1977 343 Dashboard Lights

[S10NPH]

Hi all

Gosh it's been a while since I've posted on here, oh dear

Anyway, I wonder if anyone has any bright ideas on the following problem I am having with my 1977 343, picture for a reminder:
TUJ by Siôn Hudson, on Flickr

It went in for its MOT yesterday, and failed on electrical problems, mainly to do with the dashboard lighting.

It's been intermittent for months, and obviously has become permanent once the car went to the MOT station!

Here are the symptoms:

When the ignition is on and nothing else is operating, the right indicator and fog light glows:
Ignition on by Siôn Hudson, on Flickr

When the headlights are switched on, the fog light and the right indicator both glow more brightly
Lights on by Siôn Hudson, on Flickr

When the fog light is switched on, the fog-light indicator in fact goes off!
Fog light on by Siôn Hudson, on Flickr

Main beam
Main beam on by Siôn Hudson, on Flickr

Hazard warning lights on (the illuminated lights flash)
Hazards on by Siôn Hudson, on Flickr

So clearly, this is not right. I spent a couple of hours rooting around last night, and my guess is that there is a bad earth for these particular lights which all share an earth, which means that the dashboard lights are earthing to each other rather than to actual earth. Wire 11 (12 in the manual but it's a double thickness connector on the dashboard) coming out of the multiplug above the battery gauge is the earth. In the manual, it's listed as dark brown but the reality of it is that it's a fairly insipid shade of orange like most of the wires of the car are. It disappears into the fascia wiring loom with another 30 nearly-identically coloured wires, and then I have no idea where it goes to actually earth.

Here is a wiring diagram from the service manual for the headlamp circuit, where you can see that on the dashboard all the affected bulbs share the same earth through wire 11/12.
wiring by Siôn Hudson, on Flickr

Does anybody have any idea where this wire physically earths to?

[benji1985]

The brown wires are all earth, there is an earth point under the fuse box.
Best suggestion is create a earth wire from the battery and start adding your earths to this wire, it will pin point the fault, also use a volt meter with closed circuit time and resistance, and just go through the system step by step.

Ben

[macplaxton]

I think there's an earth point behind the instrument panel above the steering column...but it's been a long time.

[S10NPH]

Thanks - I had another poke around over the weekend and learned some more. The earth wire from the dash goes into the four light and heated window switches, and earths each of them in series. The earth then travels into a multi-plug at the bottom of the steering column shroud. I am not quite sure what happens after this point, as after wiggling the multiplug the electrics started behaving themselves again.

This therefore quite possibly accords with what you say Rich, so I will have a closer look under the steering column and see if I can work out what is happening and make sure that the earth doesn't fail again.

[S10NPH]

I have not had any further luck with this. The only thing I have managed to do is learn how to dismantle most of my dashboard, because my car currently looks like this
IMG_1677 by Siôn Hudson, on Flickr

So the fault manifests itself as a bad earth in the instrument cluster. All these bulbs share a single earth, which is tab 11 of the left hand multi-plug here:
IMG_1676 by Siôn Hudson, on Flickr

I have traced that wire 11 from the instrument cluster goes into these four light switches in series:
IMG_1679 by Siôn Hudson, on Flickr

After that it goes to the nearest rectangular multi-plug under the steering column, where I think it earths the indicator stalk:
IMG_1678 by Siôn Hudson, on Flickr

After that it disappears into the wiring loom behind the clock and it's extremely fiddly to see where it goes further without taking the loom apart. I wonder if it goes then to the hazard warning switch? And then where?

I am almost certain that the same earth issue existed with my old instrument cluster, so I do not think it is the cluster itself which is at fault.

I would love to know where this mysterious wire ends up physically earthing to the metal of the car, after going through all these various lighting components: if that connector is dirty and corroded then problem solved. If not, then I can go back along the series and see if one or more of the various switches along the way is fouling the circuit.

Any bright ideas? I may have to call an auto electrician at some point, as I have spent many hours trying to get to the bottom of this but with no success. I need to get the car MOTd so I can drive it to its garage to spend the winter out of the elements.

[benji1985]

on the later models there is a 4way earth point directly under the fuse box, it might be worth looking at that and also just splice in a temp earth and see if that solves the issue?

Ben