I have replaced the Mando 55 amp 4 wire (or, pur, blk, red w/pur stripe) on my HP 500 with a Mechman single wire 170 amp marine alt.

I'm using 2/0 wire for both + and - with a 250 amp fuse between the alt and starter lug.

Using the 90 amp fuse (wht square) on the starter lug to power the circuit breaker on the engine harness.

Using the purple wire from the engine harness to excite the alt.

My question is, what do I do with the red w/ purple stripe wire?

My understanding is it went from the alt, to the circuit breaker on the engine harness, the starter solenoid, and then through a 20 amp fuse to the ignition switch. I read it was to supply power to the helm/gauges.

Post #7 has the wiring diagram. HP 500 engine harness

The way i look at things, the red purple wire starts at the 50 amp red button relay. The red wire that feeds that relay is battery power on engine side. Again, the red purple provides batt + to dash. That’s reason it is relay protected where it is …engine side.
Since you ran a different (thicker) charge wire from alt to batt + , the red purple at alternator is not needed.
You said you ran a 2/0 charge wire ?

edit in: going off my old decrepid grey matter. Ice still on my lake. Haven’t touched a boat/engine since mid october.

not sure how you are using the purple wire. a single wire alt does not need an exciter wire. just the main feed. on a single wire alt all you have is the large lead for charging and all the others are now doing nothing.

IYeah, not really sure how it goes? Alt, starter solenoid, circuit breaker, and ignition switch.

Correct,, anchor brand 2/0 awg wire for both + and ground. It’s a little overkill, but I had some leftover along with the lugs.

Thanks you

Maybe I shouldn’t have said single wire then? The mechman alt has a single wire pigtail that needs to be connected to a switched ignition source. That’s why I’m using the purple.

You then hook up the + to the post, and connect a ground wire to the body of the alt. I scraped the paint off the body at the mounting bolt. Then installed the ground wire between the alt and the washer and nut securing it in place to the bracket.

best guess is that the red/purple is originally used as a reference voltage for the regulator and now the new regulator is using the purple wire as both and exciter and reference in one like a true one wire alternator that does all that internally. you should be able to just tape off the red/purple as it is not using it for reference voltage

Hope it really is that easy. Thanks