5 replies to this topic

[ItsNotDads]

Hi all,

 

Son’s ‘87 Mayfair has a headlight issue. Nearside very bright (&hot), offside dim.

 

Cleaned bullet connectors behind grill, cleaned earth behind washer bottle - all seem ok.

 

Stuck a meter on the headlight connectors. Nearside headlight earth to earth 0v. Offside headlight earth to earth ~ 12v.

 

It’s all electric string to us - can someone point us in the next direction please?

 

Thanks,

Andy

[weef]

Appears to be that there is no earth to the offside unit. Run a tempory earth wire to this unit and see if that fixes it. 

[ItsNotDads]

Thanks. The earth wires on the dim side are showing 12v - trying to understand where to look?

 

Earths on the bright side are earthed, no voltage showing.

[alpder]

As weef says, to confirm your diagnosis briefly dab a wire between a good earth on the chassis and the earth connection of the dim bulb - if it then lights properly it confirms a bad earth connection. Obviously, don't accidentally dab a grounded wire onto any other connection on the bulb: doing that would short your 12V supply to earth (and - hopefully - blow your fuse). So take care.

 

The earth wire on the dim side will run to a point where it connects to the chassis. Somewhere between (where you've measured 12V on the earth wire) and (the earth wire's chassis connection) there will be a bad connection. Most likely where the earth wire fixes to the chassis - either where it's crimped onto the eye or where the eye's screw goes into the chassis. But it could also be elsewhere along the wire.

 

The reason you're measuring 12V+ on the "earth" wire is that (when the bulb is switched on) 12V+ runs from the battery to one side of the bulb filament. If the bulb's earth is poor, you'll still be measuring a good proportion of that 12V+ even on the other (supposedly earthed) side of the same filament, where you should normally measure 0V.

[ItsNotDads]

Thanks. Will go out and try!

[ItsNotDads]

As weef says, to confirm your diagnosis briefly dab a wire between a good earth on the chassis and the earth connection of the dim bulb - if it then lights properly it confirms a bad earth connection. Obviously, don't accidentally dab a grounded wire onto any other connection on the bulb: doing that would short your 12V supply to earth (and - hopefully - blow your fuse). So take care.

 

The earth wire on the dim side will run to a point where it connects to the chassis. Somewhere between (where you've measured 12V on the earth wire) and (the earth wire's chassis connection) there will be a bad connection. Most likely where the earth wire fixes to the chassis - either where it's crimped onto the eye or where the eye's screw goes into the chassis. But it could also be elsewhere along the wire.

 

The reason you're measuring 12V+ on the "earth" wire is that (when the bulb is switched on) 12V+ runs from the battery to one side of the bulb filament. If the bulb's earth is poor, you'll still be measuring a good proportion of that 12V+ even on the other (supposedly earthed) side of the same filament, where you should normally measure 0V.

Appreciate your help. Yup a jumper earth restores the light as you expected. Will endeavour to find the break. Not obvious and we’ve cleaned up the chassis bolt/eyes, but it’s had new wings recently and that’s probably when something got caught or damaged we suspect. Cheers again