Let me start with a Caveat, I have completely rewired the car, stripped out all the old 1989 factory harness & replace it with my own, so potentially there is nothing wrong with the alternator & the fault is in my wiring, however I don't think it is & will go onto explain why.

The car is based on a 1989 Skoda Rapid 136, when I got it back (owned 17 years ago) the wiring was not in the best state, corroded connections, relays that had clearly had water in, when I got the car started the alternator would not put out more than 13.2V & looked worse for wear. I took it to the local shop who did a refurb on it, new bearings, new bushes, cleaned up the comm, removed all the spiders & the rest of the crap in there. I took the external regulator in, he connected it all up & claimed all was good, but I'm not convinced.

On the alternator are 3 terminals the main big feed from the battery, D & DF these connect to an external regulator which has the following 4 terminals 

54 which is a fused constant live

D+ which has 2 wires, one comes from a switched live via the ignition warning light / exciter bulb, the other goes to D on the alternator

DF which goes to DF on the alternator

B- which goes to Earth 

 

Symptoms, the battery goes flat overnight (new battery) if I put an ammeter in series there is a 0.25A/250mA flowing bearing in mind this is an 'old fashioned' car it has no electronics on board, with everything switched off I would have expected zero current flow (Please confirm)

The ignition/exciter light is very dim when you switch on the ignition, (if you fully short out the lead it's full brightness), if you remove the DF lead from between the alternator & the regulator there is what appears to be the same 250mA/0.25A current flowing. 

 

What I have now done is take the alternator off the car & put it in the vice, I've then wired it up along with the regulator so there is no other wiring involved & I still get the same readings, I can spin up the alternator with a drill & it does charge, at about 13.6V but put a 55W bulb on the battery & it dropped to 12.8V 

I can take the alternator back for them to have another go, but they don't open until Monday but can anyone see a flaw in my logic, or think of any other tests I can perform, searching the interweb only seems to bring up alternators with internal regulators.

I follow your logic and think there is a fault in the rectifier whereby one of the 9 rectifier diodes or the clamp (load dump) diode has failed.

The diodes will be in the alternator body.

 

I had the same issue on a ford focus when the battery went flat overnight. When the engine was cold I put my hand on the alternator and it was warm. Diodes had failed 

Thanks guys, years ago I would have not doubted myself, but the older I get the more reassurance I need that my logic is sound.

Not a lot to do today it's a bit cold out, so I may open it up & have a probe around

Hi just remove all the alternator wires and leave overnight, If it goes flat, the alternator isn't at fault! Make sure the wires don't short out! The drain on the battery must be quite big possibly 4+ amps or more. If say you have a 70ah battery. Take an amp out for 70 hours the battery will be completely flat. Double the amps, you half the time. Regards and happy new year. Mick

1 hour ago, Greasy hands said:

Hi just remove all the alternator wires and leave overnight, If it goes flat, the alternator isn't at fault! Make sure the wires don't short out! The drain on the battery must be quite big possibly 4+ amps or more. If say you have a 70ah battery. Take an amp out for 70 hours the battery will be completely flat. Double the amps, you half the time. Regards and happy new year. Mick

I 'think' I have that covered, I put an ammeter in circuit with the alternator connected I have a 250-280mA draw, now the alternator is not on the car I have a zero mA draw 

Ok glad you have found it!

The only thing I could think of that would pull any current would be a clock or courtesy light if you’ve still got any connected. 

Took it back to the shop & he assures me it's as it should be, the only thing is I had pin 54 as a permanent live, which is actually a switched live, so this resolves the issue of the battery going flat overnight. Today I put it back on the car & with the lights on full beam, the extra LED light bar on, fog lights on, heater fan on & with the hazards flashing the battery stayed at 13.2V this was just on a fast tickover so I think it's okay, but won't know until I get it on the road to be sure.

Thanks for the input always good to ask & discuss