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MG Midget and Sprite Technical - Dashing Lamps Wiring/Earth Fault

Hey folks, hope you are all well.

I have a strange issue with my dashboard wiring which I have been trying to trouble shoot today, but I think it has beaten me!

1. Everything off, turn hazards on (just to test) and they work.

2. Turn dash panel lamps on (headlight switch on first of course) Faint glow from indicator lights on dash and ignition light comes on. Hazards no longer work.

3. Pull all panel bulbs and fault goes away if I take panel light bulbs out of tacho and speedo. Put bulb back in one, other or both of them, fault reoccurs. Panel lights on, no indicators/hazards.

4. Fault does not occur on other gauges, fuel etc. Lights in those work with indicators/hazards. Just tach and speedo.

With the dash off, I had -2v at the dash indicator bulbs when panel lights on. I messed about cleaning up earths etc and the problem seemed to cure itself (not sure how or why though!). I put the dash back on... doesn't work again! Although the indicator bulbs don't illuminate enough to be visible anymore.

Ignition light comes on with panel light if there is a bulb in the speedo or tacho.

I like to think I am OK with electrical stuff (I know some folk just don't get it!) but this has me stumped as to how and where my electricity is going!!

Thanks,
Malcolm
M Le Chevalier

Couple of clarifications.

If you take the bulb out of the tacho and put the bulb holder back in empty, it is OK.

Is the wiring and everything else behind the dash supposed to be neat?! As much as I try to tidy it all up it still ends up a mess and I just stuff it all up behind out of sight out of mind!

Malc.
M Le Chevalier

Malcolm,

Sounds like an earth problem on the gauge cases. Have you tried a supplementary earth connection?
I'm not 100% sure how the cases are earthed on a midget but... I see there are two ways of achieving a ground connection;-
1, via the mounting posts and locating tabs. This relies on the contact point to the dash being clean.
2, by attaching a ground lead to the mounting posts under the knurled locking nuts.

They all look like a rats nest under the dash!!! as long as the loom is tied up to the bulkhead a long it's length there's not much you can do...

Best of...
MGmike
M McAndrew

The cases are earthed via method 2.

So, as I now understand it... the bulb holder to case connection is good, but the earth cable to the case is bad, so the 'leccy can't find earth the proper way so finds a new route, flowing "backwards" through the earth of something else, (ignition lamp for instance) causing muchos electrical confusion.

I will try to secure the earths much better tomorrow night.

Malc.
M Le Chevalier

Are your rear lamps earthing properly to the bodywork?

Try an earth wire to the lamp bodies.

You might find that the tail lamps are finding their earths through the indicator bulbs and coming back to the indicator lamps on the dash.
Dave O'Neill 2

Thanks for the suggestion Dave, should have added, tail lamps/headlamps not attached yet. Was just testing the dash itself. Sorry.

Cheers,
Malcolm
M Le Chevalier

Malcolm said:-

"So, as I now understand it... the bulb holder to case connection is good, but the earth cable to the case is bad, so the 'leccy can't find earth the proper way so finds a new route, flowing "backwards" through the earth of something else, (ignition lamp for instance) causing muchos electrical confusion."

yep something like that ;)
If you unplug the alternator you might find the lights go out, but there again who knows with this 'leccy magic.

Try a continuity check from the gauge case and a known good earth, I suspect you may have a broken cable somewhere which made contact for a while when you were investigating.

Best of...
MGmike


M McAndrew

Brrrr! It is cold out! However, we are victorious, thanks Mike. Yeah earth problem of some sort.

Took dash off, removed earths on tach and speedo, cleaned up the connectors with wet and dry paper, cleaned up the threaded posts with a small brass wire brush and ran a tap through the thumb wheels and cleaned up their surfaces with wet and dry.

Cleaned up the four way earth bullet connector behind the dash too.

All reassembled and quickly tested and I was dazzled at the brightness of my tacho and speedo back lights! :-D and all working like it should.

Thanks again,
Malcolm
M Le Chevalier

This thread was discussed between 25/01/2015 and 26/01/2015

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