Code
P1789
JAGUAR
P — Powertrain
Ignition Supply Malfunction
AI status
Completed
Completed
100%
Causes
- Weak or discharged battery
- Blown fuse(s) or faulty power relay supplying the ECU/PCM
- Damaged, corroded or loose wiring or connector to the ignition supply pin
- Faulty ignition switch or key-operated power distribution
- Intermittent open/short in harness (short to ground or battery)
- ECU/PCM internal power input fault or water damage
Symptoms
- Malfunction Indicator Lamp (MIL) or warning lights illuminated
- Hard or no-start condition, intermittent cranking or stalling
- Loss of ECU communication on scan tool at times
- Intermittent or permanent loss of engine power or drivability issues
- Electrical accessories may behave erratically when key on
What to check
- Read freeze-frame and stored freeze data and related codes with a capable scan tool
- Check battery voltage (engine off, key off) and with key ON/engine cranking
- Inspect fuses and power relays for ECU/ignition circuits; replace or swap known-good relay
- Visually inspect ECU connectors for corrosion, bent pins, water entry or damage
- Verify good battery negative and engine/chassis grounds (clean and tight)
- Measure voltage at the ECU ignition-supply pin with key in RUN and START
Signal parameters
- Ignition supply voltage ≈ battery voltage: ~12–14.5 V with key ON (allow drop during cranking but typically >9 V)
- Supply should appear within seconds of turning key to RUN/START and remain stable (no repeated dropouts)
- No significant voltage ripple or spikes on supply line (use a scope if available)
Diagnostic algorithm
- Retrieve all stored DTCs and freeze-frame data; note ignition state and voltage values recorded.
- Confirm battery health and terminal connections; perform a battery load test if weak.
- With a digital meter, measure battery voltage at rest, then at the ECU ignition-supply pin with key ON and during cranking; record values.
- Inspect and test fuses and ignition power relays supplying the ECU; replace suspect components.
- Inspect ECU and harness connectors for corrosion, bent pins, water intrusion or damaged insulation; repair as needed.
- Check main engine/chassis ground locations and continuity back to battery negative; clean and tighten grounds.
- If intermittent, perform wiggle and backprobe tests while monitoring supply voltage and module communication; use scope for ripple/spikes.
- Trace wiring from the ignition switch/relay to the ECU following wiring diagrams; repair open/shorts discovered.
- If external modules (immobilizer/alarm) are present, verify they are not interrupting power; disconnect aftermarket devices to test.
- If supply wiring and external components test good and ECU power pin still unstable, consider ECU internal fault — consult OEM service procedures for module bench testing or replacement.
Likely causes
- Blown ECU power fuse or failed ECU power relay
- Corroded battery terminal or poor chassis ground near ECU
- Broken wire or damaged pin at ECU ignition-supply connector
- Faulty ignition switch contacts not supplying power in RUN/START positions
- Aftermarket alarm/immobilizer wiring interfering with ignition supply
Fault status
Status
Ignition supply malfunction detected — ignition feed voltage missing, out of range or intermittent at ECU/PCM.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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