MGB High beam problems

[Carlbanan56]

Hi guys, I have been having problems with my left high beam. It has been working on and off for a time. One day it would work without problems. Other days it would just randomly shut off and not come on. Today I decided to fix what I fought to be just a corroded contact surface. After removing the bulb cover I checked the contacts and I had 12 volts at the bulb cables. Must be the lamp then! But when I checked the bulb to a sper 12 volts battery lit it up. This has gotten me very puzzled. If I have power to the cables and the bulb is working, why isn't lighting up when it's plugged in?

It's a 1975 MGB

Every suggestion and insight is greatly appreciated

Young and inexperienced

/Carl

 

[YakkoWarner]

My universal answer to these kind of gremlins is - check the grounding. A bad ground would let you measure 12V at the cable (from low or high beam to ground) because the meter draws almost immesurably small amounts of current - but a normal headlamp pulls around 3-5 amps of current and a bad ground won't pass that much current, hence no light.

On every B I ever owned I added ground wires to ALL the lamp fixtures. The headlamps already have a ground wire in that 3 pin plug that goes to the bulb, but the other end goes into those bundles of bullet connectors that are behind the horns...corrosion or dirt in there can cause you all sorts of headances. Thats where I'd start disconnecting and cleaning connections.

 

[Carlbanan56]

My universal answer to these kind of gremlins is - check the grounding. A bad ground would let you measure 12V at the cable (from low or high beam to ground) because the meter draws almost immesurably small amounts of current - but a normal headlamp pulls around 3-5 amps of current and a bad ground won't pass that much current, hence no light.

On every B I ever owned I added ground wires to ALL the lamp fixtures. The headlamps already have a ground wire in that 3 pin plug that goes to the bulb, but the other end goes into those bundles of bullet connectors that are behind the horns...corrosion or dirt in there can cause you all sorts of headances. Thats where I'd start disconnecting and cleaning connections.

That sounds lika probable cause. I will check this as soon as possible. Thank you so much

 

[Sarastro]

...... I have power to the cables and the bulb is working, why isn't lighting up when it's plugged in?

Because you have an intermittent bad contact between the socket and the bulb. If it was a bad ground, you would see the same effect on both the high and low beams, since they share the same ground conductor. So I don't think that's it,at least in this case. But, regardless, the suggestion of running solid grounds is a good one.

You might fix this by spraying electrical contact cleaner into the connector, or denatured alcohol if that's all you have, then push it on and off a few times. Might help, but it probably will not stay fixed unless you replace the socket.

 

[Carlbanan56]

I checked the ground and as you predicted was it pretty weak ground. I used some sandpaper to make sure that the cables had good ground. This solved my issue, thank you!