alternator 13.2V max output

disco 1, 1996 300Tdi.

Alternator kicks out 13 and a bit Volts (and I know that is already low), but turn on air con, or even side lights and the volts (measured via a voltmeter that fits into cigar lighter from New Boat Gear - [I don't think there is much voltage drop from battery to cigar socket]) DECREASE a lot.

Firstly, aircon on, and side lights, maybe down to 12.1V, SO no charging to battery, turn anyhting else on and volts go below 12.

I think as revs increase, voltage actually DECREASES and that seems odd too.

Been like this for years and have fitted a socket in bumper for easy battery charging every night (at least during winter). It is a damn small battery for a 300Tdi, but the recommended one. I have heard of people fitting a second batt. in parallel to get over this.

But I want to run another battery for leisure purposes, not for cranking, via a voltage sensitive relay - long story - so I need to fix alternator problem (if it is a problem).

I did read somewhere but cannot find the website that on some alternators there was a bit at the back where some fiddling or replacing one part for another ups the output, but don't know if that was correct or applicable.

Any ideas where to start?
cheers

14.4v to 14.7 is the charging voltage + or - a point or two depending on how good your multi-meter is, my certified Fluke multi-meter is +/- 2%. what's yours?

I run another car with the charge voltage of 13.6 without any probs. it's the amp output that counts.

Thanks.

Just edited original post because I forgot to finish a sentence stating Voltage went to below 12.7V.

I will measure the amps.

My meters ( a few types) compare favourably with my mate's fluke, but I wouldn't use his meter in case I dropped / other way of damaging it.

Just been round to a friend that has more space than I have so I could top up fluid levels - you know that saying:"if it doesn't leak it's empty" - well, gear box and both diffs needed more sae80/90 than I've ever had to put in before.

Anyway, I'd hooked up the vsr and second battery and there were plenty of amps to share all round.

Disco gave at best 13.2V, but also shoved that at 0.5A into second battery, so maybe I can get by with doing nothing.

Driving at night with full beam and air conditioner / heater on would be a different matter - will have to find that webpage that told me which thingy in the back of the alternator I could change to up the output - or get another alternator.

Sounds to me like the alternators is on its way out as it should kick out more than that.