A/c only works on highest setting

Jason,
Please try the following Website for more precise help.

The Electric Connection [[email protected]]

The following was provided by The Electric Connection to me for a 2004 Silverado. Most GM Cars and Trucks use nearly Identical Heater A/C Control circuits.

I am adding testing instructions due to popular demand. Provided by The Electric Connection.

Testing is as follows. Disconnect the 7 wire harness connector and inspect.

If there is any sign of heat damage replace both parts per above instructions.

If there is no sign of heat damage then check the following.

With the 7 wire harness disconnected check for power on the RED wire.
It should have +12 volts at all times and it gets it's power from the 40 amp blower fuse in the under hood fuse box. (Silverado)

Next with the key on check for 12 volt power to each of the following wires as you move the blower speed switch from low to high.

Yellow / low, Position 1
tan / med 1, Position 2
lt blue / med 2,Position 3
purple / med 3 Position 4
orange / high. Position 5 Energizes Relay Coil.
red/ 12 Volts Hot at ALL TIMES which supplies power to the Relay Contact so when the Relay Coil is energized the Contact Closes supplying full 12 Volts to the Blower Motor. If the Relay Return Spring fails then in most cases gravity will hold the Relay Contact Closed resulting in not being able to turn the Blower OFF unless you disconnect the Battery or remove the 7 Position Connector.

I am not sure what Color the Ground Wire (Negative 12 Volts) is. Obviously we have given you 6 colors corresponding the 5 Speeds and +12 Volts which leaves us one unidentified wire. Trace this wire back to Chassis Ground and make sure that it has NO RESISTANCE TO CHASSIS GROUND.

If any of these signals are missing you will need to inspect the blower speed switch.

Just remember that if you do get Position 5 The HIGHEST Speed then the Relay is functional in the Resistor Module. I do not understand what Fuse you removed to re-enable speeds 1-4. Please let me know where it is located and what else it controls. If you are in fact now getting speeds 1-4 without a Fuse the Fuse you removed really had nothing to do with the Heater Fan Circuit.

Consider that you have 5 speeds available utilizing 4 Resistors arranged in Series for Speed 1, 3 Resistors in Series for Speed 2, 2 Resistors in Series for Speed 3, 1 Resistor for Speed 4 and a Relay Contact for Speed 5. You can measure Voltage at the Blower Motor for each Speed and UNDER LOAD it will be Lowest on Speed 1, correspondingly Speeds 2, 3 & 4 will be Higher and Speed 5 will be somewhere between 12 and 15 Volts with the Engie running. If speeds 1-4 work and speed 5 doesn't work then the problem has to be in the Relay itself or in the 7 Position Connector and possibly the GROUND WIRE also in the 7 Position Connector. CHECK that 7 Position Connector for signs of Heat Damage and/or Mechanical Damage. Carefully remove each Contact One at a time from the 7 Position Connector by gently depressing the small tab which retains each contact. You will have to re-bend each tab slightly to get them to stay in upon re-insertion into the housing. Make sure that there is no discoloration of each contact and that there is pressure applied to the matching contacts on the Resistor Module.

The Sqeeking noise that you hear is most likely the "Stepper Motors" that drive the Damper Doors within the Heater A/C Plenum. These are a totally different problem and not easily serviced without removal of the Plenum. If you have a Factory Service Manual and are capable of following the instructions to the letter you may be able to service it yourself. I would not recommend that you attempt it as it is very time consuming and it is possible to damage internal components which would cost more to have it re-done professionally.

Blower control modular,