Unfortunately I did the stupidest thing I could have done, I reversed the polarity of the battery!
BRAAAVOOOO!
The car no longer starts.
I checked all the fuses, those inside the right, the left and the center fuse box. None of the fuses have blown.
The ignition does not turn on and the starter does not turn.
The bulbs of the instrument cluster turn on but everything else is turned off.
It seems that everything connected to the ignition key (II position) doesn't work.
Other things work, including power windows, central locking, sidelights, and hazard lights.
I tried replacing the ignition relay (behind the radio, light blue color) without any luck.
Before this stupid thing, everything worked and the car started immediately.
I have tried looking at the wiring diagram to understand where the problem might be but I don't know what to check first
Any help would be highly appreciated, thanks.

 

Hi Antonio,

We've all done things we regret, so don't be too hard on yourself.

As far as I can recall, your '93 XJ40 does not have any megafuses that might have blown.

One possibility is that the battery itself was damaged. Do you have a way of testing it?

On the XJ40, most of the switching is on the ground side of circuits. It's late and my brain is foggy so I'm having trouble thinking of the possible implications, but my first thought is that most circuits would have relay or switch contacts open, so the positive battery voltage would probably have had no chance to damage most components. But there are still quite a few components directly connected to ground, including the Engine Control Module, Central Processing Unit, etc., and any of those could have been damaged. The starter relay logic is built into the CPU, for example.

I'll try to think of some diagnostic steps you can take, and hopefully others will have some ideas.

Cheers,

Don

 

Someone years ago said:
"Before I go through a long explanation of the starter circuit, I would say that the most likely causes of a sudden no-crank condition are low battery voltage (as previously mentioned), the gearshift lever not being fully seated in the Park position or a problem with the Park/Neutral switch or Start Inhibit Relay, a problem with the starter relay, a problem with the ignition switch."
Maybe you Don??

 

I guess the key to solve the problem is inside this electrical diagram...

I will check at first the IGNITION SWITCH under the steering wheel or maybe the inertia switch, they are the first switches connected to the ground.

 

Thanks Don for your help,
I was wrong to check the fuses.
Visually they were all ok but testing them one by one the following fuses in sector 2 (bottom left) were blown:
- A5 (Violet) 3A
- C1 (Yellow) 20A (Engine Management System Power)
Please take a look at the video...bye.

 

Congratulations, Antonio!

Your persistence paid off!

Cheers,

Don