If the blower motor in your 3.8L GM vehicle is stuck in only one speed (usually stuck in high) or no blower speeds work... then there's a good chance that the blower control module is bad and behind the problem.
Although the blower control module can not be tested with simple multimeter resistance tests... it can be tested and finding out if it's bad (or not) is not hard.
In this tutorial, I'll show you how to get it done in a step-by-step way.
NOTE: Even though this tutorial is filed in the GM 3.8L index of this website... this tutorial applies to some 1992 and 1993 Cadillac vehicles. This is because this particular blower control module was used in several models from 1992 to 1999. To find out if your particular GM vehicle is covered by this troubleshooting tutorial, check the box titled “Applies To:” on the right column.
To aid you in further knowing if this tutorial applies to your Buick, Cadillac, Chevrolet, or Pontiac vehicle... this tutorial applies to the following aftermarket blower control module part numbers:
- AC Delco 158548.
- Airtex/Wells 3A1124.
- Duralast JA1543.
- Four Seasons 20219.
- Standard Motor Products RU304.
Here are the contents of this tutorial at a quick glance:
- Basics of Troubleshooting the Blower Control Module.
- TEST 1: Verifying the Blower Control Module is Getting Battery Power and Ground.
- TEST 2: Bypassing the Blower Control Module.
- TEST 3: Verifying the Blower Speed Voltage Signal.
- Where to Buy the Blower Control Module and Save.
- More 3.8L GM Tutorials.
You can find this tutorial in Spanish here: Cómo Probar el Módulo del Motor del Soplador (3.8L GM) (at: autotecnico-online.com).
Basics of Troubleshooting the Blower Control Module
The blower control module is a solid state device unlike the old style blower motor resistors that actually use resistors.
It controls blower motor speed by varying the voltage it sends to the blower motor. Depending on what blower speed is selected from the A/C-Heater control panel, this voltage can range between 0 and 12 volts.
To be a little more specific: To determine appropriate output voltage (the voltage it sends the blower motor), the blower control module monitors a varying voltage signal (between 2-10 volts) from A/C-Heater control panel (programmer) on terminal B of the 4-terminal connector.
This varying voltage signal is the blower motor speed signal that tells it what blower speed you've just selected on the A/C-Heater control panel.
The blower control module then translates this blower motor speed signal from the A/C-Heater control panel signal and applies the corresponding voltage directly to the blower motor.
|Blower Control Module 2 Pin Connector|
|B||PPL||Battery Power From Underhood Fuse Box.|
|Blower Control Module 4 Pin Connector|
|A||PPL||Variable Voltage Output to Blower Motor.|
|B||GRY/BLK||Blower Speed Input From A/C-Heater Control Panel.|
|C||BLK||Output to Chassis Ground.|
NOTE: There's a good chance that the color of the wires described above DOES NOT match what's on your particular vehicle. This is no cause for concern since the circuit descriptions are the same for all of the GM vehicles covered by this repair tutorial.
We're gonna' start our blower control module diagnostic by checking the basics: battery power and ground. We need to make sure that these two are being fed to the blower control module.
Terminal B of the small 2 terminal connector is the one that feeds battery power (12 volts) to the blower control module (see illustration above).
And terminal C of the large 4 terminal connector feeds the module with ground (see illustration above).
Checking for battery voltage and ground can be done with a simple multimeter voltage and continuity test and in the following steps I'll show you how.
Alright, these are the test steps:
Disconnect the blower control module from both of its harness connectors and turn the key to the on position (but don't 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 module itself. The illustrations in the image viewer show the front view of the female metal terminals of the connectors.
Check for battery voltage (12 V) on terminal B of the small connector with your multimeter in Volts DC mode.
This is the circuit that feeds the blower control module with power from the under-hood fuse box and should be hot all the time.
Place your multimeter in Ohms mode (Ω) and check for continuity between terminal C and the battery negative terminal.
Terminal C belongs to the larger blower control module connector.
Your multimeter should register continuity. To be a bit more specific, your resistance value should be .5 Ohms or less.
Let's take a look at what your test results mean:
CASE 1: Power and ground were present in the indicated terminals. This is the correct and expected multimeter test result and tells you that the blower control module is getting power and ground.
The next step is to make sure that the blower motor is functioning correctly. We can accomplish this by bypassing the blower control module with a jumper wire and checking to see if the blower motor runs on HI speed. For this test go to: TEST 2: Bypassing the Blower Control Module.
CASE 2: Power and/or ground were NOT present in the indicated terminals. Double check that you're testing the correct terminals and repeat the test.
If your multimeter still does not register battery power or ground in the indicated blower control module connector terminals... then you have found the problem that' s keeping the blower motor from working or functioning correctly.
If power is missing from the small connector Terminal B, then you need to check to see if the underhood fuse is blown. Replace as necessary and re-test.
If ground is missing from terminal C of the large connector, then my suggestion to you is to repair this ground by adding a ground wire to the wire connecting to this terminal and re-test.
Resolving the issues that are preventing this power and/or ground from reaching the blower control module will solve the blower motor problem your vehicle is experiencing.