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Re: PWM blower fan resistor mod
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Posted by udderslate (more from udderslate) on Mon, 13 Aug 2007 19:11:39 Share Post by Email
In Reply to: Re: PWM blower fan resistor mod, darryl, Thu, 9 Aug 2007 05:06:56
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short story..you need a new blower motor .
quick fix to the blower motor resistor,
jumper across the blown thermal fuse .

It was just an Idea , I doubt I'll really do it , but my prototype works on a small motor , getting it to work on a 18A blower motor remains to be
done.

PWM stands for Pulse Width Modulation .
Stock uses an expensive resistor to control the motor speed .
With PWM, the full 12v is pulsed (through a MOSFET ), varying the ON/OFF ratio , as an efficient way to control the motor speed .
Low speed will have a small "ON" time , larger OFF time .
ON for half the time,OFF for the other half , will average to 6v .

If you set the fan speed by a manual knob on the dash 1,2,3,4
1 is the slowest , 4 is full 12v speed . 1,2,3 settings pass thru a
large resistor in the blower box in the "aquarium" .
Truthfully , I'm not denying it , IT IS THE BLOWER MOTOR !!!
going bad , and as it ages it draws more current , and blows the resistor .
BUT...for two of my resistors, it wasn't the resistor that blew ,
but a thermal fuse , that blew .Microtemp G4A01184c ,rated at 10A ,184C .
I merlely jumpered across it as a fix ...others have done the same ,
with long term success .

It is the ON spike , that blows this thing out ( its only rated at 10A ),
and the older blower motor is doomed to draw more and more as it ages ,
eating blower motor resistors , at about 100USD each .

Cheepie me , I had just replaced the heater core , as I was "in there" ,
getting out the foam bits , that I thought was the cause of the first
blower motor resistor blowing...I see now, it was motor , as it blew another thermal fuse in a week . I DID look at the old motor.
I thought it was OK.

Take my advice ( backed many times if you do d search )
Replace the;
blower motor,blower motor resistor, heater core .

or jumper the thremal fuse and wait it out .

Enjoy
Wayne


below are two links ;
if you build either replace the single MOSFET
with multiple low Ron specd devices ( IFRZ44N,IRFZ48N..IRL2203 )
search through the RC airplane sites
the first one I built and tested on a 2A motor.
getting it to work on an 18A blower motor remains to be tested .

http://www.solorb.com/elect/pwm/pwm1/

this one is a guy in your continent , I like his audio stuff too .
http://sound.westhost.com/appnotes/an009.htm

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