Code
P1635
DS
P — Powertrain
Ignition relay control short circuit to earth or open circuit
Views:
UK: 8
EN: 9
RU: 3
AI status
Completed
Completed
100%
Causes
- Broken or open wiring in ignition-relay control circuit
- Short to chassis ground on the relay control wire
- Faulty ignition relay (coil or contacts)
- Poor or corroded relay socket/connector or ECM connector
- Blown fuse protecting relay supply or ECM
- Faulty ECM driver transistor (internal to module)
Symptoms
- No crank or no-start condition when ignition is turned on
- No audible click from ignition relay when key is turned
- Intermittent starting or loss of ignition power
- Engine stalls or cuts out immediately after starting
- Malfunction indicator lamp (MIL)/warning light illuminated
What to check
- Record freeze-frame data and full DTC list before clearing codes
- Verify battery voltage is adequate (12.4–12.8 V at rest) and connections are clean
- Visually inspect ignition relay, fuse(s), relay socket and wiring harness for damage or corrosion
- Check for aftermarket devices (alarms, immobilisers) spliced into ignition wiring
- Measure voltage at relay power terminal with key ON
- Check continuity and resistance between ECM relay-control pin and relay socket control terminal
Signal parameters
- Relay supply terminal (battery feed): ~12 V (key ON/START)
- Relay coil/control terminal (ECM-driven): typically switches to ground or to battery depending on design; expected to change state when cranking
- Open-circuit: infinite resistance between ECM control pin and relay socket control terminal
- Short-to-ground: near 0 Ω between control wire and chassis ground when relay is not commanded
- ECM output driver impedance: low when active, high (open) when inactive — specific values are manufacturer-dependent
Diagnostic algorithm
- Read and record all stored codes and freeze-frame data. Verify P1635 is current and note any related codes.
- Inspect fuses and relays: verify fuse feeding ignition relay and ECM are intact. Swap relay with identical known-good relay if available and re-test.
- Visually inspect relay socket, wiring harness, and any splice points for damage, corrosion, pin push-out or overheating.
- With key OFF, disconnect the ignition relay and measure resistance from the ECM relay-control pin to chassis ground and to battery to detect short or open.
- Back-probe the relay socket control terminal while an assistant operates the key to START/ON. Confirm the ECM control signal changes state (voltage to ground or battery as applicable).
- If control signal does not reach the relay socket but is correct at the ECM connector, repair the harness between connectors; if it is absent at the ECM, suspect ECM driver fault.
- If short to ground is suspected, isolate sections of harness (disconnect connectors in-line) and re-check to locate the shorted segment.
- Check for intermittent faults by wiggling harness at common stress points while monitoring the control circuit and observing for code reappearance.
- Repair or replace damaged wiring, connectors or relay. If ECM driver is suspected and wiring/relay verified good, consult vehicle manufacturer guidance before replacing ECM.
- Clear codes, perform functional test (start/operate engine) and re-scan to confirm repair. Monitor vehicle for recurrence.
Likely causes
- Worn/corroded relay socket or poor connector/crimp
- Broken/open control wire between ECM and relay
- Ignition relay failure
- Short to ground from damaged insulation where harness passes through bulkhead
Fault status
Status
Ignition relay control circuit — short to earth (ground) or open circuit detected.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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