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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