He means to turn the engine/key to off. You need the thing at least on ACC to set the HVAC panel to 73º and the passenger **** to straight, before you pull the fuse.

 

 

I'm a bit confused by the term "73 degrees". If the HVAC **** was a clock what position would the **** be in?

 

OK....... Bought some manifold gauges at Harbor Freight. Rented a vacuum pump. Vacuumed the system down to -29
Held for an hour. Started to add the first of 2 12oz cans, but the compressor wouldn't turn on. Pulled the high pressure sensor plug,
and tried the old fashion way of jumping the terminals. Nothing. (I hope I didn't mess anything up, but I might have).
I read here to disconnect battery and then reconnect and start car. Compressor will turn on. It did!!!
Emptied both cans. Low side pressure was 36 and high side 230. Inside duct temperature was 54 degrees, even though I had the temp setting at 73 degrees.
Perfect!!! I won!...............Not so fast - Turned the key off. waited a couple of minutes, started up the car, and the a/c won't turn on.
Push the a/c button......light comes on, then goes out in 3 seconds. I try several times. same thing. The compressor simply won't turn on.
Now what???? I know the system is full. Not under charged or over charged. All fuses OK (checked twice with a 12v test probe)
It seems as if the compressor is not getting power. I am at a loss, and it's 93 degrees today. HELP!!!!!

 

To set the temp to 73 degrees you turn the driver's **** while looking at the hvac display. There is no pointer on the driver's **** and it will turn all the way around with no mechanical stop. If you get cold air on your legs check the mode display, there is one setting for dash air only, and another for dash AND lower under-dash air. If you see the correct dash only indication and still get under-dash air then you may have a faulty door actuator. I think the actuator is a vacuum powered device. The manual that I have is not Corvette specific in the HVAC section. I would check for vacuum at the actuator. If you have vacuum, connect a hand vacuum pump with a gauge to operate the actuator, it may have a leaking diaphragm. If it holds vacuum you can operate the actuator manually to see if the under-dash air is controlled by it. They use foam gaskets on the door flapper valves, foam can deteriorate and leak, although, if this is the problem I would live with it as long as it is cooling. You don't want a leaking vacuum system, this can cause drivability issues and other codes.

On the compressor 'fail to start', you put refrigerant in, this should trigger the low pressure switch and engage the clutch. If it does not and you have adequate refrigerant (1 lb. +) the low pressure switch may be bad, check for voltage to the switch, if it is there, then the switch is bad.

Update- The system only has one pressure sensor, it senses both low and high pressure. So, if there is at least one pound of refrigerant in the system it should send the signal to the HVAC head to start the compressor. If the sensor fails the compressor will not start. If it is cycling the clutch rapidly this indicates excessive pressure but the sensor is working. Look for the cause of the high pressure.

 

Man, is my face red. 73 degrees on the hvac display! No coffee this morning or another "senior" moment, LOL. Thanks!

 

 

There is a high and low pressure sensor switch in the liquid line right behind the radiator. Its job is to detect the presence of refrigerant turn the compressor on and off if line pressure gets too high, (fans not turning below 35 mph, condenser core clogged with dirt and leaf debris, clogged orifice, collapsed internal hose liner). If this switch is working and you have a high pressure situation, the clutch will engage and shut off rapidly, turn the blower down and radio off and listen for it. The switch can fail shorted (rare) and If it fails open the clutch will never engage. To check this switch for proper operation you need to connect a proper set of ac gauges to see if the high side pressure is exceeding limits. On a 100 degree day it will quickly reach 400 psi. I see the pressure approach 380- 390 and then decrease for normal operation. A clogged orifice will pull a vacuum on the low side gauge. When the pressure goes this high the fans should go to high speed, if they don't, check the 3 relays. You can check them by letting the engine idle (AC OFF) until it reaches 227 degrees, the fans should come on at low speed, let it get hotter, at around 235 degrees the fans should go to high speed. They will shut down after the engine temp goes below 217 degrees, and should run continously below 35mph with the ac on. You can do the fan test quicker by driving in traffic with the ac off, but the fans are hard to hear when they turn on.

If the fans work properly and the clutch still cycles on and off rapidly there is a restriction in the system that must be fixed. Don't forget cleaning the condenser core of dirt and leaf debris, this is an external airflow restriction that causes high pressures. The two cores are laying back collecting crap as the car goes down the road vacuuming the pavement clean. All this stuff builds up in the cores and is trapped there. Ordinary cars have the cores mounted vertically giving gravity a chance to pull the larger crap off when you turn the ignition off. I was amazed at the quantity of crap in my cores, it took two cleaning sessions to get most of it out along with removing the fan housing for access to the back of the radiator for washing with water. I think there was about 5 pounds of gravely dirt and several hands full of leaf debris.

 

A/C Works!!!!!!!!
Today, all I did was to turn key to on, (Not Start) , pushed the a/c button, and the light goes out in 3 seconds.
SO... started car up and got the car on ramps to check compressor connections, but just for the heck of it,
turned the a/c on, the compressor clicks on, the a/c runs!!!!!!!! Middle vent shows 51 degrees. (It's currently
85 degrees with humidity at 75%). For some reason....problem fixed, and the a/c is colder than at any time
since I bought it in 2011. I am a cancer patient, and have no extra funds for anything but necessities. Needless
to say, I am one happy camper. Thanks for all the imput from everyone!!

 

Quick question for all about the HF manifold gauge set: which set of numbers do you read? There are 4 rings of numbers around each dial. There's a note for each ring; on mine the 3rd ring from the inside says R134a (the others are different Rs). The outer ring doesn't indicate any R. I assume I should be reading the numbers on the 3rd ring?

 

Only use the numbers for 134A, the others are for home and industrial refrigerants. Be sure you purge the gauge manifold and hoses with 134A as you connect them. You don't want to inject air and moisture into your system. The safest way to do this is to connect the yellow (center) hose to a can of 134A, then crack open the valve on the supply can and let it go through the entire hose set. If you have a leak detector you can use it to tell you when the gas reaches the connection end. If the end connectors have valves also, crack them open slightly to blow air out of the car's ports. Do the high side first, then close the valve after it is connected to the car. Be careful if the car has been run in the last 20 minutes, it may spray liquid 134A on you, burning your skin. The connectors are usually quick release fittings, practice on the gauge manifold's dummy port fittings so you can connect them with minimal loss of refrigerant before doing it on the car. You will have to hold the lines while you make the connection, don't just push the valve down on the port with force, if it breaks you will have a face full of 134A and PAG oil.

Your gauge set should have detailed instructions, read them. I was lucky in this respect, everyone I grew up with knows ac procedures, it really helps to have someone experienced in this, it is easy to turn it into a disaster. You might check with your local Vette club for help.