MACS Monthly Newsletter - 2021 - MAR1

Service Reports
MOBILE AIR CLIMATE SYSTEMS ASSOCIATION ©

March 2021

By Dave Hobbs, MACS Technical Correspondent

Electric Motor Diagnostics - Easy or Hard?

Wikipedia

Diagnosing 2-Wire dc 12-volt Electric Motors
Vehicles have lots of electric motors for a multitude of
purposes. Consider the act of moving air. Electric motors move air across the radiator / condenser, move air
throughout the cabin, move air through the seats and
even pull a little air to keep modules cool and in-car temp
sensors accurate. Then there's the job of moving fluids.
Fluids that used to be pumped mechanically (mainly fuel
and coolant) are often pumped with electric motors on
newer models. Mechanical actuator motors move everything from blend doors to headlamp doors. Some even rotate the camshaft or place the vehicle's transmission into
park. And let's not forget that Charles (Boss) Kettering
invention that launched the Dayton Engineering Laboratory Company (D.E.L.C.O.) over 100 years ago: the electric
starter motor.
Automotive electric motors are all pretty much the
same, right? Not so much these days. Manufacturers
used to run a power and ground wire to about every electric motor on the vehicle. Now we see all sorts of circuits
going to them.
Rather than depend completely on your personal list of
silver bullets and fixes, let's take a quick dive into how a
few technologies of electric motors work (Figure 1). After
reading this MSR, as you observe the service manual's
descriptions of the various circuits running to today's
electric motors, you'll be better informed regarding the
diagnostics of electric motors.

Figure 1: Simple 2-Wire Brushed DC Motor. The most
common DC motor we encounter on vehicles is the
simple brush style permanent magnet motor. DC power
is applied to the brushes which creates an electromagnet within the armature. As the magnetic poles of the
electromagnetic field in the rotating armature react with
the poles of the fixed field, this causes the armature to
rotate. In some higher-powered motors, the fixed outer
field may be another electromagnet (referred to as field
coils), as in the case of an older 12-volt starter motor. As
the armature moves the brushes then move to the next
winding within the armature causing a reversal of the
magnetic polarity within it. This causes the armature to
continue moving as the process repeats. The motor's rotating armature is connected to a physical load (turbine,
lever, etc.) to move air, HVAC doors, fluids, etc.

Single Phase Brushed dc Motors
Using the term " dc " automatically applies to motor vehicles, right? For the most part, unless the electric motor has 3 orange high voltage cables connected to it (i.e.
hybrid or electric vehicle) it's single phase, right? 3-phase
only applies to high voltage motors like the motor / gen-

erator found on the Prius you've worked on, right? Not
true! But we'll get to those coming up. For 12-volt electric
motors / pumps, single phase brush style technology has

ALSO INSIDE THIS ISSUE:
3-Phase BLDC (Brushless DC) Motors (Blower Motors & Fuel Pumps) ...........................................................7

MACS Service Reports is the oﬀicial technical publication of the
Mobile Air Climate Systems Association, P.O. Box 88, Lansdale, PA
19446. The material published in MACS Service Reports expresses
the views of the contributors and not necessarily that of MACS. Every attempt has been made to ensure

March 2021

the accuracy of the content of MACS Service Reports. MACS, however, will not be responsible for the accuracy of the information published nor will MACS be liable in any way for injury, labor, parts or
other expenses resulting from the use of information appearing in
MACS Service Reports.

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