MACS Monthly Newsletter - 2020 - MAY6

possible "What Ifs?" with component and circuit
failures. Short circuits, open circuits, bad splices,
etc. The problem with following their flow charts
blindly are two-fold:
a.
Most service engineers today are more "engineer minded" than "service minded." As mentioned in a previous MSR we are seeing more and
more of the older "car guy / car gal" service engineers retiring and being replaced with young service engineers with little to no real-world repair
shop experience.
b. Based on the content in many diagnostic
charts, one gets an assumption that engineers are
unaware that many professional techs using their
service information are sharp enough to use a voltmeter, ammeter and lab scope. Much of the time
trouble tree charts seem to have techs running in
circles using only an ohmmeter to check for opens
or shorts in wiring.

Practical Advice to Get the Most Out of This
MACS Service Report

Find a set of colored highlighters and actually do
what is described in these steps. Taking your time
to highlight the various circuits and study the schematics will be a very practical use of your time - we
promise!
While using Jorge Menchu's exact system of particular colors (Figure 4) for particular types of circuits is very effective, it's not 100% necessary. The
main point to selecting colors is to use colors that
make sense to you and are easy to remember. It
doesn't matter if you can only find an orange highlighter instead of a red one. What matters is that
you start turning the maze into a workable roadmap! If your memory isn't photographic (mine certainly is not), create a color key system where each
color is recorded on the side of your schematic to
help you remember that circuit's function.
Since the print edition of the MACS Service Report is in black and white, my examples of assigning power and ground highlights required me to
use different line patterns such as long dashes, short
dashes and combinations of both (Figures 6-9). For
the Malibu example I used my first printed schematic page (Figure 6) to highlight the 12-volt B+ (battery voltage all the time), power grounds (grounded
all the time) and ignition switched 12-volts. You'll
notice in Figure 6 that within the underhood fuse
block there are a pair of 30-amp fuses supplying
B+ (highlighted with heavy font long dashes) to
the Low Speed Cooling Fan Relay and the Cooling
Fan High Speed Relay. A 10-amp fuse in the underhood fuse block handles the ignition switched 12volt feeds. I highlighted those with smaller font /
shorter dashes to all three fan relays.
Notice the third relay located in the center of each
of these diagrams? It's titled "Cooling Fan Speed
Control Relay." There are no high current fused
power sources running to this relay. This leads me
to my next task; identifying and highlighting all the
grounds in the circuit. Power grounds (grounded
all the time) come from G106 (leading directly to the
left cooling fan motor) and G101. Next highlight the
switched (relay trigger) ground coming from ODM
(Output Driver Module) inside the ECM at Pin 59
(Figure 7) which control the fan slow speed relay.
Then repeat with a different (but similar) highlighting theme for the ECM's ODM driver circuit at Pin
42. This circuit triggers both the fan speed control
and high-speed relays (Figure 8).

"What if?" Example 1

Take a moment to study Figure 7. Let's suppose
you use a scan tool to command the left and right
fan motors to operate in all the possible combinations (Right Low Speed ONLY, Left High Speed
ONLY and Left & Right High Speed). If you observe
the right fan motor ONLY works on low speed,
what would be a logical conclusion? You'll note that
the ground supply to run the right fan motor comes
from either relay terminal 87A (Fan Speed Control
Relay Off) or from terminal 87 when the Fan Speed
Control Relay is activated. When the Fan Speed
Control Relay is off, and the right fan's Low Speed
Relay is ON, the only path for ground to flow to the
right fan motor is through the contacts between the
Fan Speed Control Relay contacts 30 and 87A. From
87A you can follow the highlighted path (twin parallel dashes) all the way to the power input at the
left fan motor.
So how does the right fan motor get its ground
to operate at low speed? By getting a reduced
voltage through the left fan motor to ground. You
might think of the left fan motor 's winding (when it
doesn't have any power applied to it) as a great big
ballast (high wattage) voltage dropping resistor. So,
IF you ONLY have low speed on the right fan motor but you can operate the left at high speed, you
can conclude the speed control relay's high-speed
contacts (terminals 30 and 87) are burnt open or the
relay simply isn't functional. If it clicks, you know
the relay is getting triggered so it's likely the contacts are burnt.

Imagining the "Road Closed" Signs -
"What If?" Diagnostics

"What if?" Example 2

Study Figure 9 for a moment and consider this
scenario: you command the various speed combinations and conclude that the left fan motor won't

Service engineers who develop diagnostic flow
charts / trouble trees attempt to imagine all the

May 2020

MACS Service Reports

MACS Monthly Newsletter - 2020

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