MG TRIUMPH - Custom Fit Harnesses
Older cars such as the MGB, MGB GT, MG Midget, Triumph Spitfire, Triumph GT6 and Triumph TR6
power the headlamps through the switch on the dash. This is far from optimal. Modern cars use relays
to ensure full power to the headlamps. We offer two options to improve your forward lighting. First is a
Harness Kit that includes all of the hard to find parts. You supply the wire and build the harness
yourself. Wiring diagram is included. The second option is to purchase a complete plug-and-play
Get full voltage to your headlamps!
Custom Fit Harnesses
Our harnesses include premium components and 14 AWG GPT wiring for headlamp connections and 12 AWG GPT wire for power
connections. This is a plug-and-play headlight harness, no wires need be cut. A trigger connector uses the stock headlight wiring and
switch to control the relays that supply the power to the headlights. Our harnesses feature heavy wire, dual circuit power, full “home
run” ground wires (most models) and diode relays.
PREMIUM Custom Fit Headlamp Wiring Harness
This harness is a huge improvement over factory wiring. It utilizes relays, has heavier wire and does not require any cutting of the
factory wiring. It will provide full voltage to your stock sealed beams or upgraded H4 Hella or Cibie conversion headlamps. This allows
you to run the popular H4 60/55 and even higher 70/65 watt bulbs in your conversion.
• Two Bosch/Tyco 40 amp diode relays.
• Fused dual circuit 12 AWG GPT wire with ring terminals bolt to battery positive (+), starter (+) or voltage regulator depending on
• 12 AWG GPT wire ring terminal bolts to battery negative (-).
• 16 AWG GPT trigger cable plugs into stock headlamp connector.
• 14 AWG GPT wire to headlamp bulbs.
• Heat resistant plastic headlamp bulb connectors.
• All connectors are crimped and soldered
NOTE: If you want to run high watt bulbs you can upgrade to our Premium-Plus Harness (12 AWG wire and ceramic bulb connectors).
Before doing this you should consider your headlamp’s ability to handle the heat and your alternator output!
Checking Headlamp Voltage Drop - Do I Need to Upgrade My Wiring?
If you have an older British vehicle, YES!
Before you spend the money for upgraded headlamps and bulbs, you might want to evaluate your wiring. First, with a good voltmeter,
measure the voltage output of your alternator. With the engine off, clip the leads of the voltmeter to the alternator and tie them back so
they don't get tangled in the belts or the fan. Start the engine and run it up to about 2000 rpm. Note the voltage and shut off the
engine. Now move the positive voltmeter lead to the back of the bulb, you may need to pierce the insulation to do this, or remove the
bulb and put the lead in the bulb socket. (The bulb needs to be present as testing must be done under load.) Leave the ground lead
where it was. Now start the engine again and turn on your lights. Note the voltage, shut off the engine, remove the test leads and if
you needed to puncture insulation, seal it with silicone RTV. If your voltage drop is over 1 volt, you have some repairs to make before
you upgrade your bulbs. Look for loose or corroded connections, loose or corroded fuses or relays. Repair any problems. If you
cannot get the voltage in an acceptable range, it is time to upgrade to a new harness. It is HIGHLY RECCOMENDED that any vehicle
that does not utilize relays in the headlamp switching circuit upgrade to a harness.
For those in the Denver metro area we have a special test tool that we use to measure voltages and determine if the
headlights are power or ground switched. Give us a call or send us an e-mail if you want your car tested.
Why Use Relays?
Power for the headlights is controlled by a switch on the dash. This is NOT a great place to tap into the system, for two reasons: The
headlamp switch uses tiny, high-resistance contacts to complete circuits, and the wire lengths required to run from the battery to the
dashboard and all the way out to the headlamps creates excessive resistive voltage drop, especially with the thin wires used in most
factory installations. (18 AWG in most older British vehicles.)
In many cases, the thin factory wires are inadequate even for the stock headlamp equipment. Headlamp bulb light output is severely
compromised with decreased voltage. The drop in light output is not linear, it is exponential with the power 3.4. For example, let's
consider a 9006 low beam bulb rated 1000 lumens at 12.8 Volts and plug in different voltages:
10.5V : 510 lumens
11.0V : 597 lumens
11.5V : 695 lumens
12.0V : 803 lumens
12.5V : 923 lumens
12.8V : 1000 lumens (Rated output voltage)
13.0V : 1054 lumens
13.5V : 1198 lumens
14.0V : 1356 lumens (Rated life voltage)
14.5V : 1528 lumens
The Europeans take a slightly more realistic with their voltage ratings; they consider output at 13.2v to be "100%". The loss curve is
the same, though. When operating voltage drops to 95 percent (12.54v), headlamp bulbs produce only 83 percent of their rated light
output. When voltage drops to 90 percent (11.88v), bulb output is only 67 percent of what it should be. And when voltage drops to 85
percent (11.22v), bulb output is a paltry 53 percent of normal! It is much more common than you might think for factory headlamp
wiring/switch setups to produce this kind of voltage drop, especially once they're no longer brand new and the connections have
accumulated some corrosion and dirt.
Triumph TR6 Custom Fit Harness
Triumph Spitfire Custom Fit Harness
MGB, MGB GT Custom Fit Harness