[Penthode]

There is only one of two reasons for the lights dimming. But first, what is the ammeter reading?

1. If the ammeter indicates a heavy discharge (needle pegged negative), then there is a short circuit in the brake light circuit is providing an extra heavy load. CAUTION: DISCONNECT THE BATTERY AND LOOK AT THE BRAKE SWITCH AND WIRING TO THE TAIL LIGHT.

2. If the a meter does not show a heavy discharge, then there is a poor connection (high resistance) between battery terminal and the common distribution point shared between the headlamps and the brake circuit. The common point is generally at the head lamp switch at the base of the steering column which means the high resistance will be before this point.

If following number 2, to determine where the high resistance point is, you need a digital voltmeter to measure the voltage drop at various points.

With someone stamping on the brakes, use the voltmeter starting at the battery. Take note of the voltage to ground and then move progressively along the circuit taking measurements until you find a higher than expected voltage drop. (You will find a normal drop of 0.2 to about 0.4 volts between battery and lamp connection. If it is more than 0.5 volts, the is a bad wire or connection in the circuit). This way you can isolate the high resistance section causing the drop responsible for the dimmed lights.