Blower Motor Speed Switch Tests (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

You can easily test the blower motor speed control switch with a few tests performed with a multimeter.

In this tutorial, I'll show you step-by-step how to do it. With your test results, you'll quickly determine if the blower speed switch is good or bad.

IMPORTANT: This tutorial applies to 2.5L Chevrolet S10 pickup (GMC S15 pickup, GMC Sonoma) equipped with AC.

You can find this tutorial in Spanish here: Cómo Probar El Interruptor De Velocidades 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 may also come in handy (and compliment this one):

1. How To Test The Blower Motor Resistor (1988-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
2. How To Test The Blower Motor (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
3. 1991-1993 2.8L Chevy S10 Blower Motor Circuit Diagram.

Basics Of Troubleshooting The Blower Control Switch

Depending on the blower speed you select (LO, M1, M2, or HI), the blower speed switch channels the current it receives to the blower motor resistor assembly or to the blower motor relay.

To be even more specific:

• Moving the blower speed selector causes the blower switch to mechanically open and close a set of contacts within its assembly.
• This opening/closing of the contacts channels the incoming 12 Volts from the brown (BRN) wire to any of the other three wires of the blower speed switch connector.

Here's a further breakdown of how the blower switch does what it does:

• When the blower switch lever is in the LO position:
  • Battery power is available in terminal D of the blower switch connector.
  • The blower switch does not channel this current anywhere.
  • The blower motor runs in LO speed.
• When you move the blower switch lever to the M1 position:
  • Battery power is available in terminal D of the blower switch connector.
  • The blower switch closes circuits D and A and channels the incoming battery power to the TAN wire of the blower switch connector.
  • The blower motor now runs in M1 speed.
• When you move the blower switch lever to the M2 position:
  • Battery power is available in terminal D of the blower switch connector.
  • The blower switch opens circuits D and A.
  • The blower switch closes circuits D and C and channels the incoming battery power to the LT BLU wire of the blower switch connector.
  • The blower motor now runs in M2 speed.
• When you move the blower switch lever to the HI position:
  • Battery power is available in terminal D of the blower switch connector.
  • The blower switch opens circuits D and C.
  • The blower switch closes circuits D and B and channels the incoming battery power to the ORG wire of the blower switch connector.
  • The blower motor now runs in HI speed.

The following circuit diagram may come in handy:

• 1991-1993 2.8L Chevy S10 Blower Motor Circuit Diagram.

Over time and use, the electrical arcing that occurs when the mechanical contacts (inside the switch) open and close will pit and cover them with carbon.

These pits and carbon deposits will eventually cause one or more fan speeds to stop working.

Blower Motor Speed Switch Circuit Descriptions

Blower Switch Circuit Descriptions
Pin	Wire Color	Description
A	Tan (TAN)	Mi Speed.
B	Orange (ORG)	HI Speed.
C	Light blue (LT BLU)	M2 Speed.
D	Brown (BRN)	12 Volts (input).

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 tutorial.

Where To Buy The Blower Motor Speed Switch And Save

The following links will help you comparison shop for the blower motor control switch:

Not sure if the above blower switch fits your particular Chevy S10 pickup (GMC S15 pickup, GMC Sonoma)? Don't worry. Once you get to the site they'll make sure it does fit or they'll find you the right one.

TEST 1: Checking Continuity Of The Blower Motor Speed Switch

To start our blower speed switch diagnostic, we'll perform a couple of continuity tests with your multimeter in Ohms (Ω) mode.

You'll need to unbolt and pull the AC/Heater Control Panel (but not completely remove it) from its location on the instrument panel to access the blower switch terminals (see the photo 2 of 2 above).

OK, let's get started:

1. 1

   Unbolt and pull the A/C-Heater control panel from the dash.
   
   You'll only need to pull it out far enough to access the blower motor switch and unplug it from its connector.

2. 2

   Set your multimeter to Continuity test mode.

3. 3

   Disconnect the blower switch from its electrical connector.
   
   NOTE: All the continuity tests in this test section are done on the male spade terminals of the switch itself.

4. 4

   Set the blower switch lever to the M1 position.
   
   Check the continuity between terminals D and A.
   
   Your multimeter should register continuity.

5. 5

   Set the blower switch lever to the M2 position.
   
   Check the continuity between terminals D and C.
   
   Your multimeter should register continuity.

6. 6

   With the blower switch knob set to the HI position.
   
   Check the continuity between terminals D and B.
   
   Your multimeter should register continuity.

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

CASE 1: One or more of the circuits tested had NO continuity in the indicated blower speed switch lever position. Repeat the tests just to make sure of your test results.

If there is no continuity as indicated in the test steps above and this lack of continuity coincides with the fan speed not working on your S10/S15 pickup, then you have found the source of the problem.

Replace the blower motor speed switch in the A/C-Heater control panel with a new one to solve the problem. To comparison shop for the blower control switch, take a look at this section here: Where To Buy The Blower Motor Speed Switch And Save.

CASE 2: All circuits tested had continuity where indicated in the test steps. This is the correct and expected test result and tells you that the blower motor speed switch (in the A/C-Heater control panel) is OK.

Your next step is to make sure the blower motor speed switch is getting power. For this test go to: TEST 2: Making Sure The Blower Motor Speed Switch Is Getting Power.