UK Car Repair: Astra TD 1.7, alternator output, td 1

Question
QUESTION: hello - I have a 94 diesel astra 1.7 GM - and the battery seems to keep going flat if the car is left for any length of time. I have changed the alternator - but the problem still occurs. When I check with a voltmeter there's about 13 v going into the battery with the engine running - there is also when every thing's switched off a .2 of an amp drain - I'd guess this would be the clock and the radio. The battery's only 15 months old. The tension on the belt is OK. Do you have any thoughts?

ANSWER: Hello Nick, Have you looked in the boot/trunk after the lid is shut, as I have known the light to stay on and drain a battery. I am concerned that there is only 13v when the engine is running, as there is nothing left to charge the battery after the car uses 12-13 to run, it should be 14.5v -15.5v but the alternator is new right? unless you got it from a junk yard and it is the wrong type because Vauxhall used 3 different alternators and a diesel would need at least a 100a output (the much higher compression needs more power to turn the starter motor).My last question is; have you put an amplifier or subwoofer box in? These can take more power during running and leave little or nothing to charge the battery- let me know how it goes - Paul

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QUESTION: Thanks Paul - yes - I have checked the boot light too - not the problem. The alternator is from a junk yard but came from an identical car so should be right - I have measured output when the engine's just ticking over - maybe at higher revs it's greater I'll recheck though - no no additional woofers or anything like that - perish the thought!

Answer
Hi Nick, if the alternator output is only 13v then take it back to the junkyard and demand a replacement, as it appears you have bought an alternator that is as bad as it's predecessor. Alternators are a very common fault on all makes of cars and so buying a secondhand one is like a raffle. I did the same thing with my vectra, not to worry though. Most garages will be happy to test it for you (usually free if there is a sniff of a sale) - Keep me posted - Paul