batt light

hi all this problem started the other day when i turn the ignition on the batt light refuses to light up, the bulbs ok the only time it comes on before start up is when i put a test bulb screwdriver on the pos terminal on batt and touch the yellow wire out of the new altenator that i fitted today, as we thought it was at fault this wasnt the case the light does come on now an then for start ups but then out on the next. say it was out on the start up from work. but in the mornibg it lights up again like nothings wrong any help would be gratefull

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