Alternator failing or wiring?

[adebutter]

I’ve just completed a 350 mile trip in a T3 DG 1.9 to the Lakes.
The voltage was oddly low; hovering around 12V and just under.
I went to check the alternator and the wiring.

The blue and the thick red are fine with solid connections
The lowest wire (orange/red??) is really sorry looking. Old copper gone bad; oxidised and brittle. Just about put it back in the holder and plugged back in.
Now it still starts, battery light not flickering any more but voltage no better; if anything lower.

What is the lowest think wire for? It is perhaps crap the whole length ( I tried to strip back to good, found bad for a few inches.)
Should I re wire this initially and where to?
Please! Thank you!

[Mocki]

It’s common for the wire from the alternator to the starter motor to go high resistance, black in colour, and very brittle , it’s only 24inches long , but all the charge for the battery goe through it via the starter motor connections and the big wire from the battery to the starter .

It's a simple fault finding job

check alternator out put at alternator ( using alternator casing as earth neg(-) point and again at battery with engine at tickover
Anymore than 1v difference there is an issue

You can easily check if its the wire from the alternator to the starter by taking a reading at the starter motor connections
Changing the main battery feed is relatively easy , the original starter wire is now getting on for or is already thirty years old and will be higher resistance than it should be , the connections at each end will be poor or will have been replaced already .

[adebutter]

Super - thank you. I will find a voltage meter in the morning and first check if the alternator is giving good 14+ V.

Where does the bright red thick cable go to?  - (not the 3rd poor condition connection), please