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MG MGB Technical - Main beam vs 'Flash'

Hello,

I have a 1973 MGB. The flash position on the column switch works as expected (i.e. main beam flash with blue indicator lit on dash), however the main beam position doesn't work - this happened one evening when it was on main beam and then suddenly went off (including the blue indicator)- (regular dipped headlights and flash continued to work as normal)

I've given the contacts a gentle clean, but can't seem to get them to work again. Where else should I look in the circuit? I'm a little confused as I thought it was only the switch that was the difference between the main beam and flash circuits...

Can anyone help?

Cheers
Nat

Nat,
the column switch sounds like it might be the problem, are you sure the actual stalk is fully and correctly located into the switch mechanism, otherwise is contact being made for main beam on it?

Does the column switch work in all other positions with headlights on, i.e. indicating left and right on dipped and main beams?

IIRC flash is main and dip beams together, I should really know as I only looked at the column stalk on mine the other week.

Do disconnect your battery(s) before looking and sorting as the headlights themselves aren't fused (from factory at least) but looks like the flasher supply is.

As well as the wiring diagram in the Driver's Handbook have a look at Paul Hunt's excellent site mgb-stuff and if still stuck again here.

http://www.mgb-stuff.org.uk/electricstext2.htm#dip

http://www.mgb-stuff.org.uk/electricstext2.htm#mainbeam

http://www.mgb-stuff.org.uk/electricstext2.htm#flash
Nigel Atkins

Sorry, ignore me about the flasher having its own fused supply as I've just seen that's for earlier cars than yours - "After that [1970] they were powered from the purple fused and always live circuit."
Nigel Atkins

The headlamp flash circuit is entirely separate from the light switch circuit. So, power comes direct from the battery/alternator via a brown wire to the headlamp switch. When headlamps are selected the power goes from there via a blue wire to the dip switch and the dip switch sends it either to the dip or main beams. This circuit is not fused.

The flash switch has a separate power feed from fuse 4 via a purple wire. Operating the flash switch sends the current to the high beams and warning light (not the dip beams) by the same blue/white wire that the ordinary lighting circuit is connected to.

So to answer your question, something is interrupting the feed from the dip switch when main beam is selected. It can't be anything else or the dip lights wouldn't work and/or the flash wouldn't work. I suspect the dip switch is the problem.
Mike Howlett

This problem has to be the switch itself.

Yes the flasher circuit has a separate power supply but the stalk moves the contact that is connected to the main-beam wiring between the lighting 12v supply when in the main-beam position and the flasher supply when in the flash position. Another contact on the other side of the stalk dose much the same but just opens and closes the dip contact when appropriate. There is only one wire into the switch for headlights and one wire out for the main beam, so the problem has to be the main beam contacts not closing when in the main beam position. The attached may help, image 1 dip, image 2 main, next post flash.






paulh4

Flash position


paulh4

Nat, I had the exact same issue last week; also 73 BGT. I found that the high beam contacts were dirty, cleaned them, and the problem was solved. Yours may need a bit more than the "gentle clean", I would urge you to have a more thorough go before you dig any deeper. HTH ... John.
J P Hall

Thanks for all your replies, yes, everything else (indicators etc) works ok.

The switch itself isn't that old (maybe 4-5 years), but I will try attacking the contacts a bit more -from what I can see, the metal contacts do seem to touch like in Paul's photos.

cheers
Nat

At 4-5 year unless it was New (very) Old Stock it could be in the band of piss-poorly made.
Nigel Atkins

Bearing in mind that the lighting circuits (other than flash) are unfused, try pressing that centre contact firmly against the upper (in the photo) fixed contact.

There is a copper strap connecting 12v from the dip side to the main beam side so it's unlikely that broken. However the 12v wire is connected to the dip contact ('A' in the first attached) and one of the rivets connects that up to the strap ('B'). Then the second rivet connects the strap to the main beam contact, 'C' in the second attached.

Try checking for 12v on the strap and each of those rivets and it should locate where the problem is, which may be fixable and save the cost of a new switch. You may note from the pictures that I resuscitated this switch a couple of times before I reluctantly sprung for a new one.






paulh4

It turned out that it was a dodgy connection at the rivet at position B - fix in the photo attached!

Thanks for everybody's help!


Nat

Well done.

Thanks for reporting back.
Nigel Atkins

Well done, looks a bit like mine now :o)
paulh4

This thread was discussed between 01/11/2020 and 08/11/2020

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