Alternator question

As my battery was going flat, I suspected the alternator. My voltmeter said it was charging at 12.0v, so I bought a new, upgraded 70 amp one. Then I discovered it has different connections. I don't have time to change it tonight, so I plugged the old one back in.I checked with the voltmeter again, and now it's charging at 13.88v. I switched off the engine, and got a reading of 12.9v from the battery.
Question: Was it just a dodgy connection? Do I need the new alternator at all?

What does the voltmeter normally say? Personally I'd be happier if it was over 14v. Electrical faults are a bit variable, especially as they are developing, so it may be that the alternator is not pushing out a full charge all the time. If you've got a 200 or 300 TDi, I seem to remember there's a Buster's guide where he changes the terminals on the wires to fit a different model of alternator, so it doesn't look too hard to do. Later models like the TD5 have fancy multiplugs which would be a more involved process.

Normal charge level is between 13.8v and 14.2v Any more than 15v and the battery fry's.
Just after start up the reading may be lower but in a couple of min's should return to normal.

To check battery give the car a good run, recheck battery voltage after standing all night, anything under 12.5/6 volts is not good news but may not be terminal.
A loose spade terminal on the back of the alt is all it takes to stop the A127 style alt charging.

Get it checked foc at halfraids.