MACS Monthly Newsletter - 2021 - MAR6

tor's brushes, those minor current
fluctuations from the brushes moving from one armature commutator
bar to the next show up in the scope
pattern (Figure 5). A rhythmic pattern
is normal, but an erratic pattern may
be giving you a clue that the electric
motor is nearing the end of its service
life (Figure 6).

Pico

Figure 8: Current Ramping Challenges - PWM Motors. This PWM blower motor CAN be
current ramped because the blower PWM control circuit is EXTERNAL to the
actual blower motor. Simply disconnect X2 (left side of drawing) and provide your
own fused power and ground leads to the motor to run it at full speed. Place your
lower current (0-60 amp) inductive amp clamp around your fused power jumper
and connect it to your scope.

Figure 9: PWM Signals. Using two channels of a DSO (Digital Storage Oscilloscope)
can be an effective method to diagnose PWM controlled cooling fans and blower
motors that have a remote motor control module built on to the motor. Simply
back probe the smaller wire going to the motor control module (should be the
circuit coming from the PCM / HVAC module) with one channel (bottom trace in
this pattern) and then connect an inductive amp clamp to the 2nd channel to the
scope (top trace in this pattern). Notice that the cooling fan's PWM input is low
(off) about 90% of the time / high (on) about 10% of the time. The result is a current draw pulsing (at a higher frequency) between 36 and 60 amps. This would
be an average draw of around 45 amps. This is high speed. The reason for a 10%
duty cycle getting a high fan speed? If the motor controller lost its signal from the
PCM, the fan would default to full speed. Ever had a PCM throw a fit with communications and / or DTCs and hear the cooling fan running full speed? This default
helps keep the engine from overheating. In similar motor control applications for
HVAC blower fans, the same applies in order to keep the windshield clear during
times of fog / frost for safety's sake.

Current Ramping - What if the
Electric Motor is PWM Controlled?
It can be difficult in many cases
to capture a good current ramping
waveform on a motor that is being
pulsed on and off (PWM) at a high
frequency. See Figure 7. Hi frequency
PWM switching makes scoping the
electric motor's normal current fluctuations difficult. In cases where the
PWM motor speed control module
is an integral part of the electric motor, you won't be able to bypass it. In
other cases where the blower control module is remote and feeds the
PWM switched current to the blower
motor on a pair of wires, a module
bypass is possible in order to scope
that motor's state of health (Figure 8). Another circuit worth scoping on a PWM motor is the smaller
PWM signal command to the blower
or cooling fan module (Figure 9). In
this figure the AC 'looking' signal (top
trace) is NOT the current ramping of
the cooling fan's brushes & armature
itself. It's simply the PWM induced
overall current draw fluctuation. Most
electric water pumps are PWM with
an input signal and feedback signal.
In another example (Figure 10), an
inductive amp clamp is connected
around the power wires that feed the
water pump's internal module. Again,
the fluctuations are simply the PWM
total turn on / turn off of the motor. v

Dave Hobbs

Figure 10: Current Fluctuations on PWM Water Pump. This
Toyota Camry's electric water pump motor is drawing around 20 amps when the heater is commanded
to full heat. The current draw pulsations are indicative of the pump's duty cycle frequency and NOT the
pump's electrical health because we're not clamped
around the wire that actually runs the pump motor.

March 2021

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MACS Monthly Newsletter - 2021

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