Spitfire MkIV - Indicator wiring confusion

[Mjit]

So had to swap the indicator switch on my Spit. the other day and referenced the wiring diagram to double check connections.  Job done and everything working...but something about the diagram just isn't making sense to me,  What am I missing..?

So power comes from the battery, via the ignition switch, fuse, and flasher unit to the indicator switch.  If you flip this to indicate left power then flows from the switch to the left bulb at the front/through the night dimming relay to the left bulb at the read.  So far, so good but after the indicator switch the wire splits, with another spur going to the turn repeater bulb, and from there back to the other/non-connected side of the switch...and it's split wire that goes to the right side bulbs.

Now 12V are coming in to the repeater bulb and it lights up so must also be getting to earth, and that route to earth is via the right side bulbs.  So why don't the right side bulbs illuminate too?  They don't, and I'm sure it's all perfectly logical, but I can't see it and it's annoying me! why I can't work it out

[Colin Lindsay]

I'm going to suggest that it's not going back via the dashboard warning bulb; it's two parallel connections that both lead to the bulb and then to earth at the bulb holder. It may look like a loop but they don't link up, each earths individually at the bulb.

[NonMember]

7 minutes ago, Mjit said:

Now 12V are coming in to the repeater bulb and it lights up so must also be getting to earth, and that route to earth is via the right side bulbs.  So why don't the right side bulbs illuminate too?

Because the Borg are wrong.

The current flowing through the repeater bulb is quite small. It's a 2.2W bulb so, at 12V, that means 0.15A is required. The indicator bulbs are 21W each, in parallel. So they would normally draw 3.5A between them. They can't, because the repeater can only conduct that 0.15A due to its resistance. At such a low current, the indicator bulbs don't even get warm, so they're actually showing less than the 3ohms or so indicated. They're probably less than an ohm, in fact, so the repeater doesn't even notice that.

So you see, it all works because of the bulbs having different resistance, thus proving that resistance is definitely not futile and the Borg are wrong.

[NonMember]

2 minutes ago, Colin Lindsay said:

I'm going to suggest

Sorry, Colin, no cigar. It's just a simple bulb with no earth connection.

[Mjit]

Colin, you're suggestion is what most normal people would have done - take spurs from each side of the indicator switch, into a common wire, into the bulb, out the bulb and to earth.  It's not what Triumph decided to do in this case though!

You actually ARE powering the opposite side turn bulbs, just with so little current the fillament never gets anywhere near hot enough to emit light and is just acting as a piece of wire.  I can't decide if it's the work of a madman or a genius

[NonMember]

30 minutes ago, Mjit said:

what most normal people would have done

Where "normal" means "not involved in the motor industry and not competent to design electrical systems". In fact some versions of the Dolomite range had separate left and right repeaters but...

32 minutes ago, Mjit said:

take spurs from each side of the indicator switch, into a common wire

doing that turns them into hazard warning lights because that "common wire" makes both sets of indicators the same circuit.

[Colin Lindsay]

55 minutes ago, NonMember said:

Sorry, Colin, no cigar. It's just a simple bulb with no earth connection.

Ah well, I tried. Twas a good guess tho.

[Peter Truman]

So does that mean if you put a LED bulb in the dash indicator light it will only work with the indicators on in one direction as there won’t be continuity thro the bulb .

[NonMember]

10 hours ago, Peter Truman said:

So does that mean if you put a LED bulb in the dash indicator light it will only work with the indicators on in one direction as there won’t be continuity thro the bulb .

Yes, unless it's specifically designed to be bipolar (which is uncommon but there are some out there for precisely this reason).