Fusing Tail Lights For Safety

By Norman Nock

Originally published in the Austin-Healey Magazine, June 1996

The wiring on early British cars runs under the car and then passes into the trunk. These wires are exposed to damage from rocks and items in the trunk. The harness is attached to the body or chassis rails with clips, and if one of these clips is knocked out of alignment by foreign objects, the clip will cut into the wires inside the harness, causing a short circuit.

The wires to the taillight and to the electric fuel pump were not fused in early 50’s and 60’s cars. The SU fuel pump circuit never got fused. The tail light circuit from about 1965 on started to be fused, with the fuse installed at the factory in various locations on the car – behind the panel, under the hood, etc. Later it was incorporated into the fuse box.

The damage caused by an electrical fire can be very extensive. If you want to avoid an electrical fire (“keep the smoke in the wire”), then fuse the fuel pump and tail lamp circuits. A fuse can be installed in the SU fuel pump circuit in the white wire leading from under the hood to the pump. If you install the fuse close to the ignition switch on some models, you could be double fusing the circuit. Follow Figure 1 to install an in-line fuse in the parking/tail lamp circuit in the wire close to the head light switch.

The damage that fuses in the tail light circuit can prevent is demonstrated by what happened to many West Coast MGA owners. The MGAs came by ship to San Francisco with wooden bumpers. The local MGA distributor fit the chrome bumpers by unskilled warehouse personnel. When they joined the two wires to the license plate lamp, sometimes they reversed the red (power) and black (ground) wires, not knowing that the red wire always went to the tip of the bulb and the black wire went to the side. The lights would work until the paint on the lamp stopped being an insulator. Then one night, with the lights on, the new MGA owner had a short circuit and major harness damage (lots of smoke).

I have spent many hours under the dash repairing the damage caused to wiring harnesses by short circuits caused by damage to the tail light circuit. A little fuse can go a long way to insure that the smoke stays where it’s supposed to.

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