Problems with the electric part

I really need help. My SUV has no power, it stalls when I turn on the air conditioning, and it doesn't shift gears properly. The fuel injection pump has been checked and it's fine, and the transmission has no mechanical problems either.
The mechanics I've consulted say it's not a mechanical issue but something wrong with the computer or the electronics.

It's a 1997 Nissan Serena with a CD20 turbo diesel engine with electronic fuel injection

Did you check all the diesel filters? If I'm not mistaken, these also have a filter inside the fuel tank. - Regards, Alfredo

I know of an Alfa 145 that stalls depending on the electrical demand, whether it's turning the steering wheel, using the air conditioning, turning on the lights, or a combination of these. This is due to an unstable idle caused by a fault in the throttle body, which leads to drops in idle speed. This combination can also be caused by a damaged or dirty mass airflow sensor.

Before we start guessing or debating the sex of angels, we should connect a diagnostic tool and check the engine control unit (ECU).
The second thing would be to check the ECU's positive and ground power supplies, because considering the fuel pump has been checked and the engine stalls when a consumable (air conditioning) is connected, we need to verify there's no voltage drop in these power supplies.

Best regards.