Scott Dorsey wrote:
Michael A. Terrell wrote:

tm wrote:


Are you sure it is not already a pulse width regulator? Those transistors
look like they are TO-220 packages, not TO-3.

I've seen 20W power resistors in TO-220 packages.


It does indeed look like a TO-220 from the pin spacing (since there are no
actual transistors in the photos, just spots from which they were removed).

But if it had been a PWM device, there would have been some filtering in
there, inductors and capacitors to keep the noise from getting into the
power lines. Designing clean and quiet PWM controllers is not quite as
trivial as some folks have made it out to be.


PWM has been around longer than SMT parts.

Maybe it is transients from the motor that are causing the failures.


This is possible, if it is the transistors that are failing. I don't see
any big protection diodes in there either.

If it's a RoHS soldering issue, though, I would not be surprised.
--scott

A few car manufacturers use resistor wire in the harness leading to
the motor connection at the blower box.

This lowers the Q significantly giving you a voltage drop of course,
but it also reduces electrical noise and helps suppress the wheeling
voltages.

Blower motors in this case are normally designed to operate lets say
8 volts for example, for full RPM.

I learned this years ago when going through the pain of removing the
blower in an air box of a Chrysler product, only to find there was
nothing wrong with the motor. Symptoms led on to the fact there was
since the output of the speed control circuit was alive and happy but
no obvious connection to the motor. If I had unplugged it from the air
box before pulling that all apart, I would of noticed it.

The problem was the resistor wire in the harness which had open at the
crimp point, also in the harness.

Jamie