How To Test The Blower Motor Resistor (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

TEST 2: Verifying Blower Speed Switch Voltage Inputs

In this test section, we'll check that the blower motor resistor is receiving the selected blower speed signals from the AC-heater control panel.

We'll check for the M2, M1, and LO blower speed signals at the blower motor resistor assembly connector.

OK, let's get started:

1. 1

   Disconnect the blower resistor from its harness connector.

2. 2

   Set your multimeter to Volts DC mode.

3. 3

   Connect the black multimeter test lead to the battery negative (-) post.

4. 4

   Turn the key to the ON position but don't crank or start the engine.

5. 5

   Set the AC-heater control panel lever to VENT.

6. 6

   Select blower motor speed LO.
   
   With the red multimeter test lead, probe terminal D of the blower resistor connector.
   
   Your multimeter should read 10 to 12 Volts DC.

7. 7

   Select blower motor speed M1.
   
   With the red multimeter test lead, probe terminal C of the blower resistor connector.
   
   Your multimeter should read 10 to 12 Volts DC.

8. 8

   Select blower motor speed M2.
   
   With the red multimeter test lead, probe terminal B of the blower resistor connector.
   
   Your multimeter should read 10 to 12 Volts DC.

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

CASE 1: One or more of the terminals DID NOT have 10 to 12 Volts when selecting blower speeds LO, M1, or M2. 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 Chevy S10 pickup (GMC S15 pickup, GMC Sonoma), then you've found the cause of the problem.

Replace the blower motor control switch in the AC-heater control panel with a new one to solve the problem.

CASE 2: All 3 terminals had 12 Volts where indicated in the test steps. This is the correct and expected test result and tells you that the blower fan switch (in the AC-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 3: Using A Jumper Wire To Bypass The Blower Resistor.

TEST 3: Using A Jumper Wire To Bypass The Blower Resistor

For our final test, we'll bypass the blower motor resistor using a jumper wire on the blower resistor connector.

The purpose of this test is to check the integrity of the circuits between the blower resistor and blower motor.

IMPORTANT: Be careful that the 'jumper' you use does not permanently open the connector terminals up and create an intermittent open-circuit problem.

Alright, these are the test steps:

1. 1

   Disconnect the blower resistor from its harness connector.

2. 2

   Turn the key to the ON position but don't crank or start the engine.

3. 3

   Place the AC-heater control panel lever in the VENT position.
   
   NOTE: It's a good idea to hook up a battery charger to your pickup's battery while performing the tests and avoid discharging the battery.

4. 4

   Jumper terminals D and A together (this is the circuit that controls blower speed LO).
   
   Place the blower control switch lever in the LO speed position.
   
   The blower motor should run on high speed (it won't run on LO speed because you're bypassing the resistor).

5. 5

   Jumper terminals C and A together (this is the circuit that controls blower speed M1).
   
   Place the blower control switch lever in the M1 speed position.
   
   The blower motor should run on high speed (it won't run on M1 speed because you're bypassing the resistor).

6. 6

   Jumper terminals B and A together (this is the circuit that controls blower speed M2).
   
   Place the blower control switch lever in the M2 speed position.
   
   The blower motor should run on high speed (it won't run on M2 speed because you're bypassing the resistor).

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

CASE 1: The blower motor ran when all of the indicated terminals were jumpered together. This tells you several important things:

• That the circuits between the resistor connector and the blower motor are OK.
• That the blower motor relay is OK.

CASE 2: The blower motor DID NOT run when you jumpered the terminals in the steps indicated above. This usually tells you that the blower motor relay is bad or that you have a problem in the wiring (between the connector and the relay or the relay and the blower motor).

Although it's beyond the scope of this tutorial to test these circuits, you have eliminated the blower switch (on the A/C-Heater control panel) and the blower resistor as bad.

More 2.5L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials

You can find a complete list of 2.5L Chevy S10 pickup, GMC S15 pickup, And GMC Sonoma tutorials here:

• S10, S15, Sonoma 2.5L Index of Articles.

Here's a sample of the tutorials you'll find in the index:

• How To Test Engine Compression (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
• How To Test For A Blown Head Gasket (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
• How To Test The TPS (1987-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).
• How To Test The MAP Sensor (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma).