MACS Service Reports - 2015 - DEC3

A test light...Really? Voltmeters are the better
choice of tool, but only if you are getting a voltage
drop and wish to see how much it is. 95 times out
of 100 a fuse is either good or open. Power is either
there to the fuse or it's not, as in the case of a faulty
ignition switch. However, as we learned in the SBD
class long ago, those 5 percent of cases can really
eat our lunches. Take into consideration the following drawing representing a transparent fuse panel
and its connections and bus bars. What you would
see at the blown (open) fuse's output would be the
same regardless of whether a simple test light or a
voltmeter was being used. In the case of the fuse
with the broken leg (an old SBD class trick) representing an open or high resistance terminal / buss
bar within the fuse panel (those things really do
happen), the meter and test light would indicate a
fully open circuit, while a high resistance condition
within the fuse panel might not indicate a problem
unless the 12-volt accessory sourced by the fuse or
fuse center was powered on and working. Voltage
drops typically only occur when the device we are
checking is functioning. No current draw = No voltage drop! Always test and visually inspect fuses.

Looking at the Cross Systems Integration illustration however, we see that Ride and Handling (ABS)
overlaps Power and Propulsion (PCM) for obvious
reasons; torque management requests from the ABS
/ TCS / Stability System require the action of the
PCM to limit power from the engine so the chassis
electronics can rectify a problem that is occurring
with positive wheel slip or yaw sensor action, lateral accelerometer readings (sliding sideways) or roll
over sensor activity. Before brakes are ever applied
to correct the situation, torque management is enabled as a first step to keeping the vehicle from losing control. This can't work correctly if either system has a problem. Therefore, all related systems in
the integration loop must be checked for DTCs. See
Figure 4.
You should always perform and record the results
of an entire vehicle DTC scan whenever approaching an electronic control related complaint of any
kind. Too many systems overlap responsibilities to
ignore any module's DTCs. In this case study, the
vehicle had a pending P-Code (P0171 and P0174,
both banks lean) that made a completely different module (the ABS / Stability Control Module)
lose faith in the PCM's ability to be able to control
torque management properly if a real stability control problem were to occur. Therefore the ABS and
a DDM (Driver 's Door Module) reported the issue

Step 3: Perform Published Diagnostic System
Checks
Every HVAC technician is familiar with a system
performance check when it comes to A/C systems:
run the engine at the specified speed with the blower at the specified setting and the recirc door either
open or closed while observing the high and low
pressure gauge readings along with thermometers
at the ducts and in front of the vehicle. But what
about the diagnostic systems checks for the various
electronic controls? Most OEMs will have you determine if there are any DTCs or if communications
can actually occur with the scan tool. That's a good
start; a first base to go to before any other testing
is performed. However, do you think dynamically?
Do you consider the entire vehicle? Cross-systems
integration is a method of designing a vehicle's
various systems while taking the entire vehicle into
consideration. There are no more lone wolf systems
when it comes to electronics because so much is interconnected. See Figure 3.

CASE STUDY: 2007 Chevrolet Avalanche

December 2015

Delphi

This vehicle had the issue of a very intermittent
loss of power, accompanied by illumination of the
Traction Control / Stability Control system warning lamp. The technician working on the vehicle
was able to duplicate the customer 's complaint one
time and find a DTC in the ABS module. Looking in
the ABS module seemed like the right thing to do,
so the technician stopped there.

Figure 3: Nothing stands alone on today's vehicles. Systems related to engine
management (Power & Propulsion) have always interfaced with HVAC systems (Integrated Body).

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2015