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MG MGB Technical - Headlight horrors

Hi all,

I have an intermittent headlight failure problem. The headlights will work fine most times but then occasionally won't. This is mostly early mornings (I start work pre-dawn so need the headlights on). During the day when I test them they ALWAYS work!!!! Following advice on the BBS archive I checked and cleaned the bullet connectors behind the grill, and thought I'd solved it when I found a dodgy earth wire connection. I replaced this and of course, the headlights worked. But this morning, no go. The switch is relatively new. Any ideas on what I should try next?

Thanks
Mike
mike donaldson

you need to use 2 bosch relays,one for the low beams amd one for the hig beams.the voltage to the headlites goes thru the switch and causes a voltage drop.when you use the relays a small amount of current goes thru the switch.also in doing so you get full voltage output from the alt going to your headlites.
i was having problems with my switch like you were decribing when i put the relays in no more problems.a added bonus was the headlites were brighter also the dash lites were also.
ja adams

Have you tried this site? It's very, very good and even helps a noob like myself!

http://www.mgcars.org.uk/electrical/lighting.html

New switches and electrics aren't always reliable...I once bought a dud new battery which wouldn't hold a charge. Cold weather? This may be a moisture/condensation problem - go over ALL connectors and wiring again.

You don't NEED two relays, I've got a single heavy duty headlight relay which takes it's feed from the alternator and and picks up the OE loom just forward of the radiator. It's got dual output switched relay, which reduces wire clutter (although it's larger than the mini relays:

http://altura.speedera.net/ccimg.catalogcity.com/210000/210900/210963/products/7487672.jpg

Very easy, convenient and safe and with the other benefits ja adams describes.

SoS
Curtis Walker

Power to the headlights goes through TWO switches: the dash switch and also the stalk bright/dim control. Boths sets of connectors need to be brite and tight.



Dan Robinson

Mike
The two short looms to the headlights can be a problem - they are pretty well exposed to the elements and besides corrosion, can short and call for more current than the stalk switch can cope with - what happens is the stalk switch terminals heat up and melt the plastic which holds them in place. The stalk switch is repairable (drive out the fulcrum) but it is alarming when there is no light at all.

Relays are the best way forward.

Roger
RMW

Off topic Mike, but where in the hell is Ontario in Oz?
regards
Roger
Roger T

Hi Mike.

Lots of sound advice above.

Next time it happens (assuming you haven't fixed it by then) try gently waggling the loom in the relevant areas and operating the lighting switch and the dip switch a few times. Hopefully this will temporarily cure the fault and you will then know the area that needs attention, ie the area that you were fiddling with when the lights came on.

BEFORE you start the above, try main beam (using the dip switch, not the headlamp 'flasher'. If main beam works (but dip still doesn't) you have eliminated the main lighting switch and the headlamp earths.

Don
Don

Correct Don, and that leaves the stalk switch itself and the bullets between it and the headlamps. There is a multiblock connector at the end of the short stalk loom as well but this is unlikely to be the culprit. Take a loose live feed from the fuse box and probe it into the bullets behind the grille and then under the column cowl. When the lamps stop lighting you have found the fault. Be careful that you don't ground the lead as this could be dangerous.
Iain MacKintosh

Hi all,

Thanks very much for all the advice, I'm determined to track this down (the bloody things worked perfectly this morning!!!). I'll let you know how I get on Curtis, that site is fantastic. And Roger, I'm in Canberra, don't know why the site has suddenly relocated me to Ontario (though I'm sure it's a very nice place.)

Regards
Mike

mike donaldson

The purpose of using relays is to limit the current in the main lighting switch and dip-switch. I'd be interested to see how a single relay could do this - unless it was a three position relay.

Do both dip and main beam fail? If so does the headlamp flasher still work? If the answer to both these is 'yes' then the problem is in the brown feed to the main lighting switch or from there to the dip-switch as the headlamp flasher uses a different power source.

If dip fails but main and flasher is OK then it points to the dip-switch or the connections between it and the 4-way bullet connector by the right-hand headlight.

If there is total failure, but the headlamp tell-tale lights when the *dipped* beam should be lit, then the problem is a failure in the ground from the aforementioned 4-way bullet connector to the grounding point by the fusebox.

And don't forget - "Just because you have found one problem, you may not have found all the problems".
Paul Hunt 2

Paul, JC Whitney calls it a dual relay, I have never seen one but I assume it has dual relays inside one box. I noticed it also has fuse protection inside.
http://www.jcwhitney.com/autoparts/Product/tf-Browse/s-10101/Pr-p_Product.CATENTRY_ID:2003361/showCustom-0/p-2003361/N-111+10201+600002850/c-10101

Clifton
Clifton Gordon

If this doesn't work try copy and paste. That's the only way I could get the above link to work.
http://www.jcwhitney.com/autoparts/Browse/tf-Browse/s-10101/showCustom-0/refId-600002850/N-111+10201+600002850/c-10101

Clifton
Clifton Gordon

I clicked on the part # for additional information and got this conflicting information. I'm not sure what it is now. It looks like the relay linked in the post by Curtis Walker. I couldn't get either link to work without copy and pasting.

"HEAVY-DUTY HEADLIGHT RELAYS FOR OFF-ROAD H-4 QUARTZ-HALOGEN BULBS
Single relay has 20-amp capacity and single fuse. 12-volt. Double relay also available."

Sorry for the posts, I shot myself in the foot on these.

Clifton



Clifton Gordon

Two ordinary Bosch-style relays can be mounted up under the latch panel near the headlight wiring an be made relatively inconspicuous. I saw such an installation at MG2001 and the guy who did it did a very nice and clean job. Shoulda taken some photos of it.

If you are looking for dual relays (two separate in one package), you might go look in one of the electrical supply house catalogs like Newark/Farnell or Potter Brumfield or Bosch.
Bob Muenchausen

Our company, Advance Auto-wire, sells a relay kit for headlights. However, we have posted the wiring diagram for it on our web site, http://www.advanceautowire.com for those of you who prefer to build your own.

Whether you buy from us or build your own, we do recommend wiring the relays/fuses as shown.

Whatever you do, do not wire the relays with only one fuse protecting all the lights. Either use four fuses or use no fuses at all (as the MG was originally made).
Dan Masters

Rather than the latch panel which is pretty exposed I mounted them behind the radiator mounting panel for some protection from wet, albeit a bit warmer. To avoid cutting into the new harness I constructed a mini harness to bring the main and dip wires back from the original 4-way connector near the right-hand headlight, to relays and fuses, then back to the same position to feed the right-hand headlight spur and the rest of the main harness to feed the left-hand light. Picture at http://www.mgb-stuff.org.uk/images/mg12001.jpg. The fuses are closest to the radiator panel, the relays a bit further back. The two on the right are for other functions which is why they have integral fuses, the headlight relays not.

Couldn't get either of those links to work, but this did (for me at any rate) - http://www.jcwhitney.com/autoparts/ItemBrowse/c-10101/s-10101/p-100000183054/mediaCode-ZX/appId-100000183054/Pr-p_CATENTRY_ID:100000183054. It's a bit confusing, but I suspect the single relay has two contacts one per filament to reduce contact loading, so you would still need two relays for both beams, or perhaps the double relay.

I agree with Dan about using four fuses or none. If you use only one you could easily lose the lot, unless you have the presence of mind to pull on the flasher stalk while you come to a halt. Even one fuse per beam could be a bit disconcerting, although at least one stands a chance of having the other beam available.

Paul Hunt 2

Yes - these are dual fused, dual switched relays specifically designed for head lamp applications.

There is debate about the level of fusing which is and isn't safe, depending on your fusing goals. One fuse for each beam? Set it up so the main power feed is fused with a 30Amp inline? Fuse each side seperately, so a blown fuse only takes out one side rather than a whole beam?

The options are nearly endless. A fuse will protect your whole loom from frying and isolate a bad circuit from frying others. However, any fuse, like any electrical component is also a potential failure point and added complication. Your call at the end of the day...I went with the considered advice of my local experienced autoelectrician when deciding to not fuse the main feed from the alternator lug, but each beam is fused seperately...I'll just need the presence of mind to flip the beam switcher if I lose lights at night (while hoping it's not the main feedline about to fry my entire loom).

Assuming I survice that, I'll play the odds further and grab a lottery ticket :)
Curtis Walker

Hi all.

Curtis raises some interesting points.

Another area worthy of consideration is :

"What event is likely to cause excessive current ?".

A short to earth as a result of insulation chafing on a sharp metal edge is no.1 in my experience, as found in the gearlever mounted overdrive switch.

A light bulb can draw excessive current if a broken filament drops into a low-resistance position, but this usually burns itself out quickly.

In the event of an accident all sorts of short circuits can occur if the wiring or components are crushed, and have the potential to cause a fire.

Don
Don

With conventional MGB wiring (i.e. switched supply and common ground) one fuse per *side* isn't compatible with relays ... unless you use four relays, or the twin contact relays have two input contacts as well as two output contacts.
Paul Hunt 2

Dan Master's kit above is certainly a very nicely done and allows for a clean installation. It would save anyone a lot of the work of creating their own set up. I did mine very similar to what Paul shows in his photo, but without the harness wrap. This works OK in cars like my 68 which have the vacant space on the fender panel, but many later US cars don't seem to have as much vacant space.

Paul: The install under the lock plate I saw had the advantage that it was housed in a closed box with only wiring coming out, much like the fuse/relay boxes on modern cars like BMW or Audi. In this case, it was basically similar to Dan's unit except that the relays were mounted such that they plugged into the connector block horizontally, rather than vertically, and this allowed for a shallower and less conspicuous box.
Bob Muenchausen

Hi all,


As recommended, I went back again over all the bullet earth and headlight connections, checking and cleaning. Some that I had missed before were very dirty, some loose. The brown wire going to the dashboard switch was full of green looking muck (I hadn't peeled back the plastic cover sufficiently the first time I checked it). I should have mentioned its a 69B with floor mounted dip switch. I cleaned and checked this too ... again very grotty and loose. I have now had four days of properly working headlights (which never happened when the problem was present). Can I declare a cure and submit my findings to a major medical journal?

Many thanks for all the replies, I've learned a lot about electrics. This site has been a wonderful support for me over many years.

Regards
Mike
mike donaldson

Curtis, if you want to protect the main feed wire you can use a circuit breaker available at NAPA and others. They come in a variety of ratings and are the size of a flasher unit and are self reseting. US cars have always had a circuit breaker built into the headlight switch.
John H

Mike...congratulations - you can at least write it up as a sucessfully treated case report, pending longer term follow-up ;)

John...yes, I considered one of those, but have to admit, the wire between alternator lug and headlamp relay is short (<1m), well wrapped in electrical tape, and not in danger of chafing. As I said above, the odds of it shorting are less than my winning the lottery, so I have elected to not fuse it, as the odds of fuse failure for some other reason are in fact higher. I am however running fuses for each beam, as the odds of peripheral wire or filament shorting are somewhat higher - and less catastrophic should individual fuses go, rather than the entire lighting supply.

Paul...yeah, but it's amazing what one can do with enough fuses, connectors and wire. I'm not saying it's easy, ideal, or even safe - but it is possible :)

My point is that fuses are not the be-all and end-all of safety. Their use and placement needs to be logically considered and the risks evaluated in this light (ha, ha).
Curtis Walker

Absolutely, which is why a single circuit breaker controlling all the lights must be regarded the same way as a single fuse. They may be self-resetting, but they will only do that if the cause of the short is removed ... before you hit something.
Paul Hunt 2

This thread was discussed between 13/12/2005 and 17/12/2005

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