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Whether you’re working on a vehicle just involved in a collision or repairing one with a reoccurring electrical problem, a familiarity with strategy-based diagnostic flow charts
As the collision repair industry continues
to advance, technological break-throughs permeate every area of
a repair, including electrical troubleshooting.
Called by various names and developed alongside
high-tech tools and equipment, a strategy-based diagnostic approach
the most proficient way to isolate a problem on any electrical
system or subsystem. Gone are the days of step-by-step repairs
when dealing with complex electrical systems. Due to time constraints,
you often have to start in the middle of the process, skipping
a few charts, to find and solve the problem as quickly as possible.
Whether you’re working on a vehicle that’s
just been involved in a collision or taking on the repair of one
with an electrical problem that simply won’t go away, careful
planning and a familiarity with strategy-based diagnostic flow
charts are critical.
Power and Ground Distribution
Step one in your initial troubleshooting sequence
should come only after a complaint has been verified.
If the problem is collision related, chances
are that localized areas of damage might have involved the fuse
block, feeder circuits or relay centers – which exist under the
hood or alongside main fuse panels, are tucked up under the dash,
or are on the firewall or other underhood structural members.
General Motors routes main and auxiliary-line
"trunks," harnesses within zip-lock wiring sheath or
conduit tape, toward their destination using the best path and
keeping textbook order. However, chafing and rub through can still
account for 30 percent or more of the hard-fault warranty-related
problems. These problems often come back to haunt the dealership
– and add gray hairs to the already-receding hairlines of the
At the body shop level, a schematic of power
distribution can be invaluable while searching for a loss of power
within any circuit or for trouble points destined to cause a collision
repair customer to come back, pounding on your door with the mistaken
assumption that anything that goes wrong on his car after a collision
is directly related to the accident. In such cases, shop owners
should advise techs to first look at the fuse blocks to determine
what line is affected, possibly a result of body or structural
damage that pinched a wire and caused a fuse to blow.
After the power network has been successfully
checked out, ground-distribution diagrams can lead a technician
straight to the answer to the electrical problem. For example,
picture a vehicle with both headlamps, as well as the park and
turn circuit, all out on the side damaged in a collision. In such
a case, suspect an open in the common ground wire or possibly
a bad ground connection itself. By looking over the schematic,
you might also find that one headlamp works, thereby determining
that the ground connection and the circuit up to the splice are
OK. My bet is that you’ll probably find the problem from the splice
on up to the other lamp assembly.
Understanding the way power reaches or comes
back from any particular circuit is significant in a successful
troubleshooting routine. Technicians looking to repair problems
should provide protection for themselves and their customers by
becoming familiar with the correct procedures for disarming supplemental
inflatable restraint systems (SIRS). All service technicians should
also protect solid-state devices by understanding the hows and
whys of electrostatic discharge (ESD) damage prevention.
SIRS can be safely disarmed by referring to
specific factory manuals and by following exact procedures for
deactivating the parts of the circuit that can cause the bag or
bags to inflate. Targeting the underdash circuitry for analysis
via a test light or live test probe can lead to disaster by accidentally
deploying the air bags. Using only the correct test equipment
will ensure a simple connect/disconnect shorting pin tool installation
to allow for circuit analysis. One step further requires a column
load device that acts as a simulator for the air bag while eliminating
the explosive part of the system from the on-board checks.
ESD is a problem that kills many solid-state
electronic components during the test or removal phase of a repair.
A few precautions to employ when coming into contact with this
sensitive hardware might include a wrist strap and a good ground
connection to the technician’s body before beginning to remove
a piece of gear, such as a body or control assembly. Some techs
simply reach out and latch on to a known good ground before sliding
across a car seat or pulling a component off the vehicle. But
the experts agree that directly touching control-unit pins or
edge-board connectors is suicide for hardware not otherwise protected
from ESD. Checking for ESD with a voltmeter requires that the
ground lead be connected first to eliminate static dump from meter
Keeping new components in their static-protected
packaging until they’re required for installation is one of the
best ways to avoid injuring them and seeing the vehicle back in
the shop because of system failure on an already replaced item.
The vast array of new electrical systems is
a result of the rash of new features offered by car makers. For
example, heated mirrors – a popular feature in most new cars –
are dependent upon a good circuit and supply-side amperage, which
is routed through a relay and may serve other functions, including
the defogger switch. The left and right mirror elements are wired
parallel after the relay, so one system may not affect the other’s
function. When there’s a problem, isolating the bad portion of
this circuit may have your troubleshooters working through the
interconnected defogger circuit, which can have a defective relay
and cause the heated mirror to malfunction.
General Motors ’97 and ’98 vehicles continue
to use a body control module to centralize control over a variety
of subsystems. Multiplex feed circuits are designed to provide
a defined interchange depot for various networks within the framework
of auxiliary and main system operation. Electronic brake control,
daytime running lamps, remote door-lock functions, multifunction
headlamps, fog lamps, warning systems for seat belts, trunk-ajar
signals, courtesy lamps, theater dimming, sun-visor mirror lighting,
lighted rear-view mirrors and more are all linked through a group
of bus devices that either react directly to operator controls
or reflect a predetermined status relative to information sent
by accessory control modules or input devices. The vehicle’s main
power control module may also be an integral part of the system
when traction control is linked to throttle deactivation.
Body control module data is displayed on a
scan tool on many of these GM vehicles to allow the technician
a command and response system for exercising the module priority-function
network. If switched inputs can be properly electronically bussed,
a change of state in a system will be recorded on the scan tool.
Activating system outputs can be the best way to find out if the
body control module is capable of carrying out program instructions.
Techs should always confirm actual function operation when testing
in this state since, when the body control module commands a change
of state, the system output may be incapable of carrying out the
regimen despite a scan tool inference that such an action has
Much like its cousin, the power control module
(or engine control computer), the body control module is reprogrammable
in certain instances. Sometimes, unit replacement is directed
by the manufacturer, and self tests on initial start-up may reveal
a so called "checksum error," which may affect calculation
or control and prevent the controlled system function. Much like
the GM computer "tattle tale fifties code string," the
body control module’s diagnostic trouble code 80605 should alert
the service techs to an internal snafu.
Intermittents and Connector Problems
After rub through and direct mechanical damage
after a collision, intermittent electronic failures caused by
connector deficiency become a technician’s greatest challenge.
Disabled vehicles waiting for body parts are unlikely platforms
for diagnosing such intermittents because, many times, poor mating
of connector halves, improperly formed and damaged terminals,
chafing wire harnesses, corroded connectors and ground terminals,
and generally abused wiring may all lead techs astray in what
would ordinarily be a trouble-free analysis.
Diagnostic trouble codes set from the vehicle
body control module are invaluable when trying to confront such
problems. In such cases, many technicians prefer an active road
test and won’t finalize the vehicle estimate until they’re able
to bring up the unit to a state of worthiness that will allow
for such a road test.
Current operating procedures regarding wiring
repairs on collision damaged vehicles suggest breaking the system
down into smaller substructured bites and attacking any obvious
problems discovered during the visual inspection first. By gaining
a familiarity with strategy-based diagnostic flow charts, technicians
will become familiar with successful test routines that will lead
to a high percentage of fixes, satisfying both the customer and
the insurance company.
Often, problems created by a collision will
result in an undesirable vibration or harsh ride quality. Failure
to bring the vehicle back into spec in this particular area can
lead to all kinds of resonance-induced electronic and electrical
Be sure mounting fixtures are intact and components
are secured in the same location and fashion as the manufacturer
has specified. This is especially critical with regard to SIRS.
When inspecting for SIRS failure, don’t neglect any part of the
system, especially forward discriminating sensor mounting attitude.
Whether you’re working on a vehicle that’s
just been involved in a collision or repairing a vehicle with
a reoccurring electrical problem, a familiarity with strategy-based
diagnostic flow charts is critical. Hopefully, you won’t see the
same vehicle back in your shop suffering from a troublesome electrical
problem, but you will see a satisfied customer return for other
Writer Bob Leone, a retired shop owner,
is ASE Three-Way Master Certified and is completing qualifications
as a post-secondary automotive instructor in the vocational school
system in Missouri.
The Haunted Grand Prix
Sometimes, a valuable lesson comes from the
depths of the darkest repair experience. Such an instance came
by my bench last summer while I was working at a new-car dealership.
A 1989 Grand Prix had recently been sold.
All was well with the car … until a minor mishap occurred involving
the car and a tour bus. Though the vehicle was damaged only on
the right side, the frustrated owner swore it seemed to have taken
on an entirely new personality.
Initial checks showed no obvious problems,
though I did have to tighten up several mounting brackets both
under the dash and under the hood. In addition, the vehicle hood
hinges needed tightening, and the center console was loose.
Examining the collision-damaged area and looking
at the wiring schematic in the factory manual, I determined that
the already-performed collision repairs looked good, so I decided
to take a test drive. The test drive was uneventful … at first.
After cruising the road for awhile, the speedometer suddenly went
berserk, and the needles on the gauges began bouncing back and
forth with no logical rhyme or reason.
Safely back at the shop, I researched the
strange phenomena and found a technical service bulletin detailing
a similar problem involving the oil gauge only. The bulletin said
a new sender was available to isolate the problem on the one gauge.
I installed the sender and, as the bulletin predicted, the oil
gauge worked like a charm.
Confident I had solved at least part of the
previous problem, I took the vehicle for another test drive. Though
I witnessed no abnormal gauge behavior, I was sure the previous
instrument cluster episode couldn’t be related entirely to just
the oil sender. But dealership time schedules don’t permit lavish
diagnostic routines on nonsymptomatic cars, so after a brief check
of ground circuits, the vehicle was returned to the customer.
The following week, I fully expected the Pontiac
would be back with more problems; it wasn’t. And after a second
week elapsed with no Grand Prix at my bench, I almost chalked
the repair up to the "X-Files."
Not so quick.
By Wednesday of the third week, the Grand
Prix was sitting in front of my tool box with an even longer list
of symptoms. The vehicle speedometer was now completely dead (the
speed sensor in the transmission housing had already been replaced
at another dealership), and an annoying idle fluctuation was accompanied
by a whining noise coming from under the hood.
The noise, I determined, was an alternator
diode set screaming for help. As part of the rescue, a replacement
unit was installed. AVR tests and road evaluation showed normal
function of the system, but the speedometer was still dead – so
it was removed and sent to the GM factory rebuilding depot. The
idle seemed OK, and factory relearn objectives were easily met.
My road tests are usually short, but because
of the circumstances, I decided to take the Pontiac for a longer,
more detailed road test this time. The fact that the instrument
cluster was absent because the speedometer had been removed wasn’t
a problem – I stayed well within the traffic flow constraints
thanks to the usual road congestion. While passing a slow moving
tractor/trailer, however, I was forced to accelerate and noticed
that the vehicle’s interior had an abnormal amount of rattles.
Something suddenly dawned on me, and I immediately turned off
the main route and found a secondary road. Out of traffic, I was
able to bring the car up to higher speeds, and the vibration coming
through the floor pan was enough to shake the ash tray out of
its slides, dumping ashes onto the floor.
Back at the shop and on the lift, I spied
a recent oil leak from the transaxle on the same side a new C/V
shaft had been installed after the original collision. The shaft
had a balancing weight installed on the center section, and by
the size of the weight, I determined that this rebuilt shaft might
be the culprit of the ghostly electrical problems. The shaft was
replaced, the rebuilt speedometer installed and the road test
Later conversation with the customer revealed
that he’d noted the vibration when traveling more than 70 mph,
but felt that it was a "tire thing." Since he rarely
drove that fast, he never complained about it. This torsional
vibration of the drive shaft had apparently been responsible for
many of the ghostly, failed, sensitive electronic components on
the Grand Prix. (The drive-shaft rebuilder was later contacted
and acknowledged that the shaft returned for credit had indeed
been defective.) The car returned twice to my bench with other
electronic problems, undoubtedly still related to the previous
In this age of scan tools and high-tech gadgets,
a simple road test should be expanded to include all areas of
the customer’s ordinary driving habits. It may also become necessary
to pursue problems with the help of another shop equipped with
a chassis dyno.
And remember, every component on a vehicle
is part of a system, and each system is related to the vehicle
whole. Any part of the system, no matter how remote, can and will
affect other, more critical members. Use common sense and develop
experience along these lines. This alone can be your first line
of defense against unique and stubborn driveability problems.