Changing from a Mando Alternator to a single-wire Alternator: Wiring problems

Hello,

I'm doing a lot of work on my 1991 Bayliner Capri with a 5.0L Mercruiser V8 and I am overhauling most of the electrics.
I've just fitted a new high-output single wire alternator to replace the old one (Mando-type I think) and I don't know what to do with the purple and red/purple excitation wires.

The earth and orange wire to the starter motor have been wired correctly and the motor was starting and running OK this weekend but I think that the alternator wasn't charging because:
a) I was getting no more than 12.5V at the battery when running the engine
b) There is no increase in voltage after starting the engine, and when revving the engine.

I connected the purple and red/purple wires back to a +ve terminal but that made no change to the voltage in the system.

Should I leave them isolated or connect them back to a +ve?
Is my alternator definitely not charging?

(BTW, I noticed that the alternator belt could do with some tightening but that shouldn't be the issue as the alternator is turning with the engine.)


thanks for your help

Re: Changing from a Mando Alternator to a single-wire Alternator: Wiring problems

With the engine off, is there voltage at the Orange wire where it hooks to the Alt.?

Re: Changing from a Mando Alternator to a single-wire Alternator: Wiring problems

Don't know. Will have to check and get back to you tomorrow.

Re: Changing from a Mando Alternator to a single-wire Alternator: Wiring problems

Yes there is.

Its probably also worth pointing out that the purple and red/purple wires always have voltage, even with the ignition off. Previous owners have by-passed a lot of things in the wiring of this boat.