Most of us have followed a car and observed that every time the driver braked one or both rear lamps were extinguished and the result in either case was puzzling and rather dangerous. The cause is almost always that rust has eaten away any possible earth return from a lampholder with a double filament bulb. Triumph rear lamp holders are mounted in an area where rusting proceeds at a very high rate and can produce the following symptoms:
(a) TR2/3/3A Front: The side lamp works, but when the flasher is used the appropriate (left or right) side lamp is extinguished and the flasher filament is not energized.
(b) TR2 Rear: Ditto
(c) TR3/3A/4 Rear: The side lamp works, but when the brakes are applied the side lamps (or perhaps one only) are extinguished and the brake lamp filament is not energized.
Bearing in mind that double filament bulbs for brake/side or flasher/side lamp duty have filaments rated respectively at 18 and 6 watts, it follows that the resistance of the brake or flasher filament will be much less than that of the side lamp filament (in inverse proportion of the power ratings, in fact). You can then see that if we were to connect both filaments of the bulb in series and place them across our battery (nominally 12 volts), about 3/4ths of the voltage available (say 9 volts) would be dropped across the side lamp filament and only 1/4th (3 volts) across the brake/flasher filament. Now 9 volts is enough to make a filament light, not as bright as it should be of course, but 3 volts will scarcely make it even glow red. Returning now to the troublesome lights on the car itself, let's imagine that rust has eaten away the earth return of the rear right (RR) lamp holder on a TR2. With the side lamps only on, the current flowing to the RR side lamp filament cannot return via earth, and so passes through the RR flasher filament and finds itself then at the common connection with the front right (FR) flasher filament, through which it then passes to get to the earth return. Now, despite the fact that the current has passed through one side lamp filament and 2 flasher filaments on its journey we still find 60% of the available voltage appearing across the side lamp filament (it will in fact be a greater proportion because as the filament heats up its resistance increases) and it will light.
Now, what happens when we decide to turn right and operate the flasher switch? We apply our 12 volt supply to the right flasher filaments and the front one will be energized. At the rear we have now no potential difference between the side lamp supply and the flasher supply and no earth, and without the potential difference the current will not flow; the side lamp therefore goes out as the front flasher operates and comes back on between front flashes.
The solution is, of course, to run an earth return from the offending lamp holder (the rear assemblies have a scroll type connector that will receive a snap connector "bullet") to some part of the bodywork that has escaped the ravages of rust. If you have fitted fiberglass body panels you must of course run earth returns for all lamps mounted thereon, otherwise they'll never work. Don't forget that the headlamps take a hefty current and use a correspondingly heavy gauge of wire for the earth returns here.
Ian Cornish
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