MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Intermittent Headlight problem

'73BGT, headlight switch does not always turn the lights on. Appears to be an intermittent problem - throwing the switch a couple of times seems to work.
Just cleaned the fuse box, and will see if that does anything. Next step will be to take out the switch and clean all the contacts there.
Any other ideas/recommendations welcome,
Pat
Patrick Callan

The likely suspects are: headlight switch; dipswitch; or any number of corroded bullet connectors between the switches and the lamps.

Cleaning the fusebox won't help much, as the headlamps aren't fused.
Dave O'Neill 2

I thought that was the case, but figured I would start with the easiest part to expose and clean. Next step(s) will be along the list you mention.
I'm pretty sure you are correct about a corrosion problem, since it does not happen every time I turn the lights on.
Patrick Callan

Do the parking lights/instrument lights come on when the headlights don't? If so the problem is as Dave says. If they don't come on, or go out when you turn the switch to headlights, then the problem is where the brown wire attaches to the switch, or further back towards the solenoid from there.

Do neither dipped or main beams work? If so then does the headlamp flasher work? If so then it is the main lighting switch or dip switch. If the flasher doesn't work either, but the tell tale does glow, then the problem is bullet connectors. But to affect both main and dip it would have to be two bullet connectors. Unless it was the earth, but then the front side-markers wouldn't work but the rears would.
Paul Hunt

When the headlights come on, all lights are on - headlights, sidelights, turns, high and low beams, etc. When they do not, I believe the side-markers do come on.
I will have to go through a more rigorous step-by-step the next time it happens. It is a bit random, though, which makes troubleshooting tough.
This morning, I pulled the switch and used an electronic cleaner on all the connections. Checked the switch with a meter, and it appears to work properly - one click engages what I think is the side marker and parking light circuit, another click engages the headlight circuit. It is that second "click" that sometimes does not work.
Because I use the lights even during daytime driving, I sometimes do not realize that the headlights have not come on - until I pull up behind someone and don't see the reflections.
I'll use the above checks one by one for a while, and see what happens.
Thanks
Patrick Callan

I've just re-read your original post.

If the lights come on after throwing the switch a couple of times, it could be that the grease inside the switch has dried out. I had a similar problem with the headlamp switch in my midget. I had to dismantle the switch and clean off the dried grease.
Dave O'Neill 2

Checking the switch with a meter won't always show up the problem as the meter uses a tiny current. When you switch in the headlights the contacts are taking several amps and if they aren't up to it the connection won't work.
Mike Howlett

Right now, I'm leaning toward the switch itself - after cleaning everything, it seems to be working. However, the "flasher" switch does not seem to be working (ignition off) even though the high beam selection on that switch is working properly. Will check again with ignition on tomorrow (weather permitting).
Patrick Callan

Another possible fix is to install a pair of relays in the circuit. Frequently the switch is still passing adequate current to run a relay, especially after a bit of R&R. Tap into the headlight wiring in the engine compartment as it goes to the front of the car and use one relay to turn on the brights and another for the dipped. Plenty of diagrams out there to show the way.
David "worked for me" Lieb
David Lieb

"Checking the switch with a meter won't always show up the problem as the meter uses a tiny current."

An *ohmmeter* yes. The only reliable way to check connections including switches is with the meter on the 12v scale, measuring before and after the suspect item, when carrying it's normal current i.e. headlights.

The problem with the headlights not working at all is that the switch could not be working at all, so going to the trouble and expense of fitting relays would get you nowhere. A connection that is so bad the headlamps don't even glow dimly should be resolved at source. Fitting relays to get the extra brightness they usually deliver is then optional. And when fitting relays you need to fit individual filament fuses i.e. four as any short on that unfused wiring is going to do more damage and faster than the original circuit.
Paul Hunt

Paul,
Your theory is perfect. However, my experience directly contradicts it and, with all due respect, I tend to go with reality over theory. The headlight switch in my Midget was displaying the same symptoms reported by Patrick. The running lights always worked, but the headlights would work at times and not at other times. I pulled the switch, cleaned it, recognized the signs of wear and wished in vain that I had a decent replacement. Meanwhile, I had been meaning to install headlight relays for some time, having done so on my first Midget. I got the switch back in and it was marginally better, but still iffy. I installed the relays and never had another problem with the lights. No, there was not a glow from the lights when they did not work with the relays.

Likewise, I see no point in your "fuse per filament" concept. I fuse the supply before it gets to the relays; any short is going to blow that fuse before the rapid damage you mention. Yes, it has. My Midget already has over twice (nearly thrice) as many fuses as the factory used, but even I would not get that enthusiastic.
David "not too bright" Lieb
David Lieb

I am not as a knowledgeable as you guys with regards to electronics but I have run relays for my 74.5 MGB headlights and used 2 fuse's. One for the low beam and one for the high beam.
I had some other problems which resulted in blowing a fuse for the headlights. Luckily I was only going about 15MPH so I just stopped the car. But it made me think about what would happen if I was going a lot faster when the lights go out. If you can react quick enough I guess you could switch from high to low beam or visa versa. But if you can't well...........
I am going to be doing some re-wiring this year which will include making up a new fuse/control panel for my MG. Paul has given me a great idea, use 4 fuse's for the headlights and that way pretty well no matter what, I would still have 1 headlight.

FWIW

Bruce
Bruce Mills

Don't feel like I "fixed" anything, but after the contact cleaning, all the lights came on and functioned properly today (ignition on). Most works with ignition off - except the "flash-to-high beam."
Not sure if it's problem solved, or just wait 'til next time.

Pat
Patrick Callan

The trouble with one fuse is that if it blows you have lost all headlights, when the problem may only be with one beam in one headlight. Two is better, assuming you have the presence of mind to switch when suddenly plunged into total darkness on that mountain road! Four avoids that.
Paul Hunt

By the way David I did say 'could', which doesn't exclude your experience.
Paul Hunt

Good point there, Paul. In an urban area it is seldom Stygian darkness, but you do tend to disappear from the radar of the other drivers. I have always assumed this to be the reason that they were not fused from the factory.
David "you mean there are stars up there?" Lieb
David Lieb

I had a problem with mine ('70 B) going out one night when I was heading home, and it was no fun, though I made it ok since I was very close in a quiet area.

Since then I installed relays (with one fuse, may go to multiple since reading your post Paul). I also upgraded to Cibie lights with halogen bulbs, and now have very good lighting.

Since then I had a problem with one lamp not lighting on hi beam, which I traced back to a balky connector, so I have corrected this too.

All told I spent about $45 for the relays, connectors and fuses, and installed it in about 2-3 hours max. Worth every penny and minute to have great reliable lights.
Mike
m.c. keel

Relays (with fuses) are certainly required if uprating the headlights, even if the switch appears to be working as it should. A friend had put very high power in his and complained turning them on was like turning up the wick on an oil lamp, and switching between them gave a period of darkness. Relays solved both those problems, making illumination virtually instant, as well as noticeably brighter i.e. he was only then getting the full benefit of his new lights.

I've not bothered on either of mine, standard headlights, and anyway I'm a gentleman so don't motor at night ...
Paul Hunt

If I use 4 fuse's for my headlights, one for each low & hi beam. I would then have to run another hi & low beam wire?
Would this also require 2 more relays? I already have 2 relays for my headlights.

Thanks

Bruce
Bruce Mills

On my Advance Auto harness, designed by Dan Masters, there are no fuses on the headlights. Dan doesn't believe in them. The circuits are through relays, and the cabling is continuous from the relay to the bulb, with no connectors at all. It is also much heavier cable than the factory used.
Mike Howlett

Well, it turned out to be the switch, after all.
This time, I dismantled the switch completely to clean it - all of the contacts were gummed up. Used a contact cleaner and cloth to shine them up, reassembled, and hooked back up. All lights now working properly.
Thanks for all the info - I like the idea of eventually improving the "candle power" of the headlights, but I'm not sure I have the expertise to do all the re-wiring.
Thanks again,
Pat
Patrick Callan

Still only two relays, you simply split the output of each to two fuses, then take the output of each of those fuses to one filament each. This *does* mean you have four wires from the fuses to the filaments of course. There are already four from the connectors by the right-hand headlight to the filaments, you just extend these back to your fuses.

I parted the 4-way bullet connectors by the right-hand headlight, ran the two wires that come from the dip-switch back to the relays, then 2 wires forwards from the relays to the fuseblock, then forwards again from the four fuses back to the right-hand headlight. But instead of the right- and left-hand headlight wires being joined together at a 4-wire bullet connector there are now four individual 2-wire bullet connectors instead. I mounted the relays and fuses imediately behind the radiator mounting panel for protection from the elements, which means the extra wiring is of minimal length. By running back and forwards again as well as avoiding cutting into wiring and having to make additional joints, one could always revert ... if one wanted to.

This Advance headlight relay kit http://www.advanceautowire.com/headlights.htm clearly shows individual filament fuses. Why their power block contains relays for the headlights *without* fuses I can't imagine. There are eight fuses on the power block for other circuits, hardly a case of 'not believing in them', I suggest :o)
Paul Hunt

Thanks for the information Paul. I will hang onto it for this fall/winter project of installing a new electrical panel

Bruce
Bruce Mills

This thread was discussed between 26/09/2009 and 06/10/2009

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.