Morris Minor Owners Club

.........It's Mungo's MOT next week and I have been checking him over (with my limited knowledge) as to what he might fail on. One particular aspect that I have found is the rear brake lights. Withouit any lights on at all I can press the brake pedal and the rear brake lights light up but, when I turn on my headlights and side lights then press the brake pedal the rear brake lights don't come on . Is this a typical moggy thing ? if not what is there that I can do to rectify it bearing in mind that I have very little electrical knowledge.
Mungo is a '56 splitscreen. I would be so grateful to here from any of you experts out there.
regards Sid

That sounds like either a bad earth, or bad wiring (or both)
With a good earth there should be no interaction between brake lights and rear side lights.

Did the '56s have the multi relay for flashing brake lights / turn signals? This might be the problem if so.

Otherwise I'd go with Ray, faulty earth. But faulty earth on both at the same time?

....The Earth seems ok otherwise surely I wouldn't get any lights at all,the puzzling thing is I get brake lights showing when I don't have any lights at all on (whilst pressing down the pedal) all the other lights work fine, headlights , main beam , side lights, as soon as I put the sidelights on and press the pedal the side lights are still on but the brake lights don't show. There isn't any relay for the flashing brake lights as I still have semaphores and not flashing indicators.

....The Earth seems ok otherwise surely I wouldn't get any lights at all

that's far from technically correct.
If the lights aren't earthed, the light base goes to 12V when a bulb is on, this then passes backwards through the other filaments into different circuits and earths somewhere else.
This is 99% likely why you can get one light to work on its own but not both.

Follow the above advice and double check all the earths.
You must have seen cars where the brake lights start flashing if
the indicators are turned on and such like? This is because the
electrics will follow the easiest path back to earth so you can get
all manner of wierd results if there is not a good 'no resistance'
connection to the chassis. Apart from that,if you have a dodgy
battery there may not be enough juice left to operate the lamps
when the headlights etc are turned on. If you turn on only the
side lights do you still get the problem?

Make sure the feeds for the brake lights and side lights haven't been reversed, the brake light filliament are 21 watt so are brighter than 5 watt sidelight lights so brake lights wont show if 21 watt feed is been used for side lights.

honda90 wrote:Make sure the feeds for the brake lights and side lights haven't been reversed, the brake light filliament are 21 watt so are brighter than 5 watt sidelight lights so brake lights wont show if 21 watt feed is been used for side lights.

Along those lines, is it possible a single filament bulb has been fitted, and the central contact is touching both the offset ones in the holder?

It shouldn't be possible as IIRC the pins are offset, but if it's been wellied in it might just happen.