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monitored should draw. This is how circuits are being evaluated for performance
and the reason why related diagnostic
trouble codes (DTCs) are set when components' ohmic values fall too far outside
of specification. I'd much rather monitor a circuit's current flow dynamically
than to open the circuit and measure for
resistance statically. A comparator circuit
is used to carry out this task for the ECU's
self-diagnostic strategy (Fig. 1). It serves

Fig 1 - A comparator circuit serves
as a DVOM within the ECU to
measure cicuit voltage.

as a DVOM within the ECU to measure
voltage in the circuit under various states
of operation. In the example drawn here,
a few things can be seen:
* The circuit is of a pull-down design
(ECU provides the ground-path to
energize the circuit).
* The circuit is open, and no current
should be flowing.
* The DVOM should be measuring/
indicating source-voltage (12V) in
the circuit's current state of operation.
This is what is typically occurring
when an ECU sets a DTC about " circuitlow " faults.

THE BASIC BUILDING
BLOCKS OF DIAGNOSTICS
Viewing not only the wiring diagram but
also the theory and operation of the circuit provides for a solid understanding of
the circuit functionality and anticipation
of what the ECU expects to see on that
circuit during its current state of operation (energized or de-energized circuit).
The DVOM represents not only where
the ECU is monitoring the circuit, but

also where we would place our DVOM
to monitor the circuit ourselves. In the
example in Fig. 1, it should be obvious
that the intended state of the circuit
would have us anticipate source-voltage
at that point under the circuit's current
state of operation. A lot can be derived
from just these few pieces of data.
Remember: Having fundamental
knowledge of the individual components'
functionality (at the most basic level) can
be applied to any vehicle or system out
there. Nearly 85 percent of what occurs
in any automotive circuit can apply to any
vehicle or system. This is due to the physics involved. The remaining 15 percent is
how each manufacturer chose to make
that circuit function. Combining basic
fundamental concepts (from years of
practice) along with the tools and testing
techniques you've learned to employ can
help yield a diagnosis efficiently.

A SHIFT IN STRATEGY
To demonstrate this process, I will use an
example: A 2008 Dodge Grand Caravan
experiencing a stored DTC P0760 " overdrive-solenoid circuit fault, " along with a
transmission functioning in a defaulted
state (no upshift from second gear). The
point isn't necessarily that the vehicle
was fixed, but more so how the circuit is
being monitored and what techniques I
recruited from my experience and fundamental knowledge to prove the fault
and repair the vehicle.
I built my game plan away from the
vehicle by referencing the service information for DTC P0760. The description and operation of the system and
the wiring diagram proved to be all I
needed in my arsenal to approach the
vehicle and ask of it the questions I would
like answered.
* Why won't this vehicle upshift?
* Why is the DTC set?
* Is the solenoid functional?
* Does the solenoid have everything it
needs to function?

The ECU was anticipating seeing an
inductive kick. To have a healthy functioning solenoid, the resulting inductive
kick from its magnetic field collapsing
should be present. Said another way: If a
weak inductive kick is present, the solenoid cannot do its job properly.
The service information and wiring
diagram together tell us where to test,
how the circuit functions, what we should
anticipate seeing during a test, and what
the ECU is looking for to either verify
or condemn the circuit for functionality.

Fig 2 - By placing a lab scope at the point
indicated on the wiring diagram and
referencing it to ground, we can compare
the signature to that derived from
testing one of the known-good solenoids,
controlled by the PCM.

(Fig. 2) By placing a lab scope at the point
indicated on the wiring diagram and referencing it to ground, we could then compare the signature to that derived from
testing one of the known-good solenoids,
controlled by the PCM.
The PCM was located within the leftfront fender well of the vehicle (Fig. 3).
The appropriate connector and circuits
were identified and then probed to be
monitored while using the scan tool to
carry out a bidirectional control of the
individual solenoids. The test results

Fig. 3 - The PCM was located within
the left-front fender well of the vehicle
(shown here).
Photo credit: B. Steckler

December 2020 I Diagnostic Process I 13

Professional_Tool_And_Equipment_News_December_2020

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