dim headlights on dip only?

[iconic]

Just failed my MOT on this problem. 1965 two door saloon.

Mainbeam is bright on both lights and the mainbeam idiot light works too.

Dipbeam is dim on both lights and strangly the mainbeam idiot light is also dimly light....?

The lights have individual earths to the inner wings and both seem sound...but I also guessing that if the mainbeam is fine then the earths shouldn't be the problem as this would also give dim mainbeam?

Do the main/dip switches ever go bad?

I know there isn't too much in the way of wiring for the moggy but any suggestions where to look first would save some time.

many thanks

[bmcecosse]

Looks like a bad earth problem to me - check the earths carefully.

[mike.perry]

Check the blue/red wires at the bullet connectors from the dip switch to the headlights

[bmcecosse]

The fact that the high beam warning light is glowing tells me the dip beam lights are earthing out through the main beams. But I'm not convinced this is an MOT fail point (although it obviously needs fixing) - is there a requirement for headlights to be at a certain brightness ? I'm not aware of that......

[IslipMinor]

This is from the MOT Inspection Manual as some of the 'Reason for Rejection' for headlights:

a. inoperative, excessively damaged or
deteriorated or has a product on the lens or
light source so that the light output is well
below that required to illuminate the road
ahead

b. an obligatory headlamp shows light that is
not substantially white or yellow

Dim headlights would likely fail on both counts??

[andy.pointeer]

Also check the blue/red wires at the bullet connectors behind the headlights either side of the rad. Both sides join behind the drivers side headlight and may be a bad connection at that point. You may need to remove all bullets and clean.

[bmcecosse]

Thanks Richard - does look like they have covered it there!
I am still baffled by this one- since I would expect 'poor earthing' to effect dip and main beams pretty much equally - yet the glowing warning light does show there is current flowing in the main beam circuit when the dip beam circuit is energised. Were the lights ok before - and now suddenly are like this ? If the connections on the back of the lights have been connected wrongly - with 'main' wire on the lamp connector that should be 'earth' (and vice versa) - I wonder if that would give the result here? Has the wiring or connectors been tampered with recently? Main would work well, and dip would have to find earth through the main filaments - also making the warning light glow. Look inside the headlights when on 'dip' - are BOTH filaments in each lamp glowing - relatively dimly ?