Currently having problems and needing help! I have a 1990 LS400, I was driving home one day and it just completely shut off out of nowhere, I have already diagnosed the ECU, Ignition tumbler, Wiring harness, Fuses, Relays, Battery, Alternator, and Starter. Whenever I turn my key into the ignition position my electronics and lights and everything works, but when I go to crank the engine there’s not even a click from the starter, nor can I hear the fuel pump priming, but I can bypass the starter from the relay and it cranks the engine over, but the key does nothing, anyone have any inputs or advice?

 

Your words, "but I can bypass the starter from the relay and it cranks the engine over, but the key does nothing" indicate that the problem is caused by the ignition switch, ST fuse 7.5A, J/B4, Park/Neutral Position Switch or related harness in accordance with the circuit diagram attached. I'd check the voltage of output terminals of these parts to determine which is the culprit.

 

Ok I’ll make sure to take a look at those, but I just replaced the ignition switch which did nothing, and I checked the fuses which they didn’t look burnt but I’ll go ahead and replace them, thank you for the input I’ll make sure to keep the thread updated

 

In that case, I think that the highest possibility is the Park/Neutral Position Switch. For the confirmation, I'd try to start the engine after shifting the AT lever between P and N for many times while pushing the release button. To try to set the lever to N position is another choice.

The reason to shift many times is to try to revive the contacts by the friction.

 

Went to go and try that for a bit, didnt do much but also remembered, back then when I went to take it out of park I’d have to hold the release button down, but sometimes I wouldn’t, when I went to go and try what you said I didn’t have to press the button at all to shift it out of park, I still held it down anyways to try everything but I just now remembered that.