are.

Good Luck,
MB
 
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You learn something everyday...I wasn't even thinking about blowing up chips with the test light.

I should clarify that I don't really recommend diagnosing A/C issues with the engine running. It can be really difficult and chancy to be a DIY'er if you are not going to make the investment in the proper tools.

And A/C work has a lot of required tools...if something goes wrong you would have been ahead to get the proper tool. In that case, I just have the opinion that "it needed replacing anyways": go buy the correct tool and buy a new part. Might have cost me more, but education is expensive and I chalk it up as "a learning experience".

MB is the pro.

I'm an aspiring shade tree mechanic.

Drew
 
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Everyone,

Problem solved!!! I took my car to a new high-end used car dealership only about 2 miles from my house. They looked it over and determined that it was indeed low on Freon. They did a dye test and were able to pinpoint a loose fitting under the front of the car. I guess the vibration of the engine over the last 14 years caused the problem. I had a couple of cans of R-12 that they let me bring to keep the cost down. Now the a/c is very cold, even better than before. The eye glass is full to the brim also. I cannot tell you how relieved I was to find out that the compressor did not go bad. I am going to get into the habit of viewing the glass about once a month during the upcoming warm weather. Thank you all so much for the great advice!! This site and its members are truly awesome!!!

Sincerely,

John
'91 Red/Ivory
 
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John, Checing the sight glass is a good dea on a slightly older car.
It should look solid over 2krpms (takes two people on an NSX), but a small trace of really tiny (pinpoint size) bubbles at idle is correct.
Too much freon is worse than a few oz's low. More is NOT better.

Stay COOL,
MB
 
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Mark,

Just to clarify, when the a/c is on, I do notice slight bubbling on the side which is good. The level does disappear when the compressor dis-engages, as it should as well. Thanks again for the posts!!! I hope to make it to the NSXPO this fall and meet you. Take care.

John
'91 Red/Ivory
 
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John, Checing the sight glass is a good dea on a slightly older car.
It should look solid over 2krpms (takes two people on an NSX), but a small trace of really tiny (pinpoint size) bubbles at idle is correct.
Too much freon is worse than a few oz's low. More is NOT better.

Stay COOL,
MB

Seems like a great idea to check next time I go for a ride, just to watch things. :)
 
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