Morris Minor Owners Club

One of the rear stoplights has stopped operating when brake applied, it works as a tail, but turns off when brake applied, the other side is fine and changed the bulbs around to check it’s not the bulb.
Had a fiddle around with the wiring, but no luck.
Any ideas? Thanks in advance

I know its obvious and I'm sure you've checked, but is the bulb fitted the correct way round? Assuming it is then possibly corrosion in the bulb holder, particularly the metal ring that the pins which protrude from the bulb cap make contact with. Poor or absence of earth contact between one of those pins may possibly cause the stop light earth to return via the tail light and through the other pin on the cap.

All looks in pretty good nick, the bulb went a couple of weeks ago so I got a new and it was fine, but then noticed it wasn’t working again, checked the bulb swapped them around to find it wasn’t the bulb. Could it be the switch? Where is the switch? Wouldn’t that affect both brake lights though?

The switch would effect both brake lights. Its located on the 4-way brake pipe union near the front bottom corner of the driver's side inner wing but it's extremely unlikely to cause the symptoms you describe, which must be related to the lamp assembly or it's associated wiring or possibly even a cheaply-made bulb!

One of my bulbs is a very loose fit and, with vibration, occasionally gives trouble. I found I got better connectivity if the bulbs had oval connections on the cap base. The ones with round connections don't seem to be as good.

I'm with palacebear , bad earth....

John

Many thanks for the advice

Remove the connections from the switch and connect them together, that will then make both light up, you can then check for a bad earth or poor connection easily. Disconnect the coil as the ignition will be on to prevent damage to the coil.

You almost certainly have an earth fault. With a 5/21W twin filament bulb there are two potentially live connections but they both use the bulb holder as a common connection to earth via the bulb base. Either the bulb holder, or the earth connection to the body, is poor. The situation may not be any different with separate stop and tail lights.

I doubt the lamp is ‘going out’ completely. It is just so dim that it is not providing much, if any, light.

The voltage is shared in proportion to the resistances in any series circuit. If there is a high resistance after the bulbs, the voltage across that resistance will be different when the tail light only or both filaments are energised.

The stop light will be down on power, but possibly not so noticeable, but the voltage now appearing across the ‘earth resistance’ will be that much higher, leaving a lower potential difference across the tail filament, which will dim. In other words, the stop light is effectively (but not quite the correct term) shunting the current away from the higher resistance path.

Think about it - if the stop filament was purely feeding back through the high tail light resistance, there would be insufficient current to light the stop filament, so one earlier suggestion is a bit off-beam, I think.

The situation may well be exacerbated if there is a high resistance in the feed side of the tail lamp

Thanks for that

Or to describe the missing earth another way..

The electricity flows into the tail bulb, but at the other end it's not finding the expected path to the chassis (and hence the battery ground). But the electric then finds a path, albeit in the wrong direction, firstly through the local brake light's filament and then (crucially) through the other brake light's filament (in the "right" direction this time). And here it finds the good earth.

So the tail bulb lights up, albeit slightly dim due the extra resistance of the two brake filaments. I bet they glow dimly if you look hard. But when the electric arrives from the brake switch, this resistive earth is pulled up to 12 volts. So the tail bulb has 12 volts at both ends of its filament. So there's no flow and hence no light.

Much the same can happen happens at the front pilot/indicator cluster. When the lights interact like this, it's because the local earth has fallen off or corroded.

Mike

And you can prove the path to ground is via the opposite side's brake bulb by simply removing that bulb. I bet that doing that will stop the first side's tail bulb working at all.

The same scenario applies with a missing earth on the front indicator/pilot cluster; only this time the pairing is between the back and the front of the car, as that's how the indicator bulbs are joined.

Marvellous thanks Mike

Nice clear description of poor earths Mike
Regards John