40 amp ignition fuse keeps blowing out after turning key on.

Pull ALL THE OTHER FUSES before installing another 40 amp and turn on the key,then install one at a time until you hit the circuit that is shorted and blowing the fuse.Now you can trace that circuit for problems.

You have a short to ground on that circuit , find a wiring diagram an see what all is on that fuse . So far I see the ignition switch an starter relay ! Does it blow right away when you replace it or do you have to turn on the ignition switch ?

Blowing fuses is never an easy direct fix. Its rather a hunt needs some dedication. What I try and do is Figure out what is all on this 40 amp fuse, and one at a time remove something from the system and see if it blows. The key is knowing when it will blow for these tests to work. Keep removing what ever is on the system until it stops blowing a fuse. Then figure out how to single out items on that fuse. Whatever fuse it is the problem is on its path. Check all paths that go to items if no items are easily singles out. Replace as you find ,smash, green corrosion and shady connecters. Looks for heat by finding melted plastic connecters. These are clues. If you have a volt meter, then you will find the problem a whole lot easy, by figuring out what OHMS LAw is for whatever guage wire you are working with and a rough guess on length of known path. Using this you can find smashed and corroded wires quicker. As well as finding a short to ground and even short to powers in some cases. Good luck :)

It will help if you can identify the 40 Amp fuse that is being toasted when the key is turned.

I looked briefly at a fusebox diagram and found 3 circuits protected by a 40 Amp fuse. The Lamp circuit protects several circuits which have smaller rated fuses on each separate wire. The 40 Amp protection seems to be a safeguard if all the smaller circuits are active in total. Turning off the automatic headlights, should reduce power drain.

The next circuit is the Rear window defroster. Can easily pull 40 Amps, but once the 40 Amp fuse breaks, it should have no effect on the engine.

The last circuit, on the cars with Trailer Tow or Police package has a
40 Amp fuse on the Battery charge Relay. This is a remote possibility. But the engine should be able to run without an Alternator for several minutes using Battery power.

Have them check the fuel pressure. The Inertia switch could have tripped while the car was being repaired. It does not explain the wiring short, but can explain the no start condition.

Again, if the same 40 Amp circuit is overloading, identify the circuit with the fuse box diagram.

that is a dead short, the fuse saved you the ensuing fire.
there are 2 fuses or more... with that name..
old cars love to short things, no lie.
the fsm is the best thing to have, and alldata.com , 2nd.
the older that car the more true that is... and your car is 10x that.
a 1991
is the fuse in the cab BOX? NOT ENGINE BAY?
THE CAB FUSE , HAS these (USA cars, other are DIFFERENT)
no country stated (sigh)
ignition sw AM1
ignition sw AM2
ignitor 1
ignition coil (1 for 4cy, and 1 for v6 engine, unsaid)
on one car there is a IGN fuse 7.5 amps
and runs the ECU and main relay.
there are 3 engines, and drawing for all 3.

get the fsm and use that or login to alldata.com and hope
it covers your engine. (unstated)
22r,3vz,3F

U have a positive wire making ground

Why was the battery replaced? Theres no reason the the 100 amp fuse to blow unless the alternator is shorted or you hooked up the battery backwards. I think the 100 amp fuse goes to the alternator. If it was hooked up backwards then you may have engine computer damage.

Fuses only blow if-Old,Overheated or you have a Short circuit.
That should be any easy fix,to the extent that there aren't many 40 Amp Fuses and you will need a wiring diagram to follow the problem from the fuse to the cause. If you know what fuse it is, you can guess,but I wouldn't recommend that,it will be time consuming and maybe costly ,if you don't have a test procedure to follow and throw parts at it.
Don't replace the fuse.Their expensive and will keep blowing.

It sounds like you have a short in some wiresets. Check the wires directly connected to the ignition. If this persists, without any problems with wires, I suggest checking the amperage and voltage on your battery to make sure they are at operational levels. Anything over 14.8 volts and you start to fry electrical components and... blow fuses.

I'm sure there's a short somewhere, but I can't find it. I've even replaced the starter and it is still doing it..???????
The wiring that fuse protects is shorted to ground ! You don't know how to find a short ? Your best bet ,take it to a shop that can .
Finding The Source Of Electrical Shorts In Your Car