A/C doesn't work?

Some of these trucks had a wiring issue leading from switch to resistor. check for weak and very flexible wiring like it's been burnt up. if so there is a kit available from gm dealers or www.gmparts direct.com it's about $100 and you also need to replace the resistor. the purple wire goes to the blower and should have 12 volts. check the fuse and or blower relay under hood. see if power goes to the resistor on lower speeds it may be just a bad resistor as that controls the lower speeds other than high.

This video and guides will help you fix the problem.

https://youtu.be/uZrQCGwXfek

and

https://www.2carpros.com/articles/car-air-conditioner-not-working-or-is-weak

and

https://www.2carpros.com/articles/replace-blend-door-motor

Check out the diagrams (Below). Please let us know what happens.

Sorry isent the stuff for front blower. Check fuse and blower resistor assembly the red wire shold have 12 volts for blower. See pic.

Sorry I should have done a better job on the answer as i'v e not done this for a while so taking me a bit to get in swing of things. It is item 5 on blower housing. See pic

If you are talking about the rear your wire colors are all wrong. Check the fuse for rear blower under hood it will say rr hvac and also check ign 3 fuse in car. Then have a pro scan it for you the control may be bad.

It sounds like your blend door could be jammed open or closed. Another common problem is there is a valve that allows hot water to come through the heater core at the firewall. This could be wedged open/closed. Check to make sure it operates properly. You should be able to hear the blend door moving back and forth when you turn the climate control knob to hot/cold.

Try a system reset. Start engine, turn off climate system. Stop engine. Turn key on, but do not start. Leave for 10 seconds. Turn off and pull key out of ignition. Leave out for 30 seconds. Restart engine, turn on air conditioning. Try that and see what you come up with.

89018964 Module, HTR & BLO & A/C CONT this is an updated part # for the blower control unit if that has gone out.

Check this unit as the terminals can melt on them sometimes.

First turn key on and pull HVAC fuse inside truck for sixty seconds then reinstall fuse and see if the actuator works it might. That resets the actuators when you do that. See picture for actuator. The dash has to be removed to gain access.

Let me know how the system reset goes.

If the problem occurs intermittently I would venture to say there is a loose connection/short that is happening in the system.

Well we can't clean the cables for ya on here, so you will have to have someone else do it or do it your self. There is an actuator under the dash on the heater box. See if it is stuck yo may have to feel around for it it's electric. If it's bad it will have to be replaced or see if there is any power going to it.

The control unit that Blaze mentioned could also be the culprit since it helps determine whether the system blows hot or cold.