light in the dash of a tow truck is supposed to light up when you engage
the PTO. Here are five steps to follow when troubleshooting this
1. Make sure the light
has power supplied to it.
Because the PTO light turns on by means of a "grounding" switch
down on the PTO, it's supposed to have power supplied to the light fixture
in the dash all the time, whether or not the light is on.
2. Make sure the light
fixture itself has not failed.
PTO lights get whacked a lot. See if the bulb has burned out, or if
the socket the bulb fits into has wires or contacts that have come
apart. Test the light fixture by isolating it from the truck, then
grounding one wire and energizing the other wire. The light should
come on. I frequently see PTO lights that light up fine, but the
bulb and socket receptacle have fallen out the back of the light fixture,
leading one to think the light doesn't work.
3. Make sure the wire
has not come apart between the light bulb and the PTO.
If you've made sure the light fixture in the dash has power supplied to
it, and you're satisfied the light fixture has not failed, but the bulb
still does not light up when you engage the PTO, then make sure the wire
between the light fixture and the PTO has not come apart. This wire
goes down under the truck where road debris and a harsh environment often
dislodge the wire from the single terminal it attaches to on the PTO light
switch. This part of the PTO light circuit -- the part between the
light fixture in the dash and the PTO on the transmission -- serves to
ground the light bulb when the PTO is engaged and make the bulb light
up. This part of the circuit is not always supposed to be
"hot," but the wire in this part of the circuit does need to be
connected, not broken.
4. Make sure the PTO
light switch on the transmission has not failed.
The six-sided PTO light switch, which screws into the PTO shift cover on
the transmission, looks like a large hex bolt head with a single
electrical terminal in its center. On some PTOs, the PTO light
switch fits into a recessed hole so all you can see is its electrical
terminal. The PTO light switch grounds the PTO light and thus makes
it light up. A failing PTO light switch will sometimes make the
light come on and off when you move the PTO cable one-quarter inch up and
PTO light switches come in two
flavors: normally closed (black plastic center) and normally open
(white plastic center). If the PTO light comes on when it's supposed
to be off and goes off when it's supposed to be on, install the other
5. Consider the
possibility that the PTO light circuit is OK, and mechanical failure in
the PTO is what's causing the light to fail to come on.
For example, the PTO cable shift cover could be missing parts
inside. Or, on air-shift PTOs, a failure of air supply could make
the PTO engage less than all the way.