MACS Monthly Newsletter - 2021 - MAR2

been the standard for many decades. What do you need
to know about diagnosing them other than if they don't
spin / pump when 12-volts and ground are applied, you
need to replace them?

ty Standards) dictated that IF the blower motor control
module should ever fail, it should fail in the " full speed on "
mode. Incidentally, these same safety standards dictate
that mode door controls must always failsafe to defrost.
The idea is, if you've got problems with HVAC, at least the
HVAC is moving air at high speed to the windshield so the
driver can see where they're going.
5. NOISE / PWM: Electric motors make two kinds of
noises; audible and electrical. We hear the blower and fan
motors running. If you're hearing is good enough you can
hear an electric cooling fan, electric water pump or fuel
pump too.
a. PWM - Example from Lighting: Pulse Width Modulation has been around for decades. It's much more efficient to control 12-volts applied to a light bulb for variable
dimming, fan's air movement or pump's output by turning it on and off rapidly compared to giving the electric
motor a variable analog voltage. For the light bulb, the
PWM switching speed (frequency) needs to be over a
certain frequency measured in Hertz (Hz - times per second) where our optic nerve and brain functions can no
longer discern a flashing light. Night club strobe lights
work below 10-12 Hz. Over that frequency strobe-lighting
effects can trigger a form of epilepsy (known as photosensitive epilepsy, PSE, or light-sensitive epilepsy) if the
flash frequency is broadly in the range 16-25 flashes per
second (some people experience PSE at lower or higher
frequencies). Therefore, PWM lighting is well over those
frequencies. Many lighting PWMs are at 128 Hz which
just happens to be easy math within the digital design for
electrical engineers.
b. PWM Motors: Motor PWM frequencies are another story. The frequency range we can hear undesirable
switching is between 20 Hz and 20,000 Hz (20 kHz) with
the most effective range between 2 kHz and 6 kHz. Many
PWM electric motors (fans and pumps) are pulsed off and
on above 15 kHz with some as high as 22 kHz. The good
news is these motors are not noisy as they pulse on and
off due to their high frequency PWM. The bad news is EMI.
This is the reason you'll see noise shields running from a
remote PWM control module to a motor if the module isn't
integrated into the motor assembly. Even without PWM,
motors are electrically noisy. We've all seen that large
electrolytic capacitor that's connected between the blower motor's power wire and ground on old school blower
motor housings. Regardless of the motor's PWM switching frequency, it's the duty cycle of ON vs. OFF power sent
to the motor that controls the motor's actual run speed.
6. CURRENT DRAW & WORK: Electric motors draw
more current when they do more work and vice versa.
Take a cooling fan motor for example. It takes in air from
the HVAC system's intake and moves it through the HEBA
(Heater Evaporate Blower Assembly) and out the proper
ducts. What happens if your customer's cabin air filter is
severely restricted? The blower moves less air and the
current draw goes down. The same current reduction applies when the blower motor is switched to low speed

Here is a list of technical truisms for simple 2-wire
(power and ground) 12-volt electric motors / pumps:
1. AMPLE COOLING: Electric motors need to be
cooled to survive. For cooling fan motors this is easy -
they're in their own airflow! For pump motors (fuel, coolant, ATF, etc.) they need the fluid that they pump to stay
cool. This means a low coolant (or fuel) level can lead to
their early demise / repeat failure.
2. PROPER VOLTAGE: Electric motors run at a speed
relative to the voltage they're supplied. For HVAC blower
fans this is of course how the air flow is adjusted to the
driver's desired setting. For cooling system radiator fans,
a remote or integral blower module may be doing the job
similar to the HVAC blower motor control resistor / control module as it varies the speed of the motor to assist in
cooling system control. Motor running too slow? Check
the supply voltage. If the motor is set to high speed and
not physically turning (blowing air or pumping coolant /
fuel) like it should, you may have a voltage drop in the
circuit. Back probe the motor's positive and negative circuits while commanded to full speed. If voltage supply to
the motor is 0.5 volts or more LOWER than the battery's
voltage, it's time to move your voltmeter leads to sections
of the same circuit (power or ground) to determine where
the voltage drop actually is. If a section of wiring harness
where you measure with your positive and negative meter
leads (along the same circuit) displays over 0.2 volts per
section, that section is suspect. Note: the motor / pump
must be running for a proper voltage drop test to be conducted. If it won't run, disconnect it and temporarily wire
in a substitute load (of approximately the same current
draw value) in place of the inoperative electric motor.
3. BALANCE: Since electric motors spin on bushings
or bearings, they need balance. Just as an improperly balanced 4-stroke combustion engine needs balance, so do
electric motors. A broken blade on an electric cooling fan
or HVAC blower motor's squirrel cage fan (or a buildup
of debris inside the same fan) can create a motor imbalance that will result in (1st) noise and vibration, then (2nd)
possible performance issues, then (3rd) premature motor
failure.
4. INRUSH CURRENT: Electric motors start up with an
'inrush' of current. It takes lots more energy to get a vehicle rolling than it does to maintain a desired speed. The
same applies to any electric motor. Some electric motors,
due to age and wear are harder to start up, and therefore create a higher than normal inrush current spike.
This can damage the electronics that control them. A fuse
that protects the wiring to an electric motor will often not
blow if this current inrush is very short lived. Remember
all those Cadillac blower motors that blew their electronic
control modules? FMVSS (Federal Motor Vehicle Safe-

March 2021

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MACS Monthly Newsletter - 2021

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