TEST 2: Testing The Power Transistor's Diode
If you've reached this point you have confirmed that the blower motor power transistor's resistance on pins 3 and 4 is within specification (TEST 1).
In this test section we're going to test the diode that's in the blower motor power transistor. For this test, we're gonna' use a multimeter with diode testing capability.
If the diode multimeter test DOES NOT show a forward voltage drop, then you can correctly conclude that the blower motor power transistor is fried and needs to be replaced.
This is what you'll need to do:
- 1
Set your multimeter's dial to the diode test function (see photo 2 of 2 in the image viewer).
If you don't have a multimeter or need to upgrade yours, check out my recommendation here: Buying A Digital Multimeter For Automotive Diagnostic Testing (at: easyautodiagnostics.com) - 2
With the red multimeter test lead, probe the power transistor terminal labeled with the number 2 in the image viewer.
- 3
With the black multimeter test lead, probe the power transistor terminal labeled with the number 4 in the image viewer.
- 4
Your multimeter should register a voltage (known in tech speak as a forward voltage drop). This voltage will be less than 0.7 Volts and will usually hover around 0.4 Volts if the blower motor power transistor diode is good.
If the multimeter registers the letters OL (open loop), then the diode is bad. - 5
Now, reverse the polarity of the test leads. To be a bit more specific: the red multimeter test lead goes on pin number 4 and the black lead goes on pin number 2.
The multimeter should register the letters OL (open loop). If the multimeter registers a voltage, the power transistor is bad.
Let's take a look at what your test results mean:
CASE 1: The multimeter DID NOT register the indicated voltage or the letters OL. This test result tells you that the blower motor power transistor is fried and needs be replaced.
Take a look at the section: Where To Buy The Blower Motor Power Transistor And Save for tips on where to buy the blower motor power transistor.
CASE 2: The multimeter registered the forward voltage drop and the letters OL where indicated. So far so good since this is the correct and expected result from this test.
Your next step is to bench test the power transistor with a 1.5V AAA battery and see if the power transistor activates (or not). For this test go to: TEST 3: Bench Testing the Power Transistor With a 1.5V AAA Battery.
TEST 3: Bench Testing The Power Transistor With A 1.5V AAA Battery
If your Honda Civic's blower motor power transistor has passed with flying colors the first two tests and you're still doubting that it's good, then this last test will confirm that it's working properly or not.
This test involves applying power and Ground from a 1.5V AAA battery to two specific pins and then using your multimeter (in diode test mode) to see if the circuit between pin #4 and #2 has closed.
In real life (and in a nutshell), when the power transistor activates (because you turned on the fan switch on the heater control panel), the circuit between pin #4 and #2 closes to give the blower motor the Ground it needs to run. When it deactivates (because you turned off the fan switch on the heater control panel), the circuit between pin #4 and #2 opens and cuts the Ground to the blower motor.
Don't worry, this is a pretty easy test that you can do without too much trouble or complications and I'll show you how in a step by step way.
NOTE: For those of you with an electronics background: Pin #2 is the source, pin #1 is the gate, and pin #4 is the drain (using the photo with the labeled pins in the image viewer).
- 1
Connect pin #2 to the negative (-) side of the 1.5V AAA battery (see image 2 of 2 in the image viewer).
- 2
Connect pin #1 to the positive (+) side of the 1.5V AAA battery.
Once you make this last connection to the 1.5V AAA battery, the transistor should now complete the circuit between pin #4 and pin #2 (which is normally open). - 3
Set your multimeter to diode test mode and with the red multimeter test lead probe pin #4 of the blower motor power transistor.
With the black multimeter test lead probe pin #2 of the power transistor. - 4
Your multimeter should show a forward voltage drop of about 1 Volt (if the power transistor is OK).
NOTE: The good blower motor power transistors I tested with this method showed .0 to 0.9 Volts DC. - 5
Disconnect pin #1 from the positive side of the 1.5V AAA battery but leave the red multimeter test lead connected to pin #4 and the BLACK test lead to pin #2.
Your multimeter should now show the letters OL (open loop) to indicate that the power transistor has deactivated and has opened the circuit between pins #4 and #2.
Let's take a look at what your test results mean:
CASE 1: Pin #4 and #2 DID NOT register the indicated voltage when you applied Ground and power (from the 1.5V AAA battery) to pin #2 and #1, respectively. This test result tells you that the blower motor power transistor is fried and needs to be replaced.
Take a look at the section: Where To Buy The Blower Motor Power Transistor And Save for tips on where to buy the blower motor power transistor.
CASE 2: Pin #4 and #2 registered the indicated voltage when you applied power and Ground (from the 1.5V AAA battery) and opened when you removed power. This test result tells you that your Honda Civic's blower motor power transistor is functioning correctly.
If you have confirmed that the blower motor relay, the blower motor, and power transistor are good, then there's a good chance that your Honda has a short or an open in one of the circuits between the heater control panel and the blower motor power transistor (or the heater control panel is bad).
It's beyond the scope of this tutorial to check for any shorts or opens but I'll still suggest that you get your hands on a wiring diagram and test these circuits.
Where To Buy The Blower Motor Power Transistor
The blower motor power transistor is one of the parts on your Honda that you'll end up having to replacing sooner or later. If you're in that situation now, the good thing is that you can pretty much buy it anywhere.
I've added the links below to help you comparison shop (online). I used to buy everything at my local auto parts store, til I found out just how over-priced everything is.
Below are the links to the blower motor, blower motor relay, and the blower motor resistor (also known as the blower motor power transistor).
27%NOTE: Not sure if the above components fit your particular Honda Civic? Don't worry, they'll make sure the part fits once you get to the web site.
More 1.7L Honda Civic Tutorials
You can find a complete list of tutorials for you Honda 1.7L equipped car here: 1.7L Honda Civic Index Of Articles.
Here's a small sample of the tutorials you'll find in the index:
- Maintenance Required Light Reset (2001-05 1.7L Honda Civic).
- How To Do A Cylinder Balance Test (2001-2005 1.7L Honda).
- How To Test Trouble Code P0141 (2001-2005 Honda 1.7L).
- How To Test The TP Sensor (2001-2005 1.7L Honda Civic).
- How To Test Trouble Code P0135 (2001-2003 Honda 1.7L).
If this info saved the day, buy me a beer!
