Friend's ST1100 burns main fuses when he switches power on.

So, that is the problem. From where he should start fault finding? Bike is 1992 standard and there are no farkles bolted on.

Remove seat, LH pannier and LH side panel to inspect the mains harness underneath...
I suspect some major wiring being toasted... mains relays, connector, etc...

In addition to the visual inspection for indications of melted, overheated, burn marked areas; I have substituted a high wattage bulb jumped in place of a fuse to avoid replacing many one-time-use fuses until I've narrowed down the problem area. A very bright light indicates a short circuit (blown fuse) and will get more dim when the short is disconnected. With short circuits, my experience has been to disconnect (pull fuses) from as many circuits as possible, then sequentially reconnect and test (by light brightness) until the culprit is located. The advantage of the bulb allows movement (pull, wiggle, tap, whatever) of wires, components and other electrical connections that will just cause the bulb to get bright when the fault is located instead of replacing another fuse. Just my $.02, YMMV

Good advice in the above posts.. Also disconnect the alternator and starter and start connecting items and replacing fuses 1 at a time and see if you can isolate the bad circuit ..

v8-7 said:

Good advice in the above posts.. Also disconnect the alternator and starter and start connecting items and replacing fuses 1 at a time and see if you can isolate the bad circuit ..

Click to expand...

A slightly different approach : Or disconnect the positive battery cable and read the resistance to ground of each circuit ( pull the fuse for the circuit being tested ). Divide 14 by the fuse size. The resistance should be more than that calculation.