Code
P1690
SCION
P — Powertrain
Transmission communication malfunction
Views:
UK: 2
EN: 6
RU: 2
AI status
Completed
Completed
100%
Causes
- Damaged or corroded wiring or connectors in CAN/communication circuit
- Faulty transmission control module (TCM)
- Faulty engine control module (ECM)/PCM or other network module
- Poor ground or battery voltage (low supply or intermittent)
- Blown fuse or poor power feed to TCM/ECM
- CAN bus short to power/ground or open circuit
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Transmission may go to limp/backup mode or default gear
- No shift or harsh shift behavior
- Dashboard may show transmission communication warnings or gear indicator absent
- Other modules may report lost-communication DTCs (ABS, BCM, etc.)
What to check
- Read all stored and pending DTCs and freeze frame data with a capable scan tool
- Check battery voltage and charging system condition (must be stable during cranking and key ON)
- Inspect fuses and power feeds for TCM and related modules
- Visually inspect TCM and wiring harness connectors for corrosion, bent pins, water ingress or damage
- Check engine and transmission ground points for tightness and corrosion
- Attempt to communicate with TCM and ECM via scan tool; note if one or both are non-responsive
Signal parameters
- CAN bus recessive voltage: ~2.5 V on CAN_H and CAN_L (both near 2.5 V with key ON, idle bus)
- CAN dominant state: CAN_H ≈ 3.5 V and CAN_L ≈ 1.5 V when active (differential ≈ 2 V)
- Termination resistance across CAN_H and CAN_L: ≈ 60 ohms (two 120Ω terminators in parallel)
- Typical CAN data rates: 250 kbps or 500 kbps depending on model — mismatched rates cause comm loss
Diagnostic algorithm
- Verify battery fully charged and charging system healthy; reconnect battery if recently disconnected and confirm stable voltage
- Use scan tool to confirm presence/absence of TCM and ECM on network; document related U-codes and freeze frame
- Inspect TCM and ECM connectors: disconnect, inspect pins, clean and reconnect; check for water, corrosion, or bent pins
- Check fuses and power feeds to TCM/ECM; measure key ON voltage at module power pins
- Measure CAN_H and CAN_L at the TCM connector with a DMM (key ON, engine OFF) — expect ~2.5 V each; look for open/short conditions
- With a scope (preferred) observe CAN differential while cranking/operating to see data packets and noise or intermittent drops
- Measure resistance between CAN_H and CAN_L (power off) — expect ≈60 Ω; check for short to battery or ground on each CAN line
- If one module is not responding, disconnect other modules on the same bus one at a time to isolate a faulty node or short
- Wiggle-test wiring harness while monitoring comms to find intermittent breaks
- If wiring and power/grounds check OK and the module still fails to communicate, consult TSBs for software updates and consider reflashing or replacing the TCM/ECM as final step
- After repairs clear codes and road-test to confirm fault does not return; verify all modules communicate properly
Likely causes
- Poor or corroded connector at TCM or ECM
- Broken/chafed CAN high/low wires between ECM and TCM
- Faulty TCM (internal failure)
- Weak battery or bad main ground causing intermittent comms
- Failed termination resistor or short on CAN bus
Fault status
Status
Transmission control module communication fault detected — TCM and vehicle network are not communicating properly.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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