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With these cars being so old that voltage to me is a good circuit. There is no starter relay that handles the current, the switch itself handles the current, so I would assume contacts are just worn.

I have my car wired the same exact way. 2jzge 93-97 gs300 harness and auto ECU. 85 P type. Never had a problem with a 4k redline.

Have you tried running it with STA and NSW just not connected? My swap originally didn't hook these up and the car still ran fine without them.

Also do you have any check engine codes?

If you haven't you might want to skim this thread (1) 2JZ-GE PnP wiring guide | Toyota Celica Supra Forum

I went through a lot of bugs with my GE swap and ironed a lot of them out in the later part of that thread including STA, NSW, and the MREL for the ISCV as that is pretty specific
 
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Mk2 chassis doesn't have a starter relay. Starter wire goes straight from the ignition switch to the starter solenoid on manual transmission chassis. Branches somewhere in the harness to feed starter signal to STA on the 5mge ECU.

STA will show ground at rest on a meter just because of the starter solenoid coil. So it will show ground and then during crank will go to 12v and back to ground after letting off the starter.

The oem 5m ecu takes STA in as well. There is a STA pin on the ecu body connector behind the glove box, no reason to run a wire to the ignition switch. Black wire in the same plug that has the yellow/green temp sender wire

I've seen other people have a soft rev from STA NSW not being hooked up but I never had an issue there.

Per wilbo GE wiring pinouts

NSW
Automatic transmission: This pin is connected to Ground by the Starter Relay coil via the automatic shifter position switch when the automatic transmission shifter is in the 'N' or 'P' positions.

Manual Transmission: This pin is connected to the engine ECU STA pin inside the engine wiring loom and hence is connected to battery voltage when the ignition switch is in the CRANK position. When the engine is not being cranked this pin is connected to Ground via the Starter Relay coil

STA
Automatic transmission: This pin is connected to battery voltage when the ignition switch is in the CRANK position and the automatic transmission shifter is in the 'N' or 'P' positions. When the engine is not being cranked this pin is connected to Ground via the Starter Relay coil.

Manual transmission: This pin is connected to battery voltage when the ignition switch is in the CRANK position. When the engine is not being cranked this pin is connected to Ground via the Starter Relay coil.

You've effectively done this just by hooking those two wires to the starter wire at the column. 12V during crank and ground at rest due to solenoid coil. If it is an Auto ECU you could try connecting STA like normal and then ground NSW, but there again I never had an issue here with NSW or a soft rev.

I don't particularly think the soft rev is from STA or NSW, Keep in mind NSW does different things depending on the ECU. My auto trans is wired per how a manual transmission ecu works (bold print^) and runs just fine. I've also ran STA and NSW just not connected at all when I first did my swap and it made no difference. I would keep STA how it is and maybe mess with NSW and then go check for other things. Typically with throttle stuff I usually go towards the TPS but with no codes you're just going to have to test sensors per the TRSM
 

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If it revs to redline not under load but won't go past 4000 while driving I don't think your issue is with either of those wires. I would think the problem would be there regardless of idling or out driving if it was STA or NSW
 
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