Blower Motor Relay Test (1991-1993 2.8L Chevrolet S10 Pickup, GMC Sonoma)

TEST 2: Making Sure The Blower Relay Is Getting Ground

For our second test, we'll make sure the blower motor relay is getting Ground.

The connector terminal that delivers Ground to the relay is terminal A (see illustration above).

The Ground that terminal A provides is a chassis Ground and is available at all times.

WIRING DIAGRAM: You can consult the blower motor circuit diagram here: 1991-1993 2.8L Chevy S10 Blower Motor Circuit Diagram.

These are the test steps:

1. 1

   Disconnect the blower motor relay from its electrical connector.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

   Connect the red multimeter test lead on the battery positive (+) post.

4. 4

   Probe terminal A with the black multimeter test lead.
   
   NOTE: Terminal A connects to the black (BLK) wire of the blower relay connector.

5. 5

   Your multimeter should read 10 to 12 Volts.

Let's examine your test results:

CASE 1: The multimeter display 10 to 12 Volts. This is the correct and expected test result and confirms the blower relay is getting Ground.

Now that you've checked the and confirmed the presence of Ground, the next step is to see if an activation signal is present. For this test go to: TEST 3: Making Sure That The Blower Relay Activation Signal Is Present.

CASE 2: The multimeter DID NOT display 10 to 12 Volts. This test result tells you that the relay is not getting Ground.

Your next step is to repair the black (BLK) wire and restore its path to Ground.

TEST 3: Making Sure That The Blower Relay Activation Signal Is Present

When you run the A/C (or heater) and select the HI blower speed, the blower relay gets a voltage signal at terminal C.

This HI speed voltage signal activates the relay to channel the full battery current it receives from terminal E to terminal B.

Since terminal B is connected directly to the blower motor (via the purple wire), the blower motor will now receive full battery power and run at maximum speed.

In this test section, we'll ensure that the HI speed voltage signal is present at terminal C when you select HI speed in the AC Heater Control Panel.

Let's get started:

1. 1

   Disconnect the blower motor relay from its electrical connector.

2. 2

   Place the vent position selector in any vent position that is not OFF.

3. 3

   Place the blower speed selector in HI.

4. 4

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

5. 5

   Place your multimeter in Volts DC mode.

6. 6

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

7. 7

   Probe terminal C with the red multimeter test lead.
   
   NOTE: Terminal C connects to the orange (ORG) wire of the blower relay connector.

8. 8

   You multimeter should register 10 to 12 Volts DC.

Let's examine your test results:

CASE 1: 10 to 12 Volts are present at terminal C. This is the correct and expected test result and confirms the HI speed activation signal is present.

Your next step is to go to: TEST 4: Jumpering Connector Terminals B And E (HI Speed Circuit).

CASE 2: 10 to 12 Volts are not present at terminal C. This test result tells you that the blower relay is not getting the HI blower speed activation signal. Without this signal it won't channel full battery current to the blower motor.

With this test result you can also conclude that the blower relay itself IS NOT the cause of the blower problem.

In a normally operating system, the activation voltage signal is only available at terminal C when you set the vent select switch ON and the blower speed switch to HI. Since the activation signal is missing, you can conclude that the most likely cause of this missing battery power is one of following:

• The vent select switch is defective.
• The blower speed switch is defective.
• An open in the ORG wire between the blower relay connector and the blower speed switch.

Your next step is to test the vent select switch and blower switch.