Professional_Tool_And_Equipment_News_February_2021 - 33

to both ends of the circuit as directed by
the results of each test. What this means
is since we have no wiring diagram, start
by using a circuit tester at the fuse panel,
or in this case the circuit breaker, to determine if there is power at the source.
We connected a Power Probe and
found power at the main fuse that supplies power to the switch, which made
sense because the tail and marker lights
were operating normally. We then tested the switch to determine if there was
power on all terminals of the switch.
This switch has four terminals, which
can in some cases lose power on one side
of the switch which could cause the tail
and marker lights to work and the headlamps to not. There was power on both
sides of the switch when the switch was
in the " on " position.
Our next step was to check at the headlamp for power at the connector (there
was none). Using the Power Probe, we
applied power to the headlamp to make
sure we did not have an issue with either a
ground or a burned-out lamp. The lamps
on both sides of the truck illuminated.
So far, we have determined the switch,
fuses, and lamps are all good, so we needed to determine where the flow of current
was interrupted from the switch to the
lamps. There was a bulkhead connector
at the firewall, where we have seen corrosion issues on similar vehicles. Again,
one of the problems was we could not
easily identify which wire or which of the
three large multi-wire connectors were
for the headlamps. Since it is somewhat
difficult to observe the lamps and test the
bulkhead connector at the same time, we
either needed two technicians to test this
circuit, or try a little trick I have successfully used in the past.
Set your DMM to the audible tone
ohms setting. Disconnect the bulkhead
connector and the headlamp power wire
at the headlamp. Connect one end of the
DMM to the headlamp wire, then touch
the other probe to each pin at the engine
side of the bulkhead connector. When
you hear the tone, you have found the
correct wire. Keep in mind, that many

t This picture illustrates
corrosion on the ground cable.

q This photo illustrates
rust and corrosion on the
positive battery connection
on a vehicle from Minnesota.
Road salts used frequently
in cold, northern states can
be a factor in any electrical
system corrosion issue.

heavy duty trucks use a separate wire to
each headlamp, so you should look to see
if there is a pin for each side.
Once we found the correct pin, we
could now re-connect the bulkhead connector to determine if there was power to
the pins going to the headlamps. Our test
showed no power and using the Power
Probe we were able to determine that
when we supplied power to the pins, the
headlamps illuminated.
Now that we were able to identify
which pins at the bulkhead connector
were used, we could back trace them to

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FEBRUARY 2021

see where the loss of connection between
the switch and the bulkhead was. There
were numerous wires behind the dash,
and quite honestly, not much room to get
to each connector without completely
dismantling the dash.
Since headlamps use quite a bit of current, we assumed there would be at least
one, and quite possibly two relays for the
headlamps. We had previously looked
for relays at the fuse/circuit breaker
panel near the driver kick panel and even
though there were relays located there,
none were for the headlamps.

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33
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