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MG MGB Technical - A romantic evening ends on a flat bed

Dinner out with my wife...time to leave and no headlights...parking lights ok, turn signals ok...headlights work when they are flashed with the turn signal switch (69 MGB)...Good power to the light switch from the brown wire...mystery as the other two wires should be a red wire & a blue wire...they are red and a double red...the double red has good power from the switch...the single red seems very weak yet has power...I think the double red is the red wire to the parking lights, while the single red is supposed to be the blue wire to the headlight. As it seems weak...maybe a short downline??
Bob Dougherty

A short would cook the wiring or the switch (why oh why does everyone jump to the conclusion of a short?) unless the main lighting switch was also faulty i.e. a bad connection internally (which is probably the cause in this case).

As your car seems to have non-standard wiring there is little anyone here can say. A North American 69 *should* have two reds on the first position of the switch powering the parking lights from one and the instruments and the map light from the other, plus a blue from the second position powering the headlights.

If you have 12v on the brown and two reds, but lower than that on the other wire (whatever colour it is) then the switch is at the very least part of the problem. If the switch was OK either you would have 12v on the wire feeding the headlights, or the voltage on the brown and hence the two reds and the parking lights would also be down.
PaulH Solihull

Working via the flash is a clue. It must short out the light switch so I think thats where to look first. Short across it and see what happens, you could remove that mystery double red and splash it onto the brown and see what happens. If the headlights come on it proves the switch is at fault. If you want to check for a short first I would remove the bulbs and check to ground with the switch off or the wire pulled using an ohmeter
Stan Best

Update...I have determined the non-standard wires are thus...brown is standard brown, double red is as Paul states, for parking lights & the third wire which is a red single wire "should be" the single blue wire that powers the headlights...when I jump the brown to the red (blue!) headlight wire no headlights but a clicking noise under the dash????
Bob Dougherty

That noise is the wiring letting the smoke out! :>) Sorry, it's Autumn and Lucas jokes
abound at this time of year.
A previous owner may have added a relay which would explain the red wire. The actual headlight blue wire may be connected to the relay and the red was run from the switch to the relay. A relay is not a bad idea especially if halogen headlights are used. The relay keeps the heavy load off the headlight switch, and the relay may be at fault in this case. You will likely have to lay on your back and look (listen?) for a relay. Good Luck.


Wiring diagram shows an optional Red going to fog/drive switch. Double RG to park. N is power, U is headlamps, goes through various connectors to dipswitch, coming out as UW or UR. Gotta be a blue wire or evidence of rework there someplace!

FR Millmore

Clicking has to be a relay, but it would make more sense to have two relays - one for main and one for dip - between the dip switch and the headlights, thereby taking the load off the dip-switch as well as the main lighting switch. Routing the blue via a single relay is a bit half-hearted.

Being picky again, but the headlamp flasher *bypasses* the main lighting switch and the dip-switch to connect 12v from the fused purple circuit directly onto the blue/white for the main beams. If *this* doesn't click a relay then either there *is* only one relay in the blue, or if there are two relays the flasher must be bypassing the main-beam relay, which again is a bit naff.
PaulH Solihull

This thread was discussed between 24/09/2010 and 25/09/2010

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