Headlight issues

Technical MGB discussion
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Paulbelper
Posts: 1
Joined: Wed May 19, 2021 8:28 pm
Forename: PAUL
Surname: HODGSON

Headlight issues

Post by Paulbelper »

1975 BGT owner one week in! Came with 11months MOT but main/dip not working but flash does....any help to where to go first...I'm new to this!!
Thanks
Paul
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Bumpa
Posts: 60
Joined: Sun Dec 13, 2020 9:58 am
Forename: Mike
Surname: Howlett

Re: Headlight issues

Post by Bumpa »

Main and Dip are on entirely different feeds from the flash. The flash gets its power via a purple wire direct from the bottom fuse in the fuse box. The ordinary headlights are powered through the main light switch via a brown wire that comes unfused directly from the battery. Then a blue wire takes the power from the main switch to the dip switch. If neither main or dip are working the fault has to lie in that bit of the circuit.

Do the side and tail lights work? They also take power from the main light switch, but by a red/green wire which goes to the top fuse. Have a look at these coloured wiring diagrams from Autowire. Go to http://www.advanceautowire.com/ and click Stock Schematics on the left of the screen. You can download them as a PDF and keep them on your computer for reference. The different years are shown as separate diagrams. I think your car is on page 19, although the basic schematic is very similar for all years.

If you take off the column shrouds (four philips screws) you can see the wiring to the dip switch. If you have a multimeter check for 12 volts on the blue wire with the main light switch on - the other multimeter probe needs to be on a good earth point. If there is no power you need to trace backwards to the main switch and check for incoming volts on the brown wire. I suspect the main switch could be the culprit.

Good luck.
Mike
1969 MGB GTV8 3.9 Conversion, 1977 Triumph Dolomite 1850HL, 1971 MGB roadster undergoing restoration.
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