How do I know what gauge or size to use when replacing wires in a wiring harness.Is this the number on the the lines in the wiring diagram in the service manual thanks

Yes the number is usually the gauge size.

The sizing on some circuits is really undersized.

If the wiring size is undersize will I run in to any issues if I go to a higher voltage alternator?
Should I rewire with a lager gauge wire?
Thanks
Mike

A 14 gauge wire is (1) size larger than a 16 gauge wire. When buying spools of wire be sure & keep the same colors & that sounds elementary but when replacing wires on several circuits it's tempting to just cut lengths of wire off of the same color spool which will give you a non original color which will mess you up on down the line with future diagnosis (trust me on that one). Buy several spools of the right colors/gauges & there is a company that will sell you specific lengths of a particular color of any gauge & very likely they have wires with the tracer color along the side like some OE circuits which is very hard to come by. When I redid my 65 dart harness I set it on the floor & seperated the brass terminals from the connectors & cleaned the connectors then added 1 wire at a time so the new harness took shape on the floor next to the old one & after I added 1 wire on the new one I cut the old wire off of the old harness till it dissappeared & the new harness took shape. Take notes of what connectors go where etc. I crimp AND solder joints. NAPA has new "Packard 56" type brass terminals 725145 female and 725147 male. Use an eyeglass mini screwdriver to get the terminals out of the bulkhead/connectors without damaging the connectors. some circuits the OE gauge used is likely adequate (dash lights for instance). #1 make sure all terminals/connections are clean/tight/continuity. EDIT to answer your Q even tho OE wiring size is just barely adequate a higher amp alt will not damage anything but post what wires you are wanting to replace & we'll go from there. #1 is to keep the ammeter needle close to 12 o'clock & I do that by not using the alt to replenish a batt that is discharged & relaying electrical add ons to the alt side of the bulkhead.

Maybe this will help a little - this chart assumes the length is TOTAL length - both hot and ground combined.

Maybe this will help a little

http://www.waytekwire.com/products/ has great wire products and wire marking equipment

http://www.diyautotune.com used to have some harnesses and wire bundles that were marked every few inches with common auto wire uses (12v, ign, gnd, alt, etc) -very cool to know where a wire is connected when looking at any part of it's length

Thanks for the input on where to get the wire and about the wire sizes Mike