A/C compressor won't switch on

[efhjr]

My air conditioner only blows hot air. Contol panel is set to AC, but no cold air -- and I don't hear the compressor kick in.

The car's air conditioner control panel was replaced about a year ago (apparently it was the last OEM unit left in the US). There are no blown fuses.

Today I had the cooling system checked out by professionals and it passed all tests -- freon pressure is good, and the compressor comes on when given power from battery.

Any ideas?

[BD240Z]

I'm not an AC guy and I don't know if these systems have a valve/switch that keeps the clutch from engaging if the freon is low. I see you had the freon level checked but if it has this valve/switch and it's bad, it might prevent the voltage from reaching the clutch.

The clutch clicks on and off when you applied voltage from the battery but it doesn't when you turn the switch from inside the car. Start at the compressor and work your way back towards the switch inside the car trying to find out where the voltage is going.

Test the fuse with an ohm meter or replace the fuse, don't assume it's good. Do this first!

The "professionals" that inspected the car for you should have given you some suggestions on what to check.

Good luck with your car.

BD

[efhjr]

The "professionals" that inspected the car for you should have given you some suggestions on what to check.

They're a good shop. They just didn't know what to do with a car that's 32 years old, and were honest enough to tell me.

Thanks for the suggestions. I'll double-check and/or replace fuses next.

[zcarnut]

It is the AC relay (located on the small AC bracket with also holds the vacuum switches and the check valve) that energizes the compressor clutch through its switch contacts. The AC relay coil, in turn, has three switches in series with it: the under pressure switch (located on the receiver/drier); the ON micro-switch (energized by the AC control lever on the console); and the thermostat switch (located behind the temperature control lever). In addition the blower switch is also part of the circuit since the blower fan must be on while the AC is running (to prevent excessive pressures in the AC system). So, there are several items that must be checked to verify the series circuit is complete.

Check the easiest-to-get-to items first: The AC relay and the under pressure switch. The AC relay can be checked for both switch continuity and whether or not the coil has voltage across it. If it is suspected then it can be substituted with a good working one. The AC relay can be taken apart to inspect the contacts for pitting (very common since you are switching an inductive load) and the contact points can be cleaned with a points file.

The under pressure switch is screwed into the top of the receiver/drier. It can be tested with an ohmmeter after it is unplugged or you can short it out with a jumper wire to check it. The under pressure switch is available separately from Nissan, but it requires you to discharge the AC system first before replacing it.

The micro-switch and thermostat switch can be tested after you remove the control panel on the located between the console and the dash. Again, use a jumper wire or an ohmmeter to check them. I’m not sure about the availability of NOS parts as I have never seen these parts become defective (fortunately). Since your blower fan switch seems to be working (based on your post) I would assume that your blower switch is OK.

BTW, all of this info is in the factory service manual. I urge you to obtain one and do the troubleshooting yourself. You learn a lot more from doing than asking.

Edited July 15, 2009 by zcarnut

[DougN]

Also make sure your top control lever is set to "off" not "vent", as you will not get any cold air if you are pulling in outside air from the vent.

[efhjr]

Thanks for all the good tips and info, gents.

I'll hit the books and get under the hood this weekend.

[efhjr]

With the car off and the leads disconnected, I put my multimeter on the under pressure switch and it failed a continuity test. So there's my problem.

Now to find a new one and start saving for a few cans of R12!

[efhjr]

Uh-oh. Just paid a visit to my local Nissan stealership, and they told me that part is no longer unavailable.

[zcarnut]

Uh-oh. Just paid a visit to my local Nissan stealership, and they told me that part is no longer unavailable.

Use the 27648-U8710 low pressure switch. Cost is $10.21. The electrical connector is different (you will have to cut and splice in your old one) but it is available through Courtesy Nissan: www.courtesyparts.com

[efhjr]

Thanks for that info and link, Steve.

My car's A/C was rebuilt a few years back and is still using R12. Is there any good reason to have it converted to R134?

I ask because this will come up when I have the system recharged when the pressure switch is replaced.

[FastWoman]

The R134a operates at much higher head pressures and often doesn't get along well with existing hoses, coils, and configurations. I've had two cars retrofitted to use existing coils and hoses. The budget retrofit (Fred's no-name in an old Cadillac) blew up. The expensive manufacturer retrofit (Saturn) works fine but leaks. I've also put a complete Vintage Air 134a system in a Mustang, and of course it works great. However, that didn't involve a combination of old R12 parts and new R134a parts. I'd stick with R12 as long as feasible, as long as it works.

[efhjr]

"Blew up" is about all I need to hear, FastWoman. I think I'll stick with the R12.

I'll ask the shop if they'll fill it with Freeze 12, maybe that'll save me a few dollars.