ACTION NAPA AUTO CARE - 36
Case study: 1999 VW Jetta - A/C fuse
through ohmmeter testing, especially since the vehicle is
16 years old, and sometimes fuses just blow. In this case
however, I know the vehicle owner recently installed a
new cabin air filter, but I'm not so sure about the condition of the blower motor.
Grabbing my clamp amp along with a few hand tools,
I opened the passenger front door of the Jetta. Under the
glove compartment door there is a soft panel that can be
moved out of the way, exposing a brown wire which is the
ground for the blower motor. With the engine running,
A/C on and fan on high (setting 4), the meter measured
19.8 amps.
Blower motor amperage is not only affected by fan
speed, but also by the load placed on it. The motor is
turning fastest with the speed set to 4, but the motor is
also working harder when recirculation is turned on, and
It's relatively easy to connect an amp clamp and
measure current draw.
This brown wire is used to ground this Jetta's
blower motor.
therefore is drawing more current. That being said, why
then is the current draw so much closer together at speed
1? Since the motor is turning so slowly, the application of
recirculation does not cause as much of an effect on the
motor's current draw. Sure, it probably is working a little
bit harder, but since it's not moving as fast, the increase
in draw is negligible.
To take this diagnostic test a little further, I tested all four
fan speeds under two conditions: both with the recirculation
button turned on and off. I wanted to see what difference the
recirculation would make, if any, but also wanted to see what
the difference in current draw would be amongst the different speed settings. Here's what I found:
Fan Speed
With Recirculation
Without Recirculation
4
19.8 Amps
18.0 Amps
3
11.5 Amps
10.8 Amps
2
6.8 Amps
6.5 Amps
1
3.9 Amps
3.8 Amps
There are a few things we can learn from this chart.
First, at the highest fan speed setting the motor is drawing under 20 amps, which is much less than the 25 amp
rating of the fuse. But this chart also tells us a few things
about the relationship between blower motor speed, motor
current draw and the position of the recirculation door.
Naturally, we would expect to see current draw increase
as fan speed increases, but we're seeing a larger increase
when recirculation is commanded.
36
ACTION/NAPA * Spring 2015
The curve of the graph becomes more pronounced as fan speed increases in recirculation
mode.
Author's Note: Want to take this test a step further?
Here's a neat experiment to try: With recirculation turned
on, block the air intake with an old shirt or a stiff piece of
paper or cardboard, such as from a cereal box. What will
happen to the amp reading? Will it increase or decrease?
You might be surprised by what you find! Drop a line to
steve@macsw.org and let me know the results. ❆
ACTION NAPA AUTO CARE
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