Switch Hitter
A rebuilt JR403E transmission was installed
into a 1991 Isuzu NPR. During the initial road test it is
discovered that the charging, brake, vacuum and fuel-level
lights remain on (see Figure 1).
Besides this, there were no other
indications of a problem. The vehicle was running well and the
fuel gauge was working correctly. A quick check of all the
fuses (see Figure 2) showed that none was blown. Swapping
relays around didn’t change anything, either.
When the owner was asked whether this was a
problem before the truck came in to have the transmission
replaced, he said it wasn’t but that the alternator had
been replaced recently. Hmmm, could there be a
“connection?” (No pun intended)
A wiring diagram of the dash (see Figure 3
on page 12) was obtained with the intent of backtracking the
ground path of each bulb that was remaining on. After a good
look, a notable observation was made: Battery voltage coming
from fuse 5 was supplying power for most of the lights in the
dash. Nothing odd here, but what becomes unique and very
interesting is the ground path of the bulbs remaining lit.
The charging, brake, vacuum and fuel-level
light bulbs each have their own respective ground path that
would normally be used to illuminate the bulb should there be
something wrong with that circuit. But they each also have
their own splice and diode in their ground circuits that lead
them all into a shared circuit going to fuse 19.
Why would this be? Why would voltage be
supplied to each of the ground paths through a diode? And
isn’t it interesting that it is these four lights that
remain on, not the others? It was no surprise that when fuse 5
was pulled out, every light to which this fuse supplies power
went out. But when it was put back in and fuse 19 was pulled,
only the four lights that were remaining on went out. How odd
is that?
This would seem to indicate that somehow
fuse 19 was acting as a ground path for all four lights instead
of a power supply. After all, if each of the individual ground
paths was somehow shorted to ground, they would remain on with
fuse 19 removed. So the question was, “Where does fuse 19
get its power?” To answer this question required a
different wiring diagram (see Figure 4), and from it we can see
that fuse 19 receives power through the charging relay –
that is, if everything is working correctly.
So the charging relay is a switch hitter,
as it could provide either a ground path or a power supply.
Normally its function is to provide power. When it provides the
ground path and puts on all four lights, it means that either
the alternator is bad, the wire from terminal 1 at the
alternator to terminal 4 in the relay is broken, or the relay
is malfunctioning. This charging relay will not interchange
with the other relays, as it operates slightly differently, and
this explains why it is the only blue relay in the panel (refer
to Figure 2). 
©2006 Transmission Digest