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.
1990 Mercury Grand Marquis LSFACTORY OEM SERVICE PROCEDURE
Pinpoint Test E - Abs Pump Runs Continuously
- Cycle ignition on and off while observing pump motor operation. If pump motor operates with ignition off, go to step 2). If pump motor operates with ignition on, go to step 3).
- Unplug pump motor relay connector. Check for continuity between pump motor relay pin No. 30 and TEST pin on relay. See Figure . If continuity exists, replace pump motor relay. If continuity does not exist, verify pump motor still runs with ignition off.
- Turn ignition off. Unplug 55-pin connector from ECU. Connect Breakout Box (014-00322) and ABS Adapter (T90-50-ALA). Check for continuity between breakout box pins No. 15 and No. 60. If no continuity exists, go to step 5). If continuity exists, go to next step.
- Unplug pump motor relay connector. Check for continuity between breakout box pins No. 15 and No. 60. If continuity exists, repair circuit No. 539 (Pink/Light Blue wire). If no continuity exists, replace pump motor relay.
- With ignition off, reconnect pump motor relay connector and 55-pin connector. Turn ignition on. If pump motor runs, replace ECU. If not, verify symptom still exists. Repeat test if necessary.
Factory Service Tools
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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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