Thanks bentwrench, I've been looking through them on here but not sure which is the exact right one, would you be able to point me in the right direction?
Also the bike has had 14 previous owners and looks like a few of them have had attempts at wiring before so I have to work out their modifications too. I think I'm going to strip it back to the bare minimum to start and run without lights or anything and then add extras to the circuit myself.
Got a multimeter today anyway that makes life a lot easier!
If you haven't already downloaded it the entire manual for the 1983 seca is online. There are slight differences in the seca and your bike, but they are mainly cosmetic differences. The electrical and engine should be about 95% the same.
Go to the garage page. Click on the xs400 manuals link and it is the 3rd post down
The post looks like this but it also has the pdf attachments for the manual
Service manual for the 1982 XS400 in English, French, and German. Large download. Should be good for all DOHC models.
Chapter 1 - General Information
Chapter 2 - Periodic Inspection and Adjustments
Chapter 3 - Engine Overhaul - Part 1 - Removal and Disassembly
Chapter 3 - Engine Overhaul - Part 2 - Inspection and Repair
Chapter 3 - Engine Overhaul - Part 3 - Assembly and Adjustment
Chapter 4 - Carburetion
Chapter 5 - Chassis
Chapter 6 - Electrical
Chapter 7 - Appendices
The best electrical diagram is in chapter 7. It is the most complete and shows the connections and wire colors.