How To Test The Blower Motor Resistor (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

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.

You can find this tutorial in Spanish here: Prueba: Resistencia Del Motor Del Soplador (2.8L Chevy S10/GMC S15) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.8L Chevrolet S10 Pickup: 1988, 1989, 1990, 1991, 1992, 1993.
• 2.8L GMC S15 Pickup: 1988, 1989, 1990.
• 2.8L GMC Sonoma: 1991, 1992, 1993.

This tutorial also applies to the 2.8L Chevy S10 Blazer, 2.8L GMC S15 Jimmy. See the 'Applies To' box on the left column (desktop) or at the bottom of the page (mobile device) for more info.

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

• How To Test The Blower Motor (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How to Test The Blower Speed Switch (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• Blower Motor Relay Test (1991-1993 2.8L Chevy S10/ GMC S15).
• 1991-1993 2.8L Chevy S10 Blower Motor Circuit Diagram.

Symptoms Of A Bad Blower Motor Resistor

A failed blower motor resistor assembly in the 1988-1993 Chevrolet S10 pickup can cause several symptoms, including:

• Reduced or no airflow from the vents: If the blower motor resistor fails, it may cause the airflow from the vents to reduce or stop completely.
• Only high (HI) speed works: The blower motor resistor regulates the speed of the blower motor.
• Burning smell: A failed blower motor resistor can produce a burning smell.
• No or intermittent operation: The blower motor resistor may also cause no or intermittent operation of the blower motor.

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:

• LO.
• M1 (medium 1).
• M2 (medium 2).
• 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. M2 (medium 2) blower speed is achieved when the current passes through only one resistor.
2. M1 (medium 1) blower speed is achieved when the current passes through two resistors.
3. LO blower speed is achieved when the current passes through all three resistors.

What diverts the current to the different resistors? It's the blower motor speed 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.

Blower Motor Resistor Circuit Descriptions

As you can see in the illustrations of the blower motor resistor assembly and its electrical connector, 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

The following typical circuit diagram of the blower motor circuits may come in handy: 1991-1993 2.8L Chevy S10 Blower Motor Circuit Diagram.

Where To Buy The Blower Resistor And Save

Did you know that you can still buy the original AC Delco blower motor resistor and that it's not that much more expensive? The following links will help you comparison shop for the AC Delco (and aftermarket) blower motor resistor:

Not sure if the above blower motor resistor assembly fits your particular S10/S15? Don't worry, once you get to the site, they'll check by asking you the specifics of your vehicle. If it doesn't fit, they'll find you the right one.

TEST 1: Checking The Blower Resistor's Resistances

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.

2. 2

   Set your multimeter to Ohms (Ω) mode.
   
   Don't have a multimeter or need to upgrade yours? Check out my recommendation: Buying A Digital Multimeter For Automotive Diagnostic Testing.

3. 3

   Check continuity between terminal D and C.
   
   Continuity should exist. To be a bit more specific you'll see around 2.5 Ohms.

4. 4

   Check continuity between terminal C and B.
   
   Continuity should exist. Specifically, your multimeter should report around 0.6 Ohms.

5. 5

   Check continuity between terminal B and A.
   
   Continuity should exist. Specifically, your multimeter should report around 0.3 Ohms.

6. 6

   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).

Your next step is to go to: TEST 2: Verifying Blower Speed Switch Voltage Inputs.

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.