No Start After Relay Swap on 7MGTE Swap - Fuse Box, Coolant Sensor, and Fuel Pump Fixes

Electricaladvanced✓ OP Confirmed

Problem

1987 MK3 Supra with 1989 7MGTE swap would not start after replacing corroded relays in the driver's side kick panel. No power to fuel pump or ignition coils. Once partially resolved, car would idle poorly, not rev, and die when given throttle.

No power to fuel pump or ignition coilsEngine cranks but will not fireNo dash lights, no RPM signal, no check engine light, no door signalsMain EFI relay in engine bay fuse box getting extremely hot and blowingOnce running, engine would not idle without holding throttleEngine dies when trying to revRuns very roughWhite smoke on first warm-up

Root Cause

Three compounding issues: (1) Internal open circuit in the driver's side junction/fuse box — the AM1 circuit that feeds power through the ignition switch to gauge, wipers, engine, and turn signal fuses was broken internally due to heavy corrosion, cutting power to the ECU B+ line. (2) ECU coolant temperature sensor was not connected, causing the ECU to default to 80°C, making cold starts extremely difficult. (3) The NA chassis still had the factory NA fuel pump (190 LPH), which was insufficient for the turbo 7MGTE engine.

Confidence

Moderate confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted December 2023 on SupraForums.com