Second part of a multi-part repair post on GM truck blower systems. The blower resistor and the blower relay are located on top of the heater case. Behind the glove box.
After opening the glove box, there may be two retention systems present. Sometimes there are plastic cables that are attached to the sides of the glove box. There will be a small latch that has to be lifted so that the cable can slide out of the slot in the glove compartment. Next there will be the plastic and rubber bump stop on the right rear corner of the glove box. That bump stop has to be flexed inward to allow the glove box to drop down fully. Once the glove box is fully down it will slide off of the hinge pins.
Okay with the glove box out of the way you can see the relay and the blower resistor locations.
A closer look shows the relay to the left and the resistor harness connector to the right. Testing at the relay is as follows.
- Red wire, 12 volt battery constant power supplied from A/C (maxi fuse) in the underhood fuse box.
- Black wire, constant ground supply from location at the rear of the passenger side cylinder head assembly under the hood.
- Purple wire, power supply from blower relay to the blower motor.
- Dark Blue wire, switched power supply from the blower switch, through the blower resistor. The dark blue wire and the purple wire contacts inside the relay are connected to each other at all times, other than high blower operation.
- Orange wire, switched power from the blower switch in the high position only. Power on this wire energizes the relay coil which in turn closes the contacts between the red and purple wires. Giving full battery power to the blower motor.
Testing at the resistor is as follows.
- Yellow wire, switched power from the blower switch, on the Low speed position, to the blower resistor where it travels through the three resistor coils and out on the dark blue wire.
- Tan wire, switched power from the blower switch, on the M1 position, to the blower resistor where it travels through two resistor coils and out on the dark blue wire.
- Light blue wire, switched power from the blower switch on the M2 position, to the blower resistor where it travels through one resistor coil and out on the dark blue wire.
- Dark blue wire, variable power, from the blower resistor to the blower relay. While at rest the contacts between the dark blue wire and the purple wire are connected inside the relay. The varying power sent on the dark blue wire, goes through the relay and comes out of the relay, on the purple wire. This signal controls all of the speeds except for high.
This connector located at the right side of the glove box opening frequently burns out. When it is damaged power on the red wire or ground on the black wire will be lost. many times both wires will be burnt.
The two wires that go to the blower motor. During testing the purple wire will have varying voltage depending on the speed selected. The black wire will have ground at all times. If power is found on the black wire, while it is connected to the blower motor, it will be due to power bleeding through the motor windings from the purple wire and a Missing or Loose Ground. The common locations for the ground problem are at the rear of the passenger side cylinder head and at the connector shown above.
The ground connection at the blower motor.
This rubber cover has to be removed in order to replace the blower motor.
The orange piece in the center of the picture is the power connection to the blower motor. The color may vary.
The harness where the black wire travels through the firewall to be attached to the rear of the passenger cylinder head, under the hood.