I am looking at the Toyota factory repair manual. The wiring diagram shows that the 50 amp main fuse and the 100 amp alternator fuse are independently fed from the battery. Before we go any further, did you just remove and clean the connector that bolts directly to the battery? If so i would suggest removing the 12 mm nut that connects the TWO cables to the positive battery post. Of course disconnect the negative battery cable first. You will see that there are two connections to the battery positive post. A regular cable that goes directly to the starter and the connector that all the fuses are fed by. I would start by inspecting that assembly very carefully. The white 12 gauge wire that is connected to the 100 amp fuse goes directly to the alternator, that circuit also feeds several low draw circuits but if it's blowing the 100 amp fuse the short is going to be in the wiring harness with the 12 gauge or where the fuse block connects to the battery. a short in the smaller gauge wire would smoke and burn the wire in two before that 100 amp fuse blew. Anyway the fuse blows without the ignition switch being on, right?
I don't know how mechanically inclined you are but here's a suggestion. That 100 amp fuse is for maximum flow from the alternator. To diagnose the problem you don't have to keep blowing 100 amp fuses. You can fabricate a jumper wire with an inline 20 amp fuse that should not blow if the short is fixed and the engine is not running.

 

Hi Mor2bz
Few tests for a start
Resistance measure before fuse connecting, please do one, sometimes this box gets this white acis/salt deposite which can turn into highly conductive carbon short under the heat crested by high current.
W/o battery measure resistance between the main battery connectors. Should read infinite.
Another option
Upon starting the main ignition switch does pretty much work and can be short inside so disconnecting its connector can show too.
Another option is a stucked contactor (or a relay) in the light system which works without the switch.
Sometimes bypass wires are used in old cars and the source power is taken where "Here there are 12v constant, lets use this one"....
Got the Haynes so a model can help finding the right diagram
Good purchase, got 2 auto ones, early 90's, love'em...

 

Good recommendations but according to the owner it's blowing a 100 amp fuse. The short would have to be in pretty heavy gauge wiring or the "fuse panel" connected at the battery positive post. If the short was in lighter gauge wiring, such as 16 gauge or smaller, the wire would burn in two and the insulation would smoke before it would blow a 100 amp fuse.....
Also the owner needs to verify if the short is with the key on or off. I do have the factory wiring diagram, and i do make mistakes too, that circuit is spread over several different pages but every thing with heavy enough wiring that is "live" besides the wiring going to the alternator, which is "hot" with the key off is the heater/a/c fan wiring which is only "hot" with the key on....

 

Hi Mor, Seagray, a tour with camera will show the bug
According to the book there is no problem so we must seek out of the box
You can write to aofek54@gmail.com to begin investigation
As for 100A fuse, it is unlikely to burn and the only power source is a good battery but never under estimate the ability to bypass wires...lol
From baypass for the starter so one override the Neutral/parking switch for starting, via using a spare set of main switch connections all the way to hi power sound amplifier which is directly connected to a mid wire somewhere under the car....
50 years of cars hobbying, one see the improbable many times...

 

I usually start with the "most likely' first myself, there is always the possibility that the van has been "modified" I do have a copy of the toyota workshop manual for 1991 models with the wiring diagrams and i actually do know how to read them.
I got some pretty good schooling on the subject from the USAF. And was a light line tech for 16 years. I"m 71 so i have put in some time on the subject.

 

Hi Seagray, sorry if i gave the impression of disrespect.
Mor specified the previous done tests so ignoring the diagram was the next thing
At 67 and 71 we have seen pretty much.... Lol
Anyway, lets see if Mor finds the bug, if not we are here....

 

I'm not as confident as you in the ability of the average person to accurately do the test's. I have seen many "modifications" and done some myself and don't discount that someone has added some wiring. Usually if the problem can't be found by checking the "stock" wiring and that person is asking for help it's probably going to have to be solved by a pro. Those are in short supply who
want to work on a 30 year old obscure van. I know this is true because this is how I come by all my vans. I now deliver diesel tractors all over the country and use the vans as tow vehicles. I've gone through about a half dozen in the past 10 years all bought "dead" or limping.
P.S. When I was growing up my dad owned a salvage yard in Oklahoma, due to there being too many "midnight shoppers" we lived on the property. This was my playground growing up. Needless to say it was also the source of all my cars, my first car was a 53' Nash Rambler that i "claimed" as mine at 12.....

 

Good childhood... Lol
Well, am a mech. Engineer, cert. Electrician, software progremmer and in general anything that neeeds to fix i do so if Mor shall need it, with whatsapp video chat we shall do it....
No need for god's help..... Lol

 

I applaud you, he's lucky to have someone who will go to those lengths nto help. There is a limit to my willingness to help, i will proffer my opinion and hints but that's about the limit...

 

I suspect reversed polarity. The large spark was the determining factor. I wrote a long post about one of the worst reverse polarity experiences at my shop ever when I called the police to keep me from killing a jerk, but way off topic.

 

The simplest answer is often the correct one...