Digi-dash question

[Jcb]

Why is it that when the car is being run gently (changing up between 2-3k rpm), the voltmeter shows 12V on the dash, but when you drive a bit harder, if only have the revs at say 4000rpm for just a few seconds, the volt meter goes up to just under 15V.

When it is on 12V, there are some electrical malfunctions too. I.e say you have the headlights on, then the rev counter stops working. The headlights get dimmer themselves too. The other day, the voltage dropped to about 9V and the dash went blank and the headlights were really dim. But simply revving it up a bit solves it all out. When its running around 15V you can have all the electrics you like running and it handles it fine.

Is this a problem with the alternator/battery?

I installed the wiring for an amp not long back, but that it just the wiring, it isnt rigged up to anything.

[peester]

its worth checking some earths tbh - if the earth supply is poor - the voltage u get will be lower than it should be..
possibly some wiring isnt right - rev counter should work with lights on,.. ?
get battery and alternator tested, halfords can do it for free - but get them to use a multimeter not that stupid 'oh its buggered mate' battery tester.. lol

[Adam]

Alt sounds like its fooked to me.
Diode/Rectifier problem(it should charge the same voltage whether doing 1k rpm, or 5k rpm)

[Jcb]

But if the alternator was buggered surely the batt wouldnt charge. Yet it's been doing this fo say 2 weeks now. The battery would have gone dead long ago with no charge.

[John]

Ad, you will get some voltage fluctuation when car is running. But if you have that sort of fluctuation i'd say the alt is fooked. Is the alt belt tight?? The alternator may be charging at higher revs, but not at say idle or lower revs.
We'd need a voltage reading at idle to clarify JCB.
I looked at a 1.2 spi for a friend the other day that was charging at 10v at idle and 16v when revved!