How To Test The Blower Motor Resistor (GM 4.3L, 5.0L, 5.7L)

When the blower motor resistor fails on your GMC pickup or SUV, the blower motor will only work in one speed and in a few cases not at all.

Thankfully, testing the blower motor resistor is not that hard and in this tutorial I'll show you 4 different tests that will help you troubleshoot it and find out if its bad (or not).

This tutorial applies to several Chevrolet, GMC and Oldsmobile pickups and SUVs. To find out if your particular vehicle is covered by this repair tutorial, check the box titled “Applies To:” on the right column.

This tutorial compliments this one on testing the blower motor resistor:

  1. How To Test The Blower Control Switch (GM 4.3L, 5.0L, 5.7L).
  2. How To Test The Blower Motor (GM 4.3L, 5.0L, 5.7L).

Basics Of Troubleshooting The Blower Motor Resistor

As you're already aware, your pickup's heater control panel gives you the option of 4 different blower motor speeds.

These speeds are:

  1. LO.
  2. M1 (medium 1).
  3. M2 (medium 2).
  4. HI.

The reduction in blower fan speed is accomplished thru' resistors (inside the blower resistor assembly). ‘HI’ is the only blower speed that doesn't depend on the blower resistor to function.

When the blower motor resistor fails, 1 of the 3 or all 3 first speeds stop working. To explain this a bit further, this is due to the nature of how a resistor reduces the voltage that passes thru' it by converting some of it into heat. This process causes these resistors to get extremely hot.

Over time and use, these resistors burn up. When this happens, one or all three of the first speeds stop working and you can (usually) remove the blower resistor and see one (or several) of the resistor coils have burned and are ‘open’.

Unfortunately, sometimes the individual coils look OK, but are not letting the current pass on to the blower motor. Even when this happens, troubleshooting the blower resistor assembly is still not hard and I'll show you how in this tutorial.

The blower motor resistor's harness connector has 4 wires coming out of it. These 4 wires supply it with specific signals and in the following chart you'll find a brief description of each:

Blower Resistor Circuit Descriptions
Pin Wire Color Description
A YEL LO Input from AC-Heater Panel
B DK BLU Output to Blower Relay
C TAN M1 Input from AC-Heater Panel
D LT BLU M2 Input from AC-Heater Panel

NOTE: There's a good chance that the color of the wires described above DOES NOT match what's on your particular vehicle. This is no cause for concern since the circuit descriptions are the same for all of the GM vehicles covered by this repair tutorial.

TEST 1: Verifying Blower Speed Switch Voltage Inputs

How To Test The Blower Motor Resistor (GM 4.3L, 5.0L, 5.7L) How To Test The Blower Motor Resistor (GM 4.3L, 5.0L, 5.7L)

The very first thing we need to do is make sure that battery power is present in each blower fan speed selection.

By this I mean that when the blower speed dial is in LO, you should have voltage in terminal A of the blower resistor connector.

Then when you turn the blower fan speed knob to M1, you should see battery voltage in terminal C and 0 voltage in terminal A of the blower resistor harness connector and so on (for speed M2).

If battery voltage IS NOT present where indicated in the steps below... then the A/C-Heater control panel has a problem.

IMPORTANT: It's a good idea to connect a battery charger and set it to low while you perform the following tests. This will avoid having your pickup's (or SUV's) battery getting low on charge.

OK, these are the test steps:

  1. 1

    Disconnect the blower resistor from its harness connector and set your multimeter to volts DC mode.

    Don't have a multimeter or need to upgrade yours? Check out my recommendation: Abe's Multimeter Recommendation (found at: easyautodiagnostics.com).

  2. 2

    Turn the key to the on position and set the blower speed knob to LO.

    Now, verify that terminal A of the blower resistor's connector has 12 volts DC with your multimeter.

  3. 3

    With the key in the on position and the blower speed knob to M1.

    Now, verify that terminal C of the blower resistor's connector has 12 volts DC with your multimeter.

    NOTE: Speed M1 is the blower speed right above LO.

  4. 4

    With the key in the on position and the blower speed knob to M2.

    Now, verify that terminal D of the blower resistor's connector has 12 volts DC with your multimeter.

    NOTE: Speed M2 is the blower speed right below HI.

Let's take a look at what your test results mean:


CASE 1: One or more of the terminals DID NOT have 12 volts when the blower fan speed knob was turned. Repeat the tests just to make sure of your test results.

If voltage is not present where indicated in the test steps above and this missing voltage coincides with the blower fan speed that's not working on your pickup or SUV, then you've found the cause of the problem.

Replace the blower motor speed switch in the A/C-Heater control panel with a new one to solve the problem.

If you want to test the blower control (speed) switch, the following tutorial will show you how:

  1. How To Test The Blower Control Switch (GM 4.3L, 5.0L, 5.7L).

CASE 2: All 3 terminals had 12 volts where indicated in the test steps. This is good and is the correct and expected test result that tells you that the blower fan switch (in the A/C-Heater control panel) is OK.

Your next step is to bypass the blower resistor using a simple jumper wire. For this test go to: TEST 2: Using A Jumper Wire To Bypass The Blower Resistor.