Code
P0738
Generic
P — Powertrain
TCM Engine Speed Output Circuit Low
AI status
Completed
Completed
100%
Causes
- Open or short to ground in the TCM engine speed output wiring
- Corroded, pinched or loose connector at the TCM or downstream module
- Poor power or ground at the TCM
- Faulty TCM (internal driver failure)
- Faulty or incompatible aftermarket device tapping the speed signal
- Intermittent connection due to water intrusion or mechanical damage
Symptoms
- Malfunction Indicator Lamp (MIL) on with P0738 stored
- Erratic transmission shifting or stuck in limp/flat-shift mode
- Inaccurate or no tachometer/engine RPM display on dash
- Possible reduced engine performance or drivability complaints
- Cruise control, shift logic or related functions may be disabled
What to check
- Retrieve all stored DTCs and freeze frame data with a scan tool; note occurrence conditions
- Inspect TCM connector and wiring for corrosion, bent pins, damage, chafing and secure routing
- Verify vehicle fuses and power/ground circuits for the TCM are present and within specification
- Check for Technical Service Bulletins (TSBs) related to TCM or wiring for this model
- Use a scan tool to view live engine speed values reported by TCM and other modules (ECM/ABS) and compare
- Wiggle test harness while monitoring signal to detect intermittent faults
Signal parameters
- Type: pulsed/frequency or square-wave speed output (varies by vehicle)
- Typical amplitude: usually a logic-level pulse (commonly 0–5 V) — some legacy systems use higher amplitudes; check vehicle spec
- Behavior: frequency proportional to engine RPM (increasing with higher RPM)
- Failure indication: circuit measures close to 0 V or stuck low compared with expected pulsed waveform
Diagnostic algorithm
- Confirm stored code and note conditions (key on/engine running, vehicle speed, etc.). Retrieve related codes. Clear codes and attempt to re-create the fault.
- Visually inspect TCM and harness. Repair any obvious damage, corrosion, or loose pins before further testing.
- With ignition on (engine off) and then engine running, backprobe the TCM engine speed output terminal. Use a multimeter and/or oscilloscope to observe voltage/waveform. Compare to manufacturer spec.
- Verify TCM has correct supply voltage and a good ground at the same time as testing the output. Fix any power/ground issues first.
- If the output is low at the TCM connector, isolate wiring: disconnect downstream connector(s) and re-test to see if the output returns to normal (indicates short/load downstream).
- Check continuity/resistance between the output pin and the receiving module; check for short to ground. 7) If wiring and downstream circuits are good but output remains low, suspect a failed TCM output driver—replace or reprogram TCM per manufacturer procedure.
- After repairs, clear codes and road-test to verify the fault does not return. Re-scan to confirm no related active codes remain.
Likely causes
- Damaged or shorted harness between TCM and receiving module (most common)
- Corroded/poor connector or pin at the TCM or harness splice
- Failed TCM output driver
- Poor ground or supply to the TCM causing low output
- Water intrusion at connector causing intermittent low signal
Fault status
Status
TCM reports engine speed output circuit low (voltage/signal below threshold) — intermittent or continuous low output detected.
Repair difficulty: Medium
Diagnostic time: 1.0 - 2.5 hours
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