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1970 Mustang wiring, as well as other years of
Mustangs, can be somewhat less than adequate for
the operation of the stock headlights, much less
running the newer Halogen lamps. Often the
headlights can seem dim compared to today's cars.
This can easily be corrected and will ensure
longer life of your lighting system components
such as the headlight switch, dimmer switch, and
wiring. In the factory setup, power feeds the
headlights through the ignition switch, headlight
switch, dimmer switch, and finally the bulbs
through the factory wiring. If age has in any way
lowered the efficiency of any of these
components, dim light and component failure can
result due to component or wiring overheating.
What we have done here is install Bosch type
relays to handle the battery voltage needed to
power the headlights without straining the
headlight, dimmer, and ignition switches. The
headlights will get the full 12v power without
going through the factory system that can lower
the voltage available resulting in dimmer lights
at night. In the
schematic below, the wires are shown with the
primary colors to help you understand which wire
in the main harness is to be cut and spliced
into. RED-GREEN refers to the red wire with a
green stripe and so on. The blue wire from the
starter solenoid can be any color you wish. A
larger gauge of wire such as a 12 or 14 gauge
will carry the load better, especially if you
choose to use halogen bulbs. Some installations
even use a 10 gauge wire from the battery. Just
be sure you install the 20 amp inline fuse before
you get to the relays. The relays can be mounted
anywhere you wish out the way and out of sight.
You might even want to just mount them side by
side on a small board, painted and mounted under
the hood somewhere in the engine compartment down
in the corner behind the drivers headlamp. The
headlight harness will have to be unwrapped at
the brown connector in front of the radiator far
enough back so you can see which wires are coming
from the firewall connector and which wires go
over to the passenger side headlamp. You want to
make the cut and splice at the wires coming from
the firewall connector. Connect the relays as
shown. You will have to add wire at the point you
cut the main feeds to reach the relays. In this
type of connection, you can either solder and
heat shring the connections or you can strip the
wires back about 1/2 inch, twist the wires
together tightly and wrap with electrical tape.
Crimp or "butt" connectors can be used
however they can be bulky and make rewrapping not
look as nice. There is not much pull being put on
this particular application, therefore simply
twisting together and wrapping with electrical
tape should be adequate.
OPERATION:
With this modification, the relays act as the
switches for the headlights, sending power
directly from the battery to the lamps. The
factory switch setup simply acts as a switch for
the relays, sort of a switch for the switches.
All the factory setup does is energize the relays
when you want to turn on the headlights and the
relays then perform the actual lighting of the
lamps, bright or dim. The factory dimmer switch
still does it's job, energizing the proper relay,
bright or dim, that you desire at the time. Your
headlight switch still functions as it always has
only now the load for the headlights only goes
through the relays directly from the battery .
The inline fuse shown is to ensure power overload
or a short won't damage anything.When power is
applied from the factory setup to relay pin 85,
it grounds to the chassis and internally connects
pins 30 and 87. Power is always on pin 30 from
the battery, so when the connection is made
between 30 and 87, battery power lights the
lamps. When you have the headlights on DIM, the
DIM relay operates. When you put the headlights
on BRIGHT, the BRIGHT relay operates. That's all
there is to it. Now you can operate your
headlights in normal fashion and the factory
lighting components won't be stressed by the
load.
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