The solution was after all a very simple one.  It turns out that there are two separate filaments in the low beam headlight bulbs.  When the lights are on LOW beam (yellow wire) one of the filaments is being used.  When we switch over to HIGH beam (white wire) the other low beam filament is used.  Simple eh.

However, if (like me) when you do the visual inspection of the bulb and notice both filaments are intact you assume that there's a bigger problem (WRONG), PUT A METER ON THE PLUG FIRST and check there's 12 volts coming through it (there was on mine)!!

Tuned out that my right low beam bulb had an internal break in the housing (out of visual sight) on one of the filaments causing my Low beam right light to (APPEAR TO) switch off as I hit the high beams, not so it was just switching over to the defective filament.

Excuse my ignorance on this one, this is the first time in over 100,000 miles and seven years of ownership that I've even touched the front lights (not bad IMHO).

The dealers approach would have worked here.  "Don't analyse it nor try to fix it...... just rip it out and swap it out for a new one".  Mechanics motto 101!!!!!!!

Thanks all,
Chris Gibbs

Joel Walker wrote:

  ok, looking at bentley wiring diagram, you have three wires going to the headlights: yellow - power to low beam only, connects to terminal 56b on the back of the low beam.white - power to high beam AND low beam (when high beams are selected). connects to terminals 56a on the back of the high beam and the back of the low beam.brown - ground to both high and low beams. connects to terminals 31 on the back of each light. from what i can see (and what i recall from my square headlight days), there's a white jumper wire between the two bulbs. sounds like when you select high beam, no power is getting over to the low beam via that white wire. but the yellow wire is getting power when low beams are selected (back at the headlight relay). i'd check the back of the headlights and see if the white jumper wire is loose/frayed/corroded/missing and just clean up all the connectors, especially the white wire ones. and the brown/ground ones ... you can never have too clean a ground connection. ;) good luck!joel