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Author Topic: Voltage drop with rear lighting ???  (Read 1215 times)
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Marcus Robinson

« on: October 30, 2016, 07:42:54 PM »

Battery is good, healthy 14.4v

Getting final things checked over and fitted for the MOT friday, anyone got any thoughts on why i'd be getting such a drop for the supply to the rear side lights?

With them turned on and engine not running, it's around 10.8v reaching the wiring for the number plate lights, and this seems to give acceptable enough lighting, but thought it would be nearer 12v?...

But once dipped beams are on, it drops again to around 8.8v and the LED units i've fitted are hardly on...

Any thoughts on what I can do to improve this Huh?
Would fitting a separate relay to supply the dipped beams be the way to go, with possibly improved lighting also?


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« Reply #1 on: October 30, 2016, 08:58:17 PM »

I'm no expert on electrics (as earlier posts will have proved), but could this poor performance and voltage drop under load be down to a marginal earth? As a quick and dirty test, try rigging up a temporary earth to the rear side lights. If they already have their own earth , feel free to disregard this suggestion...   Grin

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« Reply #2 on: October 31, 2016, 12:26:53 AM »


Sorry if a bit obvious, but here goes.

Get yourself a good earth to start with for the meter you are using, run a wire from the battery if necessary.

Then work your way out from the battery to the rear in stages and note the values.

It will either be a bad earth or poor connection which is causing the issue.

Remember that there will be some voltage loss across wiring due to its resistance, but not that much!


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« Reply #3 on: October 31, 2016, 02:57:13 PM »

a separate relay for the headlights would be my first course of action, if I remember correctly they have no relays as standard so all the current flows through the column switch this does the column switch no good at all and connections get rather fried, this is especially bad if you use up rated bulbs of higher wattage (I managed to melt the internals on a Prisma  column switch years ago when I knew no better!) 
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« Reply #4 on: November 01, 2016, 10:14:51 PM »

What Peter said is how I would go for this it is probably the round connector link wires front to rear corroded or terrible earthing.

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