"Black Box Auto/Electric Question"

[John Gilmer]

Hi:

I STRONGLY suspect that the "Blower Motor Control Module" (AC/Delco
#15-71971 for those who want to "look it up") is no more. (It has failed
in the "full on" mode.)

The shot manual describes it as containing an amplifier and driver circuit.

It's "transfer" characteristics are:

Input range: 2.5 volts to 7 volts for a motor voltage (which feeds back to
the HVAC control system) for an output range: 4 to 12 volts respectively.

It just has four terminals: battery, ground, input from control system, and
motor drive (other terminal of motor is ground). It's protected by a 30amp
fuse.

I debating whether to just take the car into a repair shop, get a "generic"
replacement (which will have to be jury wired in and will cost $50+ from
NAPA), try for the OEM part-new or used; or jury rig with Radio Shack parts.

Anyway, an thoughts on what's in it? (Remember, 2.5 to 7 volts IN and 4 to
12 volts OUT.)

EMWTK

Oh, for those who like to second/third guess, it's from a 1994 Buick
Roadmaster.

 

[Wrench]

Hi:

I STRONGLY suspect that the "Blower Motor Control Module" (AC/Delco
#15-71971 for those who want to "look it up") is no more. (It has failed
in the "full on" mode.)

The shot manual describes it as containing an amplifier and driver
circuit.

It's "transfer" characteristics are:

Input range: 2.5 volts to 7 volts for a motor voltage (which feeds back
to
the HVAC control system) for an output range: 4 to 12 volts respectively.

It just has four terminals: battery, ground, input from control system,
and
motor drive (other terminal of motor is ground). It's protected by a
30amp
fuse.

I debating whether to just take the car into a repair shop, get a
"generic"
replacement (which will have to be jury wired in and will cost $50+ from
NAPA), try for the OEM part-new or used; or jury rig with Radio Shack
parts.

Anyway, an thoughts on what's in it? (Remember, 2.5 to 7 volts IN and 4
to
12 volts OUT.)

EMWTK

Oh, for those who like to second/third guess, it's from a 1994 Buick
Roadmaster.

stick it in the vice boy.

 

[John Gilmer]

stick it in the vice boy.

I wish I could say, WTF, but I'm quite close to doing that. But in it's
present state it does provide the function of connecting (when the fuse is
in place) battery to the blower motor.

 

[John Gilmer]

How big is it and did it get hot, when it used to work?

Aside from the connectors is seems to have a "plate" which is on the order
of 3"x3". From where it is mounted, it definitely could be considered a
heat source. I never observed it "working" so I don't know how hot it
really gets.

What is in it could be a "linear" analogue amplifier feeding a series
element that drops the difference between the supply voltage and the
required voltage. In which case it would tend to get rather hot and
need fins/be mounted in an airflow.

It's definity mounted on something that has fan air flowing inside it.

Or it could be a voltage controlled pulse width modulator (pwm) feeding
a series element switch (power transistor or FET). In which case it
wouldn't get very hot and could be quite small.

Could be. The motor is an slightly inductive load (I'm guessing it's a PM,
DC motor) so a "switcher" could require diode clamping. The way it was
described, however, I ASSume it's linear.

"Jury rigging" one wouldn't be that tricky, in either event. The linear
version is pretty simple - but getting rid of the heat could be a
problem. The pwm method is often used in r/c kit and there are loads of
suitable circuits around.

Me? I would go for the pwm.

Well, maybe "next time" we will buy TWO identical used cars so we can see
how things are supposed to work when something goes down.