P1643

Code

LAND ROVER P — Powertrain

Engine control module CAN / transmission control module / network operation

Brand: LAND ROVER

Type: P — Powertrain

Views: UK: 18 EN: 57 RU: 29

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Completed

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Page language: EN

Causes

Open, short or high resistance in CAN High/Low wiring between ECM and TCM
Failed or intermittent power or ground to one or both modules
Corroded or loose module connectors or pins
Missing or failed CAN termination resistor(s)
Faulty ECM or TCM internal CAN transceiver
Software mismatch or corrupted module software/calibration

Symptoms

Malfunction Indicator Lamp (MIL) or transmission warning light ON
Transmission may enter limp-home/safe mode or exhibit erratic shifting
Engine and transmission functions not coordinated (stalling, rough shifting)
Loss of certain instrument cluster messages or misreported gear position
Intermittent no-start or run issues if communication critical

What to check

Read and record all stored and pending codes from all control modules (ECM, TCM, ABS, BCM)
Verify presence of ECM and TCM on the network with a diagnostic scanner (scan tool list of modules)
Check CAN bus idle voltages: CAN High ≈ 2.5V, CAN Low ≈ 2.5V and differential ~0V; normal driven states should show expected swing (~2V differential)
Measure resistance across CAN High to CAN Low with ignition off — should be ≈ 60 ohms (two 120 ohm terminators in parallel) or manufacturer spec
Inspect connectors, pins and wiring for corrosion, damage, or loose terminals at ECM, TCM and junctions
Verify power and ground circuits to ECM and TCM (battery voltage, fused feeds, ground resistance)

Signal parameters

CAN High idle ≈ 2.5 V, CAN Low idle ≈ 2.5 V (single-ended), differential idle ≈ 0 V
CAN High during dominant state ≈ 3.5–4.0 V, CAN Low ≈ 1.0–1.5 V (differential ≈ 2.0–3.0 V)
Bus resistance across CAN H-L ≈ 60 ohms (two 120 Ω terminators in parallel) or OEM spec
Typical CAN message frequency depends on message type — some engine/transmission messages 10–100 Hz
Voltage supply to modules should be stable at battery voltage (≈12 V) with minimal drop under load

Diagnostic algorithm

Retrieve all DTCs from all modules and freeze frame data. Note occurrence pattern and history.
Verify scanner can communicate with ECM and TCM. If one module is missing, focus on that module’s power/ground and connectors.
Visually inspect CAN wiring, connectors, and junctions between ECM and TCM for damage, corrosion or chafing.
With ignition ON (engine off), measure CAN H and CAN L voltages at ECM and TCM connectors and compare to expected values.
With ignition off, measure resistance across CAN H to CAN L at a convenient access point. If outside spec, isolate sections of the bus and retest to find the faulted segment.
Verify power supply and ground to both modules (battery voltage at module power pin,
Disconnect non-OEM accessories that connect to CAN or power and retest for faults.
If physical wiring and supplies are good, perform a module presence test and re-flash or update module software per manufacturer service bulletin if an update is available.
If communication still fails and isolated to one module, bench-test or replace the suspect module (TCM or ECM) following OEM procedures and program new module as required.
Clear codes, perform road test and monitor network for recurrence. If intermittent, use a scope or CAN bus analyzer to capture events during occurrence.

Likely causes

Damaged CAN bus wiring harness near transmission or engine
Poor ground or fused power supply to TCM or ECM
Failed TCM (no response) or failed ECM
Broken/missing network termination (open/incorrect resistance)
Recent module replacement without proper programming or incompatible software

Fault status

⚠️ Status

Loss or improper operation of CAN communication between Engine and Transmission control modules — messages missing, intermittent or invalid, affecting module coordination.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5-4 hours

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P1643 — Engine control module CAN / transmission control module / network operation