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Not all transistors are created equal
Posted to Technical Theory Forum on 10/4/2010 5 Replies

This past Saturday I ran into a fault that made me do a double take. This Honda comes into the shop with a blower that sometimes works, sometimes not. The tech reported to me that it had a faulty blower motor. Well guess what, no dice. After he installed the blower he reported back to me that the blower worked for a little while then went dead again. He also made an observation that when the blower was not working the A/C compressor would not function either. I asked him how he tested the motor orginally and I was told that he had power on both sides of the motor but no action =). After a quick look see at the wiring diagram I realized the mistake that he had made.

Labscope in tow I head to the blower TRANSISTOR and hook up to both the blower control and blower speed terminals. ---data had some description of how the circuit worked but it was vague. I expected to see a square wave signal on the blower control circuit and a varying voltage return back to the controler. No dice. What I found was a solid 9v on the control signal and battery voltage on the blower speed return signal. Not convinced that the controller is faulty I changed the blower speed through the control head. The blower control signal remained at 9v. Hmmm. So I decided to play the odds and check these signals with a known good transistor. With a new part the system worked fine. I noticed as I increased the speed through the controls the control signal to the transistor would increase voltage and the blower speed return voltage would get lower as the fan got faster. A little more curious about the design I took the time to take some captures of the waveforms when the system is initally powered up (fan set to low observing signals just after key on)

Faulty transistor

[2004 Honda Accord EX, ECM/Inputs/Outputs Waveform]

Good transistor

[2004 Honda Accord EX, ECM/Inputs/Outputs Waveform]

The red and white circles show where the systems ppower up during key on transistion. A short pause then the controller goes into action.

Looking at the good signal you can see that the controller sends out near 2v to achieve low speed. The controller sees that the blower speed signal from the transistor is aceptable and in turn keeps the voltage near that level.

You can see in the faulty waveform that the controller keeps increasing the control voltage and does not get an acceptable return signal. This would inform the A/C control unit that there is no blower movement allowing the control unit to keep the compressor from turning on so as not to freeze up the evap core.

Most solid state control devices use square wave pulses to acheive a desired outcome but some don't

Jonathan from Alabama

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car Vehicle Data

2004 Honda Accord EX 2.4L

Engine2.4 L
Trans5-speed Automatic (Electronic)