TEST 2: Bypassing the Blower Control Module
Now that you have checked and confirmed that the blower motor is getting power and ground... the next step is to make sure that the blower motor runs and that's what we'll do in this test step (before jumping to TEST 3).
We can accomplish this by either by applying battery voltage directly to the blower motor or by jumpering together two specific terminals of the blower control module connectors.
In this test step we'll jumper these two terminals. The factory manual suggests that when you make your jumper wire... you include an inline fuse as a safety precaution.
IMPORTANT: Use a small diameter wire as a jumper wire or you run the risk of damaging the blower control module connectors' female metal terminals. If you use a too thick jumper wire, you may permanently open these terminals up and create an intermittent ‘open’ problem.
NOTE: If the blower motor on your vehicle is stuck on High... you can skip this test step and go to TEST 3. Having the blower motor stuck on High whether the key is on or not confirms that the blower motor runs.
Alright, these are the test steps:
Disconnect the blower control module from its harness connector and turn the key to the on position (but no need to start the engine).
NOTE: All of the tests in this section are done on the blower control module's harness connectors and not on the blower control module itself. The illustrations in the image viewer show the front view of the female metal terminals of the small and large blower module connector.
Jumper terminal B (of the small connector) to terminal A of the large connector.
The blower motor should run at full speed.
Let's take a look at what your test results mean:
CASE 1: The blower motor DID NOT run when you jumpered the indicated terminals. This tells you that that specific circuit is bad or that the blower motor is bad.
You'll need to apply battery voltage and ground to the blower motor directly to see if it runs. If the blower motor does not run, the blower motor is bad and needs to be replaced.
CASE 2: The blower motor ran when the indicated terminals were jumpered together. So far so good since this test result tells you that the blower motor is OK and not the cause of the problem.
Your next and last step is to verify that the blower control module is getting a blower fan speed signal from the A/C-Heater control panel. For this test, go to: TEST 3: Verifying The Blower Speed Voltage Signal.
TEST 3: Verifying The Blower Speed Voltage Signal
If you've reached this point, you have:
One: Confirmed that the blower control module is getting power and ground (TEST 1).
Two: That the blower motor runs (TEST 2).
Now, we're gonna' check to see if the blower control module is getting an activation signal (blower speed signal) from the A/C-Heater control panel.
Without this blower speed signal, the blower control module won't activate the blower motor.
To go into more details about this test, we want to make sure that whenever you increase or decrease the blower fan speed (on the panel) there is a corresponding voltage signal reaching the blower control module.
This is a pretty easy test that you can accomplish with your multimeter in volts DC mode.
NOTE: This voltage test is done on the blower control module connector and NOT on the module itself.
OK, these are the test steps:
Disconnect the blower control module from its electrical connectors. Set your multimeter to Volt DC mode.
Don't have a multimeter or need to upgrade yours? Check out my recommendation: Buying A Digital Multimeter For Automotive Diagnostic Testing (found at: easyautodiagnostics.com).
Probe the terminal labeled with the letter B (of the 4 terminal connector) with the red multimeter lead (see illustration above).
Ground the black multimeter lead on a good ground point (or directly on the battery negative terminal).
Have a helper turn the key on and increase/decrease the blower speed on the A/C-Heater control panel.
Your job is to see the voltage your multimeter registers.
The voltage should increase/decrease between 2 to 10 volts DC as your helper increases/decreases the blower speed from inside the vehicle.
NOTE: The voltage on your particular vehicle may not go all the way down to 2 volts and may not go all the way up to 10 volts... But if the signal is present you will see a varying voltage moving up and down and this will be enough to confirm the signal is present.
Let's take a look at what your test results mean:
CASE 1: The voltage increased/decreased as your helper changed the blower speed. This is the correct and expected test result and lets you know that the blower control module is getting the blower speed signal from the AC heater control panel.
This test result also tells you that the blower control module is bad and needs to be replaced because it's not responding to the A/C Heater control panel's command to start the blower motor. I'll explain in more detail: You have already:
- Confirmed that the blower control module is getting power and ground (TEST 1).
- Confirmed that the blower runs when the blower control module is bypassed (TEST 2).
- Confirmed that the A/C-Heater control panel is sending a blower speed signal (this test section).
Taking into account all of these specific test results, you can conclude with confidence that the blower control module is bad and needs to be replaced to solve the blower motor issue. NOTE: If your blower motor was stuck in HIGH, then this test result also confirms that the blower control module is bad and needs to be replaced.
If you'd like to save a few bucks on the blower control module, check out the section: Where To Buy The Blower Control Module and Save.
CASE 2: The voltage DID NOT increase/decrease as your helper changed the blower speed. Double check that you' re testing the correct terminal on the blower control module connector and repeat the test.
If the multimeter does not show an increase/decrease in the voltage signal (as your helper changes the blower speed from the A/C-Heater control panel)... then this test result eliminates the blower control module as being defective.
Why? Because without this varying voltage, from the A/C-Heater control panel, the blower control module won't activate the blower motor.
Although it's beyond the scope of this tutorial to test the A/C-Heater control panel or the circuits between the panel and the blower control module, my suggestion to you is to get a wiring diagram of your particular GM vehicle and check the continuity between the blower control module connector and the heater control panel connector for opens or shorts.
Where To Buy The Blower Control Module And Save
The blower control module is a pretty expensive part that usually runs over US$150. There's a good chance that you can buy it online for a whole lot cheaper than somewhere local.
The following links will help you comparison shop for the blower control module and the blower motor:
More 3.8L GM Tutorials
If you enjoyed and/or found this ‘How to’ article helpful... I have written several more 3.8L specific test articles. You can find a complete list of 3.8L tutorials here: GM 3.8L Index of Articles.
- How To Test The Fuel Injectors (GM 3.8L).
- How To Do A Fuel Injector Noid Light Test (GM 3.8L)
- How To Test The 3.8L GM Crank Sensor With A Multimeter.
- How To Test The Cam Sensor -P0341 (GM 3.8L).
- How To Troubleshoot A No Start (GM 3.8L).
- GM 3.8L Ignition Control Module and Crank (3X, 18X) Sensor Test (at: easyautodiagnostics.com).