mx5 alternator testing

[RichardH]

my 1.8 mx5 engine 1999 is not charging, i suspect the alternator is faulty but there isnt much about testing them , i have 12v on the heavy cable which comes from the battery via main fuse, i would expect this to rise to 14v ish on startup but it doesnt.
the plug onto the alternator has two wires a grey and a grey/red, i have 5v on the grey and nothing on the grey/red, from the web i reckon the grey/red should have 3 v AC ???? as a signal to the ecu. before i cough up for another alternator anyone any ideas