How to Test the Blower Motor Resistor (2.8L Chevy S10/ GMC S15)

The blower motor resistor can be easily tested with a set of simple multimeter resistance tests (especially since it's very accessible).

So, if you suspect that the blower motor resistor, in your 2.8L Chevy S10 (GMC S15), is bad... I'll show you how to test it in this tutorial.

Contents of this tutorial at a quick glance:

  1. Basics of Troubleshooting the Blower Motor Resistor.
  2. TEST 1: Checking the Blower Resistor's Resistances.
  3. TEST 2: Verifying Blower Speed Switch Voltage Inputs.
  4. TEST 3: Using a Jumper Wire to Bypass the Blower Resistor.
  5. Where to Buy the Blower Resistor and Save.
  6. More GM 2.8L Tutorials.

En Español Puedes encontrar este tutorial en Español aquí: Prueba: Resistencia del Motor del Soplador (2.8L Chevy S10/GMC S15) (en: autotecnico-online.com).

The following two tutorials on how to test the blower motor and the blower motor switch complimente this one:

  1. How to Test the Blower Motor (2.8L Chevy S10/ GMC S15).
  2. How to Test the Blower Control Switch (2.8L Chevy S10/ GMC S15).

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.

It's the blower motor resistor's job to reduce (or increase) the blower motor speed by reducing the amount of current that reaches the blower motor.

This is accomplished by the fact that the blower resistor assembly is made up of three individual resistors wired in series. In a nutshell, this is how blower speed is controlled:

  1. When current passes thru' only one resistor... you get M2 (medium 2) blower speed.
  2. When current passes thru' two resistor's... you get M1 (medium 1) blower speed.
  3. When current passes thru' all three resistor's... you end up with LO blower speed.

What diverts the current to the different resistors? It's the blower switch (specifically: the blower speed position you have set it to).

NOTE: The only blower motor speed that does not depend on the blower resistor assembly is HI speed.

As you can see in the photo at the beginning of this tutorial, the blower motor resistor's harness connector has 4 wires coming out of it. These 4 wires supply it with specific signals. Here's a brief description of each:

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

There are two basic ways of testing the blower motor resistor assembly. One is to do a resistance test of the assembly. The other is to bypass the resistor and indirectly confirm it's defective.

TEST 1: Checking the Blower Resistor's Resistances

How to Test the Blower Motor Resistor (2.8L Chevy S10/ GMC S15)

The first thing we'll do is to check continuity of each resistor that makes up the blower resistor assembly. We'll do this with a multimeter in Ohms mode.

The illustration above will help you identify the blower resistor terminals you'll be testing in the test step below.

NOTE: Remember, the only speed that is not controlled by the blower resistor is HI speed. So, if your Chevy S10 (GMC S15) has a ‘no HI blower speed’ problem... the blower resistor is not behind the issue.

IMPORTANT: You need to make sure the blower motor works before you start the blower resistor tests outlined in this tutorial. Testing the blower motor is easy and you can find the tutorial here: How to Test the Blower Motor (2.8L Chevy S10/ GMC S15)

OK, these are the test steps:

  1. 1

    Disconnect the blower resistor from its harness connector and set your multimeter to Ohms () mode.

    Don't have a multimeter or need to upgrade yours? Check out my recommendation: Buying a Digital Multimeter for Automotive Diagnostic Testing (found at: easyautodiagnostics.com).

  2. 2

    Check continuity between terminal D and C.

    Continuity should exist. To be a bit more specific you'll see around 2.5 Ohms.

  3. 3

    Check continuity between terminal C and B.

    Continuity should exist. Specifically, your multimeter should report around .6 Ohms.

  4. 4

    Check continuity between terminal B and A.

    Continuity should exist. Specifically, your multimeter should report around .3 Ohms.

  5. 5

    Check continuity between terminal D and B. Then terminal D and A.

    Continuity should exist. Specifically, between terminal D and B you should see around 2.9 Ohms. Between terminal D and A you should see about 3.1 Ohms.

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

CASE 1: Continuity exists between all indicated terminal in the test above. This test result tells you that the blower motor resistor is OK (not defective).

My suggestion to you (if you're having a blower motor speed issue) is to do the performance tests found in TEST 1 and TEST 2 in the next page.

CASE 2: Continuity DID NOT exist between two of the indicated terminals in the test above. This test result tells you that the blower resistor is defective and needs to be replaced.