Car electrical experts required...

lordrobs

lordrobs

lordrobs

Soldato
Joined
30 Sep 2003
Posts
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Location
Norwich
Ok my car has always done this and it doesn't seem to be an issue but I'm just wondering why it does it...

When pulling up with my lights on as the revs drop to a steady idle I often see a small but noticable dimming of my headlights. Simmilarly if I pull up with my wipers on they will noticably slow too.

Any thoughts?
 
Wouldn't have thought it would be alternator, as the power would still be able to be drawn from the battery.

I don't know what else it could be though!!
 
None of my previous cars have ever done it, apart from my alfa when it had a bad earth and a dying battery
 
Its because the voltage drops as the engine speed drops to idle. Alternators need a certain amount of revs to supply 13.8v a lot of them need to be slightly above idle to produce that, once up a little of idle they will produce normal operating voltage then.
 
Howard said:
Yes, at a lower voltage than the alternator produces, hence dimmer lights and slower wipers
Click to expand...
I know this, but thought the battery would be able to supply enough current. Never seen lamps dim when the revs drop on any of my cars.
 
Drop in voltage means a drop in current. If the battery doesn't have enough powahs or currents then everthing will dim/slow down etc.

My headlights don't dim and my wipers don't slow down and I drive an old Ford, so it shouldn't happen on a newish Skoda.
 
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Batteries are 12V alternator output around 13.7V.

You need a differential to charge the battery. If the balance is opposite so the alternator output is lower than the battery thats when the battery light comes on.
 
This happens in my mitsi only they return to full brightness. For example if you're out of gear and blip the throttle, on return to idle rpm there is some dimming but over the space of about a second it will return to normal without any change in the engine rpm. You'd never notice it out on the road, only parked up against a wall.
If I wanted to try adding/replacing an earth strap would I see better results from the alternator to the body or the battery to the body?
 
You may find your idle is dropping a little low. This happened on my 306 - as the revs dropped it would fall to around 800 then recover to 900-950 where the Haynes said it should be - as this happened the lights dimmed. A quick clean of the ICV (Idle control valve) sorted it out for a while.
 
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