My 1981 440LTD blew the main 20A fuse!

So Im zipping home the other night and with no warning, boom - shutdown all power lights out motor dead. I pull over and everything is dead. After all the obvious checks at battery terminals, I find the main 20A fuse blown. Tried putting another in but still all dead when turning the key. I noticed the fuse getting hot, so I removed it before it blew again. After I got it home, I started trying to troubleshoot it. I ended up opening the headlight where all the wires meet to investigate about checking the ignition switch. The schematic says that the main 20A lead goes to the ignition switch and the rectifier. So I find the area around the opening on the back of the headlight is all chewed up from years of wires rubbing. so I move the wires around and free them up not touching the light housing and then put in another fuse to fine that all appeared OK now. Anyway, I did not find a badly damaged wire, so Im still wondering if I got a bad component still maybe. Any advice on how to be sure? What readings should I get with a meter when checking the wires from the fuse box. None of the other two 10A fuses blew, only the main 20A.

I had the same problem several years back. Chewed wires in the headlight shell which the previous owner had wrapped with hockey tape which fell off and allowed bare wire to short to inside of headlight shell blowing main 20 amp fuse.
More than likely that what you found was the source of you problem.

If your fuses are the old style glass type with metal ends then while you are in there you should use contact cleaner on the fuse holders as they often corrode get hot causing fuse to over heat and blow. Many of us have replaced fuse box to the more modern auto style .

Using a meter you should see battery voltage at main fuse and continuity from one side of a fuse to the other.

Thanks for the reassurance and insight. I was hoping someone had this happen so I could relate. My first thought was to remove the headlight housing and properly protect the opening by padding the edges and also to go and address inspecting each of the wires. But after taking a hard look, I am not too keen on disturbing all the connectors to get the headlight housing totally off. Im afraid I may create new problems. Instead, I plan to use rubber sheets custom cut and wrap them around wires either in groups or individually where I can then put the whole ball of wax (wires) in a Styrofoam sleeve so that none of it touches the edges of the headlight housing ever again.

Ohhh, I dont think I'd put styrofoam in behind the headlight. It gets pretty hot in there. I think styrofoam could melt and start a fire.
There's a good electrical tape (cant remember the correct nome offhand) with silicone which activates when stretched and fuses to itself and would be a better option. Try google.

Good Point. Didn't think of it like that. Maybe i'll just use a piece of automotive grade water hose as a protective sleeve. The rubber I am using is thin and very flexible and rated for 1000V. easy to wrap around and tape.

OK, So I got the problem all resolved and been riding around now a few days on the old KZ. I ended up getting a rubber hose to wrap around all the wires going through the rear of the headlight housing to eliminate any future rubbing or contact. I also found that there was a problem with fuses overheating due to tarnished fuse holders in the plastic box. I polished those babies up with some fine sand paper and replaced all the fuses with new. I also am keeping all the spare fuses with polishing paper in my tool bag. I guess its just another maintenance item to take care of when I check my battery. No biggie. Since I got this bike some 15 years ago, this was the first time I ever looked at the fuses, so my bad I guess.

check out my post on fuses. I'm doing away with the glass fuses on my bike with a small fuse panel that takes modern ATC fuses. going to mount it to the plastic rear inner fender, I don't have a stock air box so have plenty of room for it ..plus some extra fuses for add on stuff.
one more place that can cause the same issue is the wires coming out of the turn signals/marker lights. I had it happen to me.......was fine till I put LH blinker on....them pop goes the main fuse.