TEST 2: Verifying Blower Speed Switch Voltage Inputs
As you already know, the blower motor connector has four wires. Three of those four wires are battery power input wires.
The fourth wire, a dark blue (DK BLU) one, carries the reduced current to the blower motor relay and ultimately to the blower motor.
The blower control switch determines the desired blower motor speed by redirecting battery power to the corresponding blower motor resistor terminal.
In this test, we'll select a specific blower motor speed and check if the corresponding blower resistor terminal has battery power (10 to 12 Volts) present using a multimeter in Volts DC mode.
OK, these are the test steps:
- 1
Disconnect the blower resistor from its harness connector.
- 2
Set your multimeter to Volts DC mode.
- 3
Connect the black multimeter test lead to the battery negative (-) post.
- 4
Turn the key to the ON position but don't crank or start the engine.
- 5
Set the AC-heater control panel lever to VENT.
- 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
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
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 the voltage is missing in the indicated test steps and it matches the speed of the blower fan that isn't working on your Chevy S10/GMC S15 pickup, you can generally conclude the blower motor speed switch is faulty.
The following tutorial will help you test the blower motor speed switch:
CASE 2: All 3 terminals had 12 Volts where indicated in the test steps. This is good and is the correct and expected test result that tells you that the blower fan speed switch (in the A/C-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
If you've reached this section, you have confirmed blower speed switch is sending battery power to the correct resistor connector terminals.
In this final test, we'll temporarily bypass the blower resistor by connecting a jumper wire to specific terminals of the blower resistor connector.
The purpose of bypassing the resistor assembly with a jumper wire is to check the integrity of the electrical circuits between the resistor and the 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
Disconnect the blower resistor from its harness connector.
- 2
Turn the key to the ON position but don't crank or start the engine.
- 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
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
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
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 speed switch (on the A/C-Heater control panel) and the blower resistor as bad.
More 2.8L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials
You can find a complete list of tutorials for the 2.8L V6 Chevrolet S10 pickup, GMC S15 pickup, and GMC Sonoma in this index:
Here's a sample of the articles, you'll find in the Index of Articles:
- How To Test The Fuel Pump (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Test Engine Compression (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Test The EGR Valve (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Test The Starter Motor (1988-1993 2.8L V6 Chevrolet S10 Pickup, GMC S15 Pickup).
If this info saved the day, buy me a beer!
