• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGA dim bulb on MGA

Share this page

bobmga62

bobmga62

Jedi Hopeful
Offline
Hi.. right hand front headlamp is real dim, compared to the left.

bought a new bulb and installed, but problem is still there..

Must be wiring.. I thought if it was a ground problem, it would be either on or off, not dim.. but before I go chasing everything, should I start with the ground..


footnote: if you are like me, you probably have everything in duplicate, cause you buy the part first, then find its always an opposite problem..


thanks for the help


Bob
 
Bet it's th' GROUND!!! Run another wire.
 
Gday Bob, I'd be checking the contact points(on bulb & back shell) are good.
 
When having a dim headlight problem, check to see if both the high and low beam lights are dim. If one setting works fine, and the other is weak it is a power feed problem. This would be the wire from the headlight branch connection to the headlight unit itself. The cause would probably be a dirty connection.

If the headlight is dim in both high and low beam conditions, then you have a bad ground at the dim headlight. The reason why the bulb is so dim in this configuration is because electrical current is going through 3 headlight filaments in series. For instance, let's say that you are in the low beam condition. The electrical current will go to both headlights, however in the headlight with the bad ground the electrical current will flow through the low beam filament, and seeing that it cannot go to its ground, the current then travels through its high beam filament. The electrical current now backtracks through the high beam wiring to the opposite headlight bulb through the high beam filament and then to its ground.

In actuality, when you have a bad ground to just one headlight, all 4 lighting filaments are turned on. However, 3 of them are in series, two in one headlight, and one in the other. The voltage drop to the 3 light filaments in series is about 4 volts, and that is why they are so dim. The one bright filament has all 12 volts. Of course, it is nearly impossible to look at the headlight filaments and see the individual filaments, because of the fluted glass and the way that the bulb is designed - you would be lucky if you could see it. Tailights and brakelights can have the same type of problem.
 
thank you guys...

I will start with the simplest, the connections..
actually I bought some kind of grease for the headlights to ensure a better connection ( my saab kept blowing bulbs) and work backwards from there...

r
 
Do you have a multimeter? A voltmeter is quite a valuable tool for diagnosing electrical failures/problems.

A good multimeter can be purchased at Sears, Radio Shack, etc. for under 30 dollars.

Then you can perform a voltage drop test to pinpoint the failure/problem.

Now don't let the idea of performing a test you may have never done before scare you. Google how to perform a voltage drop test and you will pull up a number of sites that walk you through it.

In essence you are looking for supply side voltage(battery power) to the user, which here would be the bulb. So by taking the ground terminal and securing a good ground(battery terminal) and positive terminal and probing supply to the bulb base you should have within 1 volt of battery power. If not check, clean and tighten connections. Then make sure that the voltage on the ground side of the bulb base is less than .5 volts. Reason why is the voltage needs to be "consumed" by the user(bulb) for the light to be brightest. If it is not getting a good path back to the battery light will be dim.hth
 
In my previous post, I completely forgot about the high beam indicator light. When you have a bad ground to one of the headlights, the indicator light will turn on when your headlights are in the low beam position. The indicator light will be very dim, of course, and you might actually see it illuminate when it is really dark, headlights turned on, and your instrument lights turned off. Of course this doesn't have anything to do with troubleshooting your problem, I just couldn't let my previous post stand without adding this little morsel of information. I believe that the first 2 posts were spot on as to where your problem exists. As a matter of fact you probably already resolved the problem by now.
 
Back
Top