[Trevor]

The symptoms described indicate an alternator fault which has been reported here many times. However the exact cause is rather obscure.

The dash indicator lights test circuit, incorporates blocking diodes to eliminate a sneak circuit which would otherwise exist. In an event involving faults within the alternator rectifier network, alternating current is induced on line. The result is pulses of reverse voltage, which pass the blocking diodes in the test circuit and energise the indicator lights.

A simple voltage test will not necessarily record this fault, as the alternator can be capable of supplying the required current with one diode in the network faulty, and the regulator to compensate.

Some long time ago I requested that someone with this fault, experiment by switching their multi meter to AC and then check for any reading, which could confirm the exact problem existed.

Again I say, "any takers on this one."