The above diagram shows how to modify the trucks brake wiring to control the trailer brakes while using
an exhaust brake. The dark line are new wires you will install.
Locate and cut the white wire, circuit #17, in the corner of the connector under the steering colume. You
may test that you have the right wire before cutting by sticking a pin through the wire and taking a voltage reading while
the brake pedal is on and off. The voltage will follow the brake pedal application. You will use both side of this wire latter
so cut it to allow later connections.
I mounted my relay on the 3/4 in pipe that crosses below the dashboard using a tie wrap through the screw
whole.
Get a 12 volts source from fuse #1, circuit # 140 an orange wire, in the fuse block next to the drivers door.
This goes to post #87 on a relay.
Attach a wire to the output side of your exhaust brake switch and attach it to post
#85 on the relay. Ground post #86.
Attach a wire to the side of the white wire, circuit #17, that you cut that does
NOT now have voltage on it when you hit the brake pedal. This side is the feed to the directional signal switch.
Attach the other end of this wire to post #30 on the relay.
Attach a wire to the hot side of circuit #17 and attach
the other end to post #87A of the relay. The drawing shows this wire coming off from the wire that feeds the Center Light
on the truck. That is the same point electricaly I just had picked this point up earlier from the center light when I
installed my trailer brake controller. You will agreee once you see how hard it is to get to the brake switch as
it is hidden up over the steering colume. Drawing also shows the wire going to the trailer brake controller wiring but does
not show the rest of the trailer brake controller wiring. I am assuming you have alrerdy completed thst installation and just
show the one wire from referance.
When completed your brake light will work as original with the voltage to the directional
switch going through the Brake Switch and the Normaly Closed Contacts of the relay. When you use the exhaust brake the relay
closes the Normaly Open Contacts of the relay and feeds the brake lights without appling the trailer brakes but allowing
you to coast with the exhaust brake and trailer lights on. This is a fail safe design in that even if the relay fails your
normal brake light operation will still work.
