car charging light

[beedz]

hi all, new to the website and in need of some advice (usual story i guess!)

my 82 mk1 fiesta is beginning to beat me! (goodness knows what i'd do with the electrics of a new car!)

the battery charge light is coming on faintly. so I check the battery earth (the fault last time!), then the battery volts just over 12, then the 2500revs volts - around the 14 mark. so i begin to get confused

the next thing i notice is, the higher i rev, the brighter the light glows.

also, it hesitates on initial start-up (as does the rev counter )

i check the volts from the blue lead from the alternator to the instrument cluster - showing the charge from the alternator fine. i check the earth from the cluster - fine.

as i understand it (could be wrong!) if the volts from that blue line go lower than the battery volts - it lights up to warn you. so how come it's getting 14v and still lighting up? a short circuit?

that's about as far as it got - any ideas much appreciated

 

[simoniris]

Battery knackered? Not accepting the charge?

 

[beedz]

no mate, the battery shows just over 12v when left, holds its' charge fine

 

[Spark123]

Faulty circuit in the alternator causing the lamp to illuminate, earth fault on the cable connecting indicator lamp to alternator, dirt/damp on connector causing tracking across to earth...

 

[TicklyT]

worn/loose alternator drive belt? (took me a couple of weeks to spot that one once )

A fully charged 12 Volt battery will give 13.8 Volts with no load (2.2 volts per cell)
I would expect to see a bit more than 14 volts when it is charging.

I believe the lamp is effectively in series with the alternator field winding, and lights up when the alternator isn't spinning fast enough to produce enough output to power it's own field coils.

 

[^neo^]

Faulty circuit in the alternator causing the lamp to illuminate, earth fault on the cable connecting indicator lamp to alternator, dirt/damp on connector causing tracking across to earth...

I agree. Resistor pack on the alternator is probably the culprit.

 

[Stivino]

TicklyT said,

"A fully charged 12 Volt battery will give 13.8 Volts with no load (2.2 volts per cell"

Typo methinks, 13.8 isn't possible, did you mean 12.8?

 

[beedz]

sorted it - you wouldn't believe it!

a fuse was furred up and only allowing 6ish volts through to the instrument cluster (so the signal volts from the alternator was lighting the bulb from the other direction!)

ah well, sorted now cheers for the help