February 2023 - 22

TOOL BRIEFING
accessing the harness, another unexpected
discovery was made. The vehicle appeared
to be taking on water.
The wires making up the harness were
p A voltage drop test revealed the RPSD could not function because it only had 2.5V supplied to it.
Therefore, a voltage drop test couldn't be
conducted. However, if one were to load
the circuit manually, a test for true circuit
integrity can accurately be determined,
and this may uncover the reason the system
is not functioning.
Leonard chose a load similar in current
consumption to the circuit to be tested. The
white wire leading from the under-dash fuse
block supplies voltage to the RPSD control
unit at terminal B1. Although it cannot be
seen in the picture, the voltage originates
from the underhood fuse block across a 10A
fuse. Leonard chose an 1157 bulb to see if he
could force the fault to the surface. If this circuit
managed a lot more current (like that of a
starter motor), the test load would have to be
heavier to stress the circuit's wiring properly,
as this test is intended to do.
With the connector removed from the
RPSD control unit, the 1157 lightbulb was
substituted (as the load) and was supplied
a known-good ground to allow the bulb to
operate. This test used the existing RPSD
control unit's voltage supply circuit to energize
the light bulb. As the bulb illuminated
(dimly), a voltage-drop measurement was
taken across it. Knowing that this bulb
should be the only load in this series circuit,
one would expect the bulb to drop
all of the voltage (approximately 12.5V),
and illuminate brilliantly if the circuit is
functioning as designed. However, this
bulb is only dropping 2.5V. The result of
the test tells us a few things:
* 10V dropped somewhere before the
bulb (in the power supply circuit).
* Further disassembly is required to pinpoint
the source of the voltage drop.
* The wiring diagram will help determine
the most logical place to begin disassembly.
DESTINED FOR DISASSEMBLY
The last thing a diagnostician wants to do
is disassemble anything for testing. That is,
unless it is justified. The results of the loaded
voltage drop test Leonard conducted
conclusively pointed to a significant highresistance
fault on the voltage supply side
of the system. More importantly, there is
no doubt where he has to disassemble for
further testing (no wasted time, no guesswork).
The significance is Leonard will not
have to backpedal, as he is assured to find
the source of the fault if he simply trusts
and follows the test result.
However, an excellent strategy he will
employ will be to reference the wiring diagram
to determine the component locations.
Using this information, along with how the
vehicle has been altered to accommodate the
mobility-assist features, should allow him to
focus his further efforts and avoid wasted
time. This is working smarter, not harder.
The modifications to the original
vehicle wiring harness occur below the
floor of the second-row seating. The voltage
source circuit in question comes from
the passenger side kick panel. The RPSD
control unit is located in the right-rear
quarter panel. Leonard chose to remove
the interior second-row floor to inspect the
harness where the mating of the original
and conversion harness are made. Upon
22 PTEN FEBRUARY 2023 www.VehicleServicePros.com
separated to allow Leonard to inspect the
circuit in question. To his delight, an ugly
green corroded section of wire was exposed
due to its submersion in the water-logged
floor. The corroded wire was exposed
because the insulation was compromised
during the installation process. The wire
appeared to be sliced by either a razor blade
or some other sharp object.
Leonard " jumpered-voltage " past the bad
spot in the harness and proved the system
to function before making the necessary
repairs. This is never a bad idea, as sometimes
more than one fault exists simultaneously.
It is better to know ahead of time
than to commit to repairing a single fault,
only leading to a partial fix of the symptom.
Reflecting on what happened makes
it quite simple to understand if we revisit
the original wiring diagram. The red " X "
is where he found the voltage drop. By substituting
a bulb (so he could energize the
circuit), it was seen that the bulb dropped
only 2.5V (leaving 10V being dropped/
wasted elsewhere in the supply side of the
circuit). Thinking of it like this may simplify
further: The right power sliding door
control unit could not function because it
only had 2.5V supplied to it.
Leonard discovered that a door seal was
displaced, which caused the water intrusion.
It was properly mated to the body of
the van, and the damaged section of the
wire was isolated, replaced, and properly
sealed. The existing wire harness was
placed in conduit, properly routed, and
secured to prevent future issues. The van's
conversion system was fully restored to
proper functionality, and the customer
left happy.
BRANDON STECKLER
is the technical editor of
Motor Age magazine. He holds
multiple ASE certifications.
He is an active instructor
and provides telephone and live technical
support, as well as private training, for
technicians all across the world.

http://www.VehicleServicePros.com

February 2023

Table of Contents for the Digital Edition of February 2023