[ROVERNET - UK] relay

Jonathan Hewison Jonathan.Hewison at vuw.ac.nz
Wed Oct 20 22:18:28 BST 2004

Thanks for that. Sounds like I will rebuilding my alternator this

Jonathan Hewison
Engineering Technician
Faculty of Architecture & Design 
Victoria University of Wellington
Te Whare Wananga o Te Upoko o Te Ika 
New Zealand. 
-----Original Message-----
From: JOHN HALL [mailto:J.HALL11 at COMCAST.NET] 
Sent: Thursday, 21 October 2004 10:02 a.m.
To: rovernet at lyris.ccdata.com
Subject: Re: [ROVERNET - UK] relay

Yes. If the car doesn't have meters, the idiot light is the only way to
let you know if the battery is charging or not. The relay is measuring
the current going to the battery from the alternator. It compares the
polarity difference between the battery and alternator. If the
alternator output is higher than the battery, the relay is held on and
the light is off. If the battery voltage is higher then the relay
relaxes and the light is on. That is why the light goes on when the
ignition key is on but the engine is not running as when starting up. If
the light doesn't come on when starting it is usually a faulty relay.

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