Battery light on dash

[david9121]

Fault was originally batt light on all the time, the faulty alternator was repaired, no problem!! However,noticed fault before connecting blue wire. I have scoured the forums and haven't found one quite like this!
So when the blue wire is disconnected from the alternator I believe the Batt light on the dash should go out? (Wiki) It doesn't!
So went to Dash and found 6.8 volts at back of bulb. When dash is disconnected at connector and voltage checked on blue wire at connector, nothing! therefore I presume the power for the Batt light is coming from somewhere in dash PCB wiring?? Any ideas and has anyone had anything similar?

[Oldiebut goodie]

You should be testing the earth - you are testing it back to front. The light comes on when there is an earth path after the bulb which is then negated when the alternator puts out enough voltage to equal the positive side of the bulb.

[MidLifeCrisis]

I bet you have a split charge relay - right??

When you disconnected the blue wire from the alternator that cheeky electricity found a new path to ground through your split charge relay!! (That's probably also why you saw that funny voltage because the voltage was being divided between the battery warning light and the relay)

So if you listen carefully you will probably be able to hear the split charge relay click when you turn the ignition key to the first position (i.e. before you start the engine)

Reconnect your blue wire to the alternator and I bet it all works fine ......

[david9121]

Thanks for the replies, your right it does work now but I wondered how the light would come on when blue wire disconnected from the alt. by the way where do I find the slit charge relay??

[MidLifeCrisis]

by the way where do I find the slit charge relay??

Depends on what type of interior you have - but it's usually next to the main starter battery (ie under the seat in a petrol)

[AdrianC]

Thanks for the replies, your right it does work now but I wondered how the light would come on when blue wire disconnected from the alt.

The way the charge light works might explain it.
The light is in the circuit sending +12v TO the alternator. It does that to "switch the alternator on" - excite the magnetic fields, which is how the alternator actually generates electricity. If the alternator is working, then there's no voltage difference between that +12v feed, and the bit of the alternator it's connected to, so no current flows, so the light stays off. If the alternator isn't generating electricity, then that other end is effectively earthed, so current flows, so the light comes on.

by the way where do I find the slit charge relay??

Without wanting to sound glib, the easiest answer is wherever the person who built your camper wiring put it... Find the main +ve lead to your leisure battery, and follow it back.