Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1991 Lexus LS 400FACTORY OEM SERVICE PROCEDURE
Es250 Power Circuits
- Ensure ignition is off. Disconnect connectors from ECU. Remove locks so ECU connector can be easily backprobed. See Fig 1. Reconnect ECU connectors.
- Turn ignition on. Using a DVOM, measure voltage between ground and ECU terminals BATT, B1 and E1. See Fig 2. Battery voltage should be present.
- If battery voltage is not present, check 40-amp MAIN fuse and 15-amp EFI fuse. Also ensure wire harness is not open or shorted. If fuses and wire harness are okay and conditions indicate ECU may be faulty, substitute a known good ECU. If condition improves, replace original ECU.
Factory Service Tools
Use the Manual With the Right Hardware
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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