MACS Monthly Newsletter - 2021 - MAR5

Dave Hobbs
Ford

Figure 6: Current Ramping " Possible " Faulty Motor. This 2-wire DC brushed fuel pump motor is on its way to being out of service in most
cases. Notice the irregular pattern of current fluctuations between the brush to commutator bar connections. Tech Tip: simply observing
a waveform like this on a motor is insufficient justification to replace it. Only replace it if there are intermittent symptoms involving motor
operation / performance COMBINED with a suspicious current ramping waveform such as the one in this illustration. Experience is a
great teacher. Many technicians have seen irregular patterns like this on electric motors and those same motors lasted several more
months or even years. As they say, " If it's not broke, don't fix it! "

Figure 7: Current Ramping Challenges - PWM Motors. Looking at the connector pin out on this Ford Fusion Hybrid traction battery
cooling fan indicates that the power to the actual motor is applied by a PWM motor control circuit built into the blower motor. B +, Ground AND PWM input plus a module feedback (FB) circuit means you won't be able to place your amp clamp
(and scope) around the actual blower power circuit to current ramp the motor. You can, however, catch the overall current
draw by scoping the circuit breaker supplied power on pin 1. Always check for lint / pet hair on these motor's blower cages.
lier. In most cases, if the electric motor is not functioning and has power and ground applied to it but still won't
work, replace it. But what if the electric motor is intermittently not working? It could be the motor control electronics, the power circuit, the ground or the motor itself causing the intermittent inoperative condition. Aside from the
obvious inspection of all related connections (and voltage
drop testing on the power and ground circuits), an inductive low amp clamp connected to an oscilloscope, along
with a fuse adapter to gain access to difficult to find power
circuits (Figure 4), can provide some valuable insight into
the electric motor's condition.

March 2021

Advanced drivability techs have been " current ramping " fuel pumps with a good degree of success. What
can we learn from their examples and how can we apply this knowledge to HVAC related electric motors? The
current draw on HVAC actuator motors is typically very
low, so we'll stick to the higher current HVAC blower motors and cooling fan motors. If one of those motors is a
simple 2-wire connection (power and ground) coming
from a relay or blower resistor network, you're going to
see a similar waveform to a fuel pump. Brushed 2-wire DC
single direction electric motors are pretty much the same.
As the armature has current applied to it through the mo-

MACS Service Reports

MACS Monthly Newsletter - 2021

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2021