99 Suburban A/C Not working

Check for battery voltage on the dark blue/yellow wire in the connector at the AC clutch with engine running and AC on. If no voltage, yank the AC relay and check for battery voltage on terminal 86 in the relay socket. If no battery voltage on that terminal, check fuse 21. If yes, check for ground on terminal 85 in the socket. If no ground, then PCM isn't commanding the AC clutch on. Check for voltage on the dark blue/yellow wire at the low pressure switch and good ground on the black wire. The PCM provides power to the low pressure switch which then goes to ground. The PCM is looking for a large voltage drop if pressure is above 34-38 psi. If pressure is below that, the switch will be open, so the PCM won't see a voltage drop and will prevent AC clutch operation. The high pressure switch should be closed at pressures below 270-330 psi

It may need freon.if it is to low it will not kick on.

POSIBLE LOW IN FREON OR YOU HAVE A ELECTRICAL PROBLEM

ADD Freon. It's a R134 system and the pressure is too low. Trust me add Freon and it will work fine. With a low amount of Freon if you jump the low pressure switch at the accumulator it will be very cold but will not have enough pressure to kick on the low pressure switch.

Assuming you have the system adequately pressurized with gas, check out the low pressure cut off switch. If it is in the open position no current will reach the clutch windings and the compressor will not operate.

If the compressor kick's in when you jumper the relay socket there is no problem with the electrical to the compressor an the compressor isn't locked up . You have a control side problem , the PCM - engine computer is suppose to energize the relay if no other problems are present . The proper way to check this would be to hook up a scan tool an check pressure input's to the PCM an is the PCM getting a request from the HVAC control panel to turn on the A/C compressor . AutoZone is wrong as usual , There can be codes without the check engine light being on . o'reilly's is wrong as well DTC P0530 (LU3, LR4, LM7, LQ4 and L18)
The powertrain control module (PCM) or engine control module (ECM) monitors the high side refrigerant pressure via a A/C refrigerant pressure sensor. When the pressure is high the signal voltage is high. When the pressure is low the signal voltage is low. When pressure is too high the PCM will not allow the A/C compressor clutch to engage.

Action Taken When the DTC Sets
The malfunction indicator lamp (MIL) will not illuminate.
Diagnostic Aids
The air conditioning (A/C) compressor clutch will not engage under the following conditions:
• The A/C high side line pressure is over 2957 kPa (429 psi).
• The A/C low side line pressure is under 138-172 kPa (20-25 psi).
• Throttle angle is at 100 percent.
• Engine speed is more than 5500 RPM.
• Engine coolant temperature (ECT) is more than 121°C (250°F).
• Ambient air temperature is less than 5°C (40°F).
• Engine is idling at a low unstable RPM
Compressor clutch engagement cannot be used to determine status of the low pressure switch. The low pressure switch is one of several inputs to the HVAC control module for A/C request authorization. A/C request is one of several inputs to the powertrain control module (PCM) that control A/C compressor clutch engagement.
To accurately determine what pressure the A/C low pressure switch opens and closes at use Kent Moore GE-47742 which will allow the technician to measure the switch point pressure at the switch.
Using a scan tool, monitor the "low pressure switch" status while monitoring the "low side" pressure at the switch to determine the switch points of the low pressure switch.
The low pressure switch "connector seal" must be removed before plugging it into the switch for testing. The "plunger effect" of plugging the connector with seal into the switch induces a pressure on the back side of the switch, this pressure will skew the opening/closing characteristics of the switch 5-10 psi until the pressure bleeds off. The time required for the connection induced pressure to bleed off can take 20 minutes or longer

Do you have the proper diagnostic tools ? Factory scan tool , A/C gauges ? You best bet , take your vehicle to a qualified repair shop !!

check for low voltage or fusible link getting hot

The power to run the compressor clutch coil goes from the battery, to the AC Clutch Relay, somewere in the engine compartment fuse box, then it goes to the positive side of the AC compressor clutch coil; goes thru the coil, activating the compressor, and then goes to complete the circuit through another switch (high pressure protection switch) in the back of the AC compressor and finally reaching negative or Ground.

Disconnect the AC compressor clutch connector and using a 12 V bulb with 2 wires (test lamp) check the following:

1.- connect 1 lamp wire to battery neg; with the other check both pins at connector with the AC in the RUN (AC and ignition switch on ) you should have power in one side.

2.- connect 1 lamp wire to battery positive and with the other check both pins at connector, lamp should light getting the ground from the clutch connector thru the high pressure compressor switch to ground.

On the other side what makes the relay close and let the power flow to the clutch works with the low pressure switch, (if the freon pressure is too low it opens and kills the ground to the relay coil and that kills the compressor), that low pressure switch is in the low pressure side of the freon tubing, the fat pipe that comes from inside the firewall into the accumulator/dryer, a big aluminum cylinder sitting on the right side of the engine compartment in GM cars.

To know if the low press switch is the cause you can unplug it and connect both legs with a paper clip for a minute or two, no more, you don't want to damage the compressor. anyway if jumping the switch makes the compressor come alive and kick in, you either have low freon pressure or a bad switch; it's time for the pressure gauges and check how the compressor is pumping and that is better left to a pro.

By the way you said that your system is a R-12 system, have you considered switching to R-134a the new "ozone friendly" freon? to do that you need to evacuate the system with a recycling machine and pull vacuum in your AC, if it holds the vacuum for a while, then you can be sure that there are not leaks and then dump 1.5 or 2 pound (check the labels in the engine compartment) of R-134a into your system. it will be a good idea to put some fluorescent leak detecting dye in the system to check for leaks, just in case..

Check for power and ground to left (large) radiator fan with test light. If power and ground to connector are ok, replace faulty fan.

the system has a definite leak and should be fixed then the system should be recharged there is a pressure switch that keeps the clutch from engaging when the system is low on freon which keeps from burning up the compressor