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Headlights + Dim

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mark75b

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Hi I have a question. When stopped and at idle speed (650-700 rpm )my headlights dim and turn indicators slow down.
I recently put in a headlight harness but had this problem before and after.
Any ideas ? Mark
 
I would check my battery very carefully. It sounds like your alternator is putting out OK but the battery voltage is low, even after being charged by the alterator.

You could also have something other than the lights drawing down the battery. Perhaps a worn-out heater fan running? Something on that order.

Check battery voltage with everything off. Check it again with the ground connection off. Be sure you have the battery electrolyte filled to the top of the plates. This is VERY important and can cause a host of troubles!

Guinn
 
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I would check my battery very carefully. It sounds like your alternator is putting out OK but the battery voltage is low, even after being charged by the alterator.


[/ QUOTE ]

I agree; I would check and clean all of the battery and alternator connections. They need to be clean and tight.

I recently replaced the main battery cable on my MGB. The cable was showing corrosion all the way back up into the insulation - not good. The replacement cable (by British Wiring) was a bit smaller in gauge. I cleaned up the old one and then laid them side by side and tested the resistance of each; the new one had literally HALF the internal resistance of the old one even though it was a smaller gauge.
 
Thanks for the input I would not have come up with thoes ideas. That gives me something to work on now all I need is time. Thanks again.
Mark
 
Nobody mentioned ground points... They have given me more grief with lighting than cables/generators/alternators. Check and clean the ground points. The black wires. /ubbthreads/images/graemlins/wink.gif
 
/ubbthreads/images/graemlins/iagree.gif I agree with Doc. Your chances of solving a lighting problem is better if you make sure ALL the ground connections are clean and tight. If I have a lighting problem, I always check the grounds before I go any further. Usually it solves the problem. [Notice I said usually]./ubbthreads/images/graemlins/rolleyes.gif PJ
 
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Nobody mentioned ground points...

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Mea culpa... /ubbthreads/images/graemlins/tongue.gif

Not sure about the headlights, but the taillights don't normally have seperate grounds; they just use the screws on the fixture frame to ground the light to the body. I'm not a fan of this method, so I created ground pigtails for my taillights and ran them to the boot catch.

Headlights are similar, but I soldered on ring terminals and connected the grounds to the hood buffer pads (I'm using Cibie H4 lights w/ relays).
 
At 650 rpm, your alternator will hardly be outputting anything - and so the battery is having to supply most if not all of the required voltage. Added to this are voltage drops due to the natural attrition of tired wiring and long wire lengths.

If you measured the voltage at your headlights in this scenario, it's likely to be 12V at optimal best (given you're runing a 12V battery), which results in dim light, as most bulbs have optimal voltage ratings at 13 volts and above. This extra voltage is usually mostly supplied via the alternator's continuous output. It is a common misconception that the battery supplies the electrical equipment on your car - this is mostly the job of the alternator when the engine is running (and whose job is also to maintain the battery charge).

The battery's main job on the other hand is to store charge for when the alternator is either inadequate (e.g. at low revs and when providing the massive starting crank) or not on (running your hazard lights etc.).

So, to summarise, at low revs, it is normal, even with a modern alternator, to experience slight dimming of lights. This will be made more noticable if you're using high power bulbs designed to operate above the 12V output of the battery, and/or have less than perfect wiring.

Hope this helps!

EDIT: notwithstanding the other good points made above about making sure grounds, fanbelt and wiring are in good condition, and battery terminals are clean and tight.
 
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