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Installing Turn Signals into an M37
Here are instructions for installation of civilian stop/tail/ and turn
lights into an M-37 3/4 ton truck (or, suitably modified, any other similar 24V
vehicle). These lights add quite a margin of safety when operating your truck,
over the original. However, I designed this set-up so that nothing original
needs to be removed, and the system here can be easily removed and
re-installed, if desired. When the lamps of this system are removed, none of
the modifications are easily spotted. These part numbers are for a 24V system,
however, all of the parts have a 12V equivalent.
Please E-mail me about any problems you run into, and with any comments
(positive or negative) you have. I appreciate all feedback.
Wire: 10 gauge, stranded copper wire: (Note: These colors
correspond to those on a Signal Stat®, 900 series, 7 wire switch.)
- Purple, 40 feet
- Gray, 30 feet
- Green, 25 feet
- Black, 25 feet
- Red, 20 feet
- Black/gray, 15 feet
The wire lengths given here are much longer than actually needed. But
stretching just doesn't work well.
Switch: 7-wire turn signal switch (such as Signal-Stat® Model
- 1, 24V pilot bulb (#356)
- 3 spade connectors (10 gauge)
- 6 "T" taps (10 gauge)
- 13 butt splices. I have used the new screw together type, called
Posi-Lock® Connectors. These are expensive, but well worth it, as they make
a very positive connection, and can be disconnected and re-connected. This is
important if you would like to remove these lights for a show or rally. They
also allow you to easily connect wires of unequal size, which this project
- Several feet of plastic wire loom (1/2" and ¾" diameter)
- 1, 24V turn signal flasher (Signal-Stat® #87114)
- 2 inches of foam double stick tape.
- 2, Single face, turn signal/marker lamps, with black housing,
Grote®, #53052; (These only come with red lenses)
- 2, Amber lenses, (Grote®, #91583)
- 2, 24V, dual filament bulbs, (#1662)
- 2, Black metal tail lamp brackets with grommets (Grote® #40720)
- 2, 24V, sealed round stop/tail/turn lamps (Grote® #40209R) with
pigtail (Grote® #94993)
- 4, 7/16" x 1" bolts, with flat washers, lock washers, and nuts
- A handful of cable ties
FABRICATE YOUR WIRING HARNESS:
Cut a 30 foot length of the purple wire. Mark one end as the "front".
Tap the remaining 10feet of purple wire to it, at a point 14 feet from the
front. The point of this tap will be referred to as the "midpoint" from here on
out. Run the 10 foot spur of purple wire off to one side.
Start at the front of your harness, and run the green wire along the
purple to the midpoint. Secure it to the purple wire with tape temporarily, in
just enough spots to keep them together. Leave the remaining 11 feet free, and
run it off to the opposite side as your purple spur. Starting a point 5 feet
from the front, run your red wire parallel to your green. Tape it to the green.
Starting at the rear of the purple wire, run your gray wire to the mid-point,
then along the red and green. Start at a point 5 feet from the rear, and run
your black wire parallel to the gray. Finally, start at the end of the four
wire spur. Run your black/gray wire to the midpoint. Leave the remainder free
from the midpoint.
All parallel wires should be joined with tape occasionally.
INSTALL YOUR LAMPS:
If your truck has a winch, there are two holes in the lower edge of the
radiator grill (brush guard). If you have no winch, remove the bolts that
secure the bottom of the radiator grill to the top of the lower grill. Drill
these holes out to 7/16". Replace the 1157 bulbs in your front lamps with 1662
24V bulbs, then install the amber lenses, in place of the red. Install the
lamps into the holes you just drilled, then install the included nut on the
stud of each. The lower two, of the three screws that secure the lenses will
probably be touching the grill struts.
Take the metal brackets, for the rear lights. Use them as a pattern to
mark and punch spots in the flange of sheet metal, parallel to the ground,
below the tailgate. Take care to put these holes far enough forward, so the
washers and nuts will clear the inner surface of vertical sheet metal, above
the flange. These brackets should be installed as far as possible to their
respective sides. Insert the bolts up, through the brackets, and the holes.
Secure with the flat washers, lockwashers, and nuts. Place the grommets into
the brackets, and install the lamps. Despite the fact that the lamps have a
side marked "TOP", turn that side down. This will leave the receptacle for the
pigtail toward the top. Snap the pigtail in place, and secure the white wire
from them under one of the bracket nuts, to ground the lamp.
INSTALL THE TURN SIGNAL SWITCH AND FLASHER:
Open the top of the turn signal switch, and replace the standard 12V
bulb with the #356, 24V bulb, for the indicator lights. Install the turn signal
switch, on the steering column, as per the instructions that accompany it.
Install a spade connector on the blue and yellow wires of the switch. Feed the
wires through the dash, next to the steering column. Use wire loom as necessary
between the switch and the opening in the dash.
Install the flasher, with the prongs facing up on the top of the flange
of sheet metal, created by the edge of the bottom of the dash being bent
parallel with the ground. Place the flasher below the steering column on this
flange. Affix the foam double-sided tape to the bottom of the flasher, but do
not peal the backing from the other side yet.
Using a "T" tap, install a piece of wire into the switched side of your
main ignition switched. Install a butt splice on the other end. I installed an
in-line fuse in this wire as well.
Do not plug the spade connectors onto the flasher yet. Wait until the
rest of the wiring is in place.
INSTALL YOUR WIRING HARNESS:
Take the mid-point of your wiring harness under your truck, at a point
on the passenger side frame rail. Temporarily secure the midpoint to the frame
Throughout the wiring process, use wire loom as needed to protect your
harness from any sharp edges, and at any points where it passes through frame
Take the spur, consisting of the black, red, green, gray, and black/gray
wires, toward the engine compartment, then feed it through the firewall. You
can either use one of the pre-existing holes for the wiring, or drill a new
hole, and fit it with a grommet. Run these wires under your dash, and affix
them to the corresponding wires of the switch.
Back under the vehicle, take the black/gray wire, which is loose. Locate
the wire from your stop light switch to the original stop light. It runs from
the switch, near the brake pedal, under the truck, across a cross member, to
the passenger side of the frame. If you have any doubt as to whether you have
the right wire, insert a straight pin through the insulation, then test it with
a meter. It should be dead, until the brake pedal is depressed (Make sure your
stop lights are switched on at the dash switch). Once you've located it, trim
the black/gray wire to length, and "T" tap it to the stop light wire, at the
passenger side frame rail.
Next, run the 10 foot, purple wire spur across the frame cross member,
then up the driver's side frame rail to the dimmer switch. Locate the incoming
headlight power wire (Not either the high or low beam only switch output). Trim
the wire to length, and "T" tap to the headlight power.
Now, take your rear purple, black and gray wires. Run them along the
passenger side frame rail, to the rear of the truck. This will entail threading
them between the frame, and the passenger side of the fuel tank, then out, in
the vicinity of your newly installed passenger side rear lamp. Trim the black
wire to length, and use a Posi-lock connector to connect it to the lamp. "T"
tap a short (3" is plenty) spur into the purple wire; and Posi- lock this to
the lamp as well. Continue the purple and gray wires to the Driver's side lamp,
trim to length, and Posi-lock both to the appropriate wires of that lamp's
Take the red, green and front purple bundle, to the front end of the
truck, along the passenger side frame rail. It is easiest to run up into the
engine compartment, and then back out, at the front of the passenger side
splashguard. I went out through existing gaps. However, it would probably look
nicer, and lessen the chance of damage to the wires, if you drilled a hole in
the splashguard, and installed a grommet for the wires to pass through. From
this point on, the wires should be in wire loom, both to protect them, and to
maintain the aesthetics. Trim the red wire to length, and Posi-lock it to the
passenger side front lamp, red wire. . At the proper point, "T" tap into the
purple wire, run another 3" spur, and Posi-lock that to the black wire of the
passenger side lamp. Run the green, and remaining purple wires, down and along
the cross member below the radiator, then up to the driver's side lamp. Be very
aware of the PTO drive shaft, if you have a winch. Trim both to length and
Posi-lock the green to the left lamp's red wire, and the purple to the lamp's
Go back to the cab, and connect the two wires from the turn signal
switch, to their appropriate spade fittings on the flasher. Then connect the
spade from your main power to the flasher. Now peal off the backing on the
two-sided tape, and place the flasher on the metal edge.
Your system is now complete. With your headlights on, you should have
rear taillights, and front marker lights. Turn signals are self-explanatory.
With the stop light switch, or the headlights on, you should have brake lights.
If everything tests satisfactorily, use cable ties, and wire loom to put the
finishing touches on your system.
By using the Posi-lock connectors, if you ever desire to return your
truck to military lighting only, you can disconnect the lamps, and un-bolt them
from the truck, and re-connect them easily, with the same connectors.