TEST 2: Making Sure That The Blower Relay Activation Signal Is Present

How To Test The Blower Motor Relay (1991, 1992, 1993 2.8L Chevy S10/ GMC S15)

When you've got the A/C (or heater) running and you select the HI blower speed, the blower relay gets a voltage signal on terminal C that activates the relay.

Once the relay is activated, it channels full battery current from terminal E to B.

Since terminal B connects directly to the blower motor (via the PPL wire), the blower motor now gets full battery current and runs at its maximum speed.

In this test section we're gonna' make sure that the blower relay is getting the blower HI activation signal.

Let's get started:

  1. 1

    Disconnect the blower motor relay from its electrical connector.

  2. 2

    Place your multimeter in Volts DC mode.

  3. 3

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

  4. 4

    Place the blower speed selector in HI.

  5. 5

    Turn the key on but don't start the engine.

  6. 6

    Terminal C of the blower relay should have 10 to 12 Volts DC. Here's how to find out:

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

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

Let's examine your test results:

CASE 1: 10 to 12 Volts are present at terminal C (in other words the activation signal is present). This is the correct and expected test result.

Your next step is to go to: TEST 3: 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 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 turn 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:

  1. The vent select switch is defective.
  2. The blower speed switch is defective.
  3. 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.

TEST 3: Jumpering Connector Terminals B And E (HI Speed Circuit)

How To Test The Blower Motor Relay (1991, 1992, 1993 2.8L Chevy S10/ GMC S15)

If you've reached this point, you have made sure that:

  1. The blower relay is getting power and Ground (TEST 1).
  2. The blower relay is getting its activation signal on terminal C of the relay connector (TEST 2).

Now we're gonna' bypass the blower motor relay and activate the blower motor in HI speed.

This test simply involves jumpering together blower relay terminals B and E to get the blower motor to run in HI speed.

IMPORTANT: Before testing the blower motor relay you must first make sure that the blower motor itself is OK. This tutorial will help you test the blower motor (if you haven't already): How To Test The Blower Motor (2.8L Chevy S10/ GMC S15).

WIRING DIAGRAM: You can find 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

    Jumper terminals B and E of the blower relay connector together using a jumper wire (see illustration above to identify the terminals).

    Terminal B has a purple (PPL) wire connected to it. Terminal E has a red (RED) wire connected to it.

  3. 3

    The blower motor should run in HI speed as soon as you jumper both female terminals of the connector.

Let's examine your test result:

CASE 1: The blower motor ran in HI speed. This is the correct and expected test result.

If the blower motor does NOT work in HI (with the relay connected to its connector), but does work when you jumper terminal B and E, then you can conclude that the blower relay is defective and needs to be replaced.

If the blower motor does not work in LO, M1, and M2 but does work in HI (with the relay connected to its connector), then your next step is to go to: TEST 4: Jumpering Connector Terminals B and D (LO, M1, M2 Speed Circuit)

CASE 2: The blower motor DID NOT run in HI speed. This usually means that the purple (PPL) wire between the relay connector and the blower motor has a problem.

Your next step is to check the continuity of the purple (PPL) wire. If the PPL wire does not have continuity, then you've found the problem that's causing blower HI speed to not work. Repair the open in the PPL wire.