I have an older Husqvarna 42" riding mower that keeps blowing a fuse (40A). Never had a problem with it before now.

I was mowing last evening and it just up and died. First thing I checked was that fuse and yep...blown. Put a new one in and it started up and ran fine for one lap and then died again. Fuse blown.

I went through this video this morning (it's for a Craftsman, but I have the exact same wiring on my Husky) and the switch, dash, engine, and chassis wiring harnesses all check out fine.

I put a new fuse in and she started right up and ran for about a minute and blew another fuse.

I'm stumped. Anybody got a clue where to look next?

If you have an old round auto headlight, put two wires on it and test it to light up with the battery.
Then put the wires where the fuse goes, one at each of the terminals at the fuse holder.
When a fuse blows, you have a power going to ground.

The test headlight will light up if you have a short to ground.
Move the system wiring around until the light goes out, or unplug components if need be.

I have a lighted tester thingy. I unhooked the negative terminal to the battery and hooked one end of the tester to the wire and one to the terminal, then turned the key on. It lit up indicating a short...somewhere. But everything tested before checks out. Am I missing something?

Some rodent likely chewed a wire over the winter and it's shorting to something metal on your tractor

I don't have a wiring diagram, but would unplug everything on that circuit one component at a time, voltage regulator, switches, engine, etc. Mice decided to chew something under the engine shroud could be possible.

According to the video I referenced, if no voltage is present at the fuse terminals with the switch unplugged, the short is in the switch itself. Just checked that again, and this is the case. I guess I'll try a new switch and see if that works.

My guess is that somewhere in the wiring there is a worn spot in the insulation where the conductor is partially exposed. When the engine vibrates, this comes in contact with a path to ground and the fuse does what it is supposed to do. Whether wear was caused by a rodent's incisors or simply rubbing against the frame is unimportant. Inspect every inch of the wiring, pay double attention to where it runs along metal or near wiring harnesses, look for paint erosion as well where the wire might have been rubbing against a painted surface. Usually, this situation can be fixed by reattaching the wire to a harness, electrical tape, shrink wrap, etc.

There are also "slo-blo" fuses that might get you through intermittent shorts.

I put the new switch in and that appears to have been the issue.

Temporarily install a 40a circuit breaker.

With the mower in the darkest place possible start moving the wiring harness around while watching for any arcing to ground.

Years ago my ‘82 Harley would occasionally lose all electrical power and almost immediately come back on. Moved wiring, tapped on components, shotgun diagnostics so I replaced the main circuit breaker and the ignition switch, the latter was very high on the list as a couple of terminals were loose on the terminal board.

Riding one night on a street in a neighboring town I hit a bump, bike went dead. This one stretch of roadway was dark with no streetlights and no other traffic. So I start moving wiring and there was a crimp butt connector with heat shrink tubing that appeared to be tight about three inches from the battery positive terminal. That wire was pretty much immovable being so short and below that it’s tied to the frame with a zip tie. Once I moved it, saw a small arcing through the yellow heat shrink, there was my problem. Got it to make contact, rode home and replaced it, problem solved.