alternator not charging, exciter wire problem?

[some_guy]

leaving the cliffhanger property last week my alternator started playing up.
when i was moving round the campsite, at idle all the warning lights where on but as soon as i got it over 1500rpm they went out and all was fine
then got half way back to the main road and the alternator started overcharging, 13.5v at idle and peaking at 18v at 3000rpm

to stop the batteries from cooking i disconnected the alternator from the battery and drove into cobar without it

went and saw the auto elec the next day and he reckon the reg had gone, but wanted $300+ to fix it. can get a new alternator for that so ended up just driving home (11 hour trip back to Melbourne) on what was in the batteries (thanks phill and all the mutley motorsport guys for making sure i got home)

pulled the alternator apart monday and got the reg replaced buy the local auto elec (i just took the stator assembly to them and got them to solder the new reg on)
while they were doing that checked the continuity between the slip rings and that the brushes looked alright

got it all back in last night and the warning lights are on with the car running and the alternator isnt charging

started checking all the wiring next and found something
disconnected the plug off the back of the alternator and checked the voltage at both wires. both the thick white wire (sense) and the thin white/red (from warning light) have 11.5v (full battery voltage, i drove 11hours with no alternator remember)

but if i leave it plugged into the alternator the thick wire still has 11.5v, but the thin wire only has roughly 2-3v
2-3v wouldnt be enough to excite the alternator and start it charging i'd think

the question i have though is why would the voltage drop so much when plugged into the alternator?
or is that normal and theres something else going on that i've missed

the car is a 91 td42 GQ with a 90Amp alternator and vac pump from patrolapart

Glenn