I have a 1999 GMC 3/4 ton truck. My air conditioning doesn't work

negative even though there are two air conditioning controls inside your suburban is still only one AC unit

small trucks like that are considered 1/4 ton. silverado 1500 1/2 ton an silverado 2500 3/4 and up. same goes for gmc

Look at the front of the tenisinor there is a squre slot in it a 3/8 ths extention for a 3/8 ths drive scoket wrench will fit in it. Put the extention on the wrench and put the end in the slot then push toword the engine this will move the tenisitoner and release the tention and then you will be able to remove the belt.

Take radio or climate control panel out and check to see if connector came off during radio installation.

You have to remove the wheel and the brake caliper. If it has antilock brakes disconnect the antilock break sensor wire up by the frame and get the wire free form the holders. Remove the big nut on the end of the axel shaft with a 1-3/8 inch socket, now turn the steering all the way to the left and losen the two bolts behind the hub with good 18MM socket. They are usually very tight be carefull not to round the head of the bolts use a good quality 6 point socket. Turn the bolts out about 6-8 turns and stop now turn the steering all the way to right do the same with the other two bolts. Now, take a air hammer with a punch end in it and hammer on the heads of the bolts, a little at time, on all the bolts, evenly until the hub is loose. If you don't have a air hammer a hammer and punch or drift will work. Then remove the bolts the rest of the way and pull the assembly off. Now you have to drive the wheel studs out and remove the rotor. I just described it on 3/4 ton a 1/2 ton might be easier but the principle is the same, I think there are only 3 bolts holding the hub on and the bolt sizes may differ. Also I think you can pull the rotor right off once caliper is out of the way. Sorry about that.

i think that your A/C system is almost emty of freon (134a)

it could be the A/C compression switch and here is a place that sells only GM parts Schram auto and truck parts 1-800-292-1032 and for diagrams pictures and step by step instructions to replace it try autozone.com or your local library

Check under the hood for deteriorated small vacuum hose coming off the intake over to the heater box area.
Repair the vacuum hose.

It should be 3/8 allen wrench to remove pads & 13 or 9/16 metric to get off calipers. I have a GMC Suburban & I have to have both metric & standard to work on it.

The clutch in compressor is controlled by a pressure switch. Start there.