Mk4 Rear Lights (Number plate and brake lights)

[Spitfire74]

Hi all

Been doing a bit of work on my Mk4 Spitfire recently. One of the jobs that needed doing was to fit a wire from the right hand number plate light to the connection that goes to the rear lights. So I did that earlier. When I went to test it neither number plate light now works...despite the left hand one previously working. The bulbs are both fine. So my first question is does anyone have any idea as to why this might be? The rear lights are still working when I turn the lights on but neither number plate lamp now works.

 

Also I noticed a yellow/green wire dangling from under the dashboard. Having had to work under there recently to fix my ignition lamp I judged that I must have knocked it and not realised. I traced it to the brake light switch on the brake pedal (see below photo) but can't work out where the loose end is meant to connect to, it has a female spade connector on the loose end. When I have tested the brake lights by depressing the pedal they both come on even with this cable disconnected. Also, strangely, if I turn the rear lights on and then press the brake light the right hand brake light illuminates but the left hand light turns off and the left hand brake light does not illuminate...!?

Hope someone is able to help! Thanks in advance!

Nick

[RobPearce]

The last one is easy. Brakes applied with side lights on extinguishes LH light almost certainly means there's no earth connection on that bulb.

The number plate lights issue could also be a bad earth connection.

There are two wires on the brake light switch. One goes only to the brake lights, the other is part of the "green" circuit - fused ignition switched live. I didn't think there would be a yellow tracer on either (and Mr Haynes doesn't show one). The only diagram that shows a double-wire on the switch is for an early LHD loom, where that extra wire would feed the reverse light switch.

[Spitfire74]

Thanks Rob, I will check the LH bulb for an earth and the number plate lights too. Although there is only the one cable from each of the bulbs, through the rear panel that then goes to a double bullet connector and then connects to the LH rear lamp, so would the earth be the same as for the LH bulb, essentially same fault for both issues, LH rear lamp is not earthed?

Yes, I had looked at Haynes and Owners' Manual wiring diagrams and could not see a GY wire from the brake switch hence the confusion haha! Strange, perhaps a PO has installed a LHD loom for some reason! The connector on the brake switch does have both the Green cables attached (one shares a terminal with the GY wire) so that is clearly why they work without it being attached! I will double check to make sure my reverse lights are working though!!

Thanks again. Nick

[RobPearce]

The rear lights on a MkIV Spitfire are earthed through the clips that hold the bulb holder into the reflector and then through that to the bodywork. It's a truly awful design and a regular source of trouble.

The number plate lights changed at some point - I think on 1500s - to two separate dome-top things. Those are equally nasty in different ways. It's a while since I looked in detail at the housing on my GT6 but I think it is earthed through the bumper. It certainly looks that way in the manual.

[drofgum]

Nick,

The green/yellow wire is the heater fan low speed wire. Look at 53 and 54 in the diagram below. I tried to link it, but wound up with the diagram on the page.

Cheers,

Paul

[RobPearce]

That may well be what the GY colour code is supposed to be but it's absolutely NOT what the wire Nick found does because that's not where it's been fitted. Also, the key does not line up with the diagram.