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MG MGB Technical - Headlight Problem
|When driving off the other evening and in need of my headlights I noticed that both high beams did not work. I haven't checked yet to see if by BIG coincidence they both burned out at the exact same time.|
Both low beams work fine, and when switching to high beam the blue light on dash does come on but no high beam headlights.
I also noticed that when I toggle between parking lights to headlights that the dash lights dim considerably.
So when parking lights only are on, the dash lights are marginally readible. When I switch to headlights, they are just about out completly.
Working from the assumption that both headlights didn't burn out at the exact same time, doesn't this imply a weak ground somewhere? But if a ground/earth problem, where - since the low beams work fine, except for the dash lights getting even weaker.
If it is that the headlights burned out together, why the further dimming of the already weak dashlights when switching from parking lights to low beams?
Thanks for any assistance.
|I think the low beams working rules out the earth as the problem. The headlamp earth is in the middle of the slam panel on the underside of course.|
The blue tell tale is directly in parallel with the main beams so i would look at the wiring from the dip switch to the main beams as a start. The dimming may be due to volts dropping as the extra load comes on. however if this happens when the main should be on then it's prabably an earth current effect. Taking a shorting wire from the dash toi the battery earth will show this up.
|Looking at the wiring diagram from advanceautowire.com, page 15 (which happens to correspond to my '73) there is a brown wire which comes off the starter solenoid and supplies all the power to the headlights(both high and low beams), marker lights, and dash lights. The dash lights are dimming due to low voltage reaching them. I bet there is corrosion in your headlight switch or at one of its terminals, creating a high resistance. When you increase demand for current by switching on the headlamps, and again using the highbeams, the added resistance causes the dash lamps to go dimmer and dimmer. I would expect the blue indicator lamp to dim as well, so I am having a tough time figuring that out. Depending on your year of car, my explanation may not be as valid.|
Do you have normal-brightness marker lights when the high beams are on? If not, that supports my theory. If the marker lights are normal brightness, then there must be some other explanation.
You may also get these symptoms if you have a short after the switch but before the load, in the high beam circuit. However, I wouldn't expect the dash lamps to do dim with the low beams on, and might expect a fire before too long...
On my car, while installing a radio, I inadvertently disconnected a purple wire from its other three wires (connected together via a bullet connnector) and this resulted in my highbeams not working when I hit the flash-to-pass switch. I doubt this could have anything to do with your high-beam issue, but I suppose there could be two faults at the same time.
Not sure what year your car is, but on mine, when you disassemble the headlamp switch, you will find a spring and a plastic piece with a copper face, all of which will likely want to fly all around the room as soon as it comes apart. If you can, disassemble it in a plastic bag to catch the parts when they do go flying. Clean up the copper part and the dimples that it makes contact with inside the switch, then clean up the terminals on the outside of the switch. I used light sandpaper. Maybe use some dielectric grease to keep it from corroding soon again. I didn't find it that tough to reassemble the switch. You may also put relays into the marker lamp circuit and the headlamp circuit, to remove the load from the switch itself, and extend the life of the switch. May even brighten all the lamps noticeably.
|My vote is for either the input to the double bullet to bullet joint where the high beam feed splits in two, or the joint just before it (single bullet to bullet). I had both my high beams go out (but not dash light)and traced it back to this little chap.|
They both sit under the slam panel as mentioned and collects all the grot sprayed up from the car in front. Of course it never rains in Abingdon, so that design drawback would never have occured to them....
|Dave Smith GT|
|Unlikely to be an earth/ground as dash lights and headlights use different earthing points. It sounds like you have two problems. As the dash tell-tale lights on main beam it shows you have some voltage at least coming out of the dip-switch, so I'd suspect one of the bullet connectors between there and the headlights. The most likely one is that by the right-hand headlight, as the wire (blue/white) coming in to that feeds both and it gets the worst of the weather and corrodes, giving bad connections. There is another in the cabin but only on cars with the floor-mounted dip-switch.|
The dash lights dimming as the headlights are turned indicates there are *more* bad connections either in the main lighting switch of the brown 12v feed to it.
That still leaves questions as to why the main beam tell-tale works when the headlights don't if there are main feed connections bad enough to cause the dash lights to dim right down, and also how you have any dipped beams if the dash lights are dimming down. But a voltmeter in various parts of the circuit around the main and dip switches, using parking lights only as well as switching between dip and main, should reveal all.
This thread was discussed between 26/08/2008 and 28/08/2008
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