Testing the blower motor resistor isn't difficult and in this tutorial I'll show you how to. With your test results, you'll quickly determine if the blower motor resistor is good or bad.
IMPORTANT: This tutorial applies to the 2.5L Chevy S10 pickup (GMC S15 pickup, GMC Sonoma) with air condition. The blower resistor for the no-AC pickups is different.
Contents of this tutorial:
- Basics Of Troubleshooting The Blower Motor Resistor.
- Where To Buy The Blower Resistor And Save.
- TEST 1: Checking The Blower Resistor's Resistances.
- TEST 2: Verifying Blower Speed Switch Voltage Inputs.
- TEST 3: Using A Jumper Wire To Bypass The Blower Resistor.
- More 2.5L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials.
You can find this tutorial in Spanish here: Cómo Probar El Conjunto De Resistencias Del Motor Del Soplador (1987-1993 2.5L Chevy S10 Pickup, GMC S15 Pickup, GMC Sonoma) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 2.5L Chevrolet S10 Pickup: 1987, 1988, 1989, 1990, 1991, 1992, 1993.
- 2.5L GMC S15 Pickup: 1987, 1988, 1989, 1990.
- 2.5L GMC Sonoma: 1991, 1992, 1993.
The following tutorials will help you test the blower motor and blower motor speed switch:
- How To Test The Blower Motor (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- Blower Motor Speed Switch Tests (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
Basics Of Troubleshooting The Blower Motor Resistor
The AC-heater control panel gives you the option of 4 blower motor speeds. These are:
- LO.
- M1 (medium 1).
- M2 (medium 2).
- HI.
To achieve speeds M2 through LO, the blower motor resistor reduces or increases the amount of current that reaches the blower motor. Specifically:
- The higher the current that the blower motor resistor allows through to the blower motor, the higher the speed of the blower motor.
- The less current that the blower motor resistor allows through to the blower motor, the lower the blower motor speed.
The blower resistor accomplishes this current reduction through its three series-connected individual resistors. In short, this is how the fan speed is controlled:
- If current flows through only one resistor, you get M2 fan speed.
- If current flows through two resistors, you get M1 fan speed.
- If current flows through three resistors, you get LO fan 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.
Where To Buy The Blower Resistor And Save
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
We can easily check the individual resistors that make up the resistor assembly with a multimeter in Ohms mode.
The figure above of the blower motor resistance terminals will help us perform the resistance tests.
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 must ensure that the blower motor is working before beginning the blower resistance tests outlined in this tutorial. Testing the blower motor is easy and you can find the tutorial here: How To Test The Blower Motor (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
OK, these are the test steps:
- 1
Disconnect the blower resistor from its harness connector.
- 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
Check continuity between terminal D and C.
Continuity should exist. To be a bit more specific you'll see around 2.5 Ohms. - 4
Check continuity between terminal C and B.
Continuity should exist. Specifically, your multimeter should report around 0.6 Ohms. - 5
Check continuity between terminal B and A.
Continuity should exist. Specifically, your multimeter should report around 0.3 Ohms. - 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 itself 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.
