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"Black Box Auto/Electric Question"

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.

Reply to
John Gilmer
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stick it in the vice boy.

Reply to
Wrench

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.

Reply to
John Gilmer

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

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.

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.

"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.

-- Sue

Reply to
Palindr☻me

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.

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

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.

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.

Reply to
John Gilmer

Well, you could just find someone who owns one and ask to look at his.

If it is analogue, quite possibly all that has failed is the series pass transistor - probably just a darlington. That would explain the "full-on" fail mode - it has just punched through a C-E short.

If you could get the thing apart, it may be a very easy fix - the amplifier section may easily be fine. Just the series pass needs swapping out.

The fuse probably doesn't limit the series-pass device to its safe operating area. You may want to wire the amp out to an external series pass device that will be nice /n easy to change, next time.

Much cheaper than buying a new unit - if it is only the darlington that has gone buy-byes.

-- Sue

Reply to
Palindr☻me

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