Headlamp relays

Details

Written by Stephen Tickell

Magnette headlamps (in common with other British cars of the 50s/60s) receive power via the dashboard and foot-dipping switches, resulting in a significant voltage drop due to resistance in the switch contacts. The effect on light output is actually more than might appear, so a drop of only 2V can result in a nominally 55W bulb receiving less than 40W.

The most common solution is to power the headlamp main and dipped filaments by separately switched supplies direct from the battery/dynamo, using relays, which have much lower contact resistance.

There are generic kits for headlamp relays such as this one from Moss. They avoid the need for crimping tools as the relays are already wired with bullet connectors to go between the existing wiring loom and headlamp wiring (plus a heavy-duty terminal for power) However they may have too much cable length for a tidy job, or possibly not enough in some situations

Some installers place the relays on the bulkhead as this is the most common position for original equipment. However they can also be mounted on the inner wing near the normal junction of the main loom and the headlamp tail, which avoids the need to extend cables on at least that side - although it will require extra cables to the other wing. Unswitched power can conveniently be taken from the fusebox - if the screw terminals have space for another wire - or from the regulator box

The original headlamp wiring is not fused, so it's a matter of choice whether the relay supply should be fused - the kit above does include it. A fuse would reduce the chance of a short-circuit causing fire, but could itself introduce some voltage drop across the fuseholder.

With well-terminated connectors the voltage measured directly across the bulb terminals should be very close to battery voltage (typically 0.25V less), so light output is maximized