P1736

What Does Code P1736 Mean?

DTC P1736 indicates a malfunction within the electrical circuit of the second gear switch, as detected by the Powertrain Control Module (PCM) or Transmission Control Module (TCM). This code signifies that the control module has identified an electrical anomaly in the signal pathway from the transmission’s second gear pressure switch or range switch. The second gear switch is an integral component, providing crucial feedback to the PCM/TCM regarding the engagement or disengagement of the second gear ratio, often by monitoring hydraulic pressure within a specific circuit or direct mechanical position. The PCM/TCM continuously monitors the voltage signal or resistance across this circuit, expecting specific electrical characteristics corresponding to commanded or actual second gear operation. When the module detects a voltage reading outside its calibrated parameters – such as a continuous open circuit (no signal), a short to ground (signal always low), a short to voltage (signal always high), or an implausible signal that contradicts other sensor inputs (e.g., vehicle speed, engine RPM, throttle position) – it interprets this as a circuit malfunction and triggers DTC P1736.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination: The “Check Engine” light will be activated on the instrument cluster.
• Harsh or erratic shifts: The transmission may exhibit noticeable jolts, delays, or unexpected gear changes, particularly when shifting into or out of second gear.
• Transmission “limp mode” activation: To prevent potential internal damage, the PCM/TCM may force the transmission into a default limp-home mode, limiting available gears (e.g., stuck in 3rd or 4th gear only) or disabling specific shift solenoids.
• Delayed or absent second gear engagement: The vehicle may experience a significant lag when attempting to engage second gear, or may bypass second gear entirely.
• Incorrect gear display: If the vehicle is equipped with an electronic gear position indicator, it may display an incorrect gear, remain blank, or flash erratically.

What Causes the Code P1736?

• Faulty Second Gear Pressure Switch/Sensor: The most common cause, where the switch itself has an internal electrical failure, such as an open circuit, short circuit, or degraded internal resistance, preventing it from sending an accurate signal.
• Wiring Harness Issues:
  • Open circuit (break) in the signal wire between the second gear switch and the PCM/TCM.
  • Short circuit to ground in the signal wire.
  • Short circuit to battery voltage (B+) in the signal wire.
  • Frayed, chafed, or damaged wiring due to heat, vibration, or physical abrasion.
• Corroded or Loose Electrical Connections: Poor pin tension, corrosion, or contamination at the second gear switch connector or at the PCM/TCM connector can interrupt the signal path.
• Internal Transmission Mechanical Failure (Indirect): In rare instances, a severe mechanical issue preventing actual second gear engagement could lead to the switch not activating, which might be interpreted as a circuit fault if the PCM/TCM expects activation. However, the code specifically points to a circuit issue.
• Faulty Transmission Control Module (TCM) or Powertrain Control Module (PCM): Less common, but an internal defect within the control module itself could prevent it from correctly processing the second gear switch input or providing proper reference voltage/ground.

How to Diagnose and Troubleshoot

Diagnosis of P1736 requires a methodical approach using a scan tool and a digital multimeter (DMM).

1. Retrieve DTCs and Freeze Frame Data: Connect an OBD-II scan tool and confirm the presence of P1736. Analyze associated freeze frame data, which captures critical engine and transmission parameters (e.g., vehicle speed, engine RPM, throttle position, transmission fluid temperature) at the moment the code was set. This context is vital for understanding operating conditions.
2. Visual Inspection:
   • Perform a thorough visual inspection of the transmission wiring harness, paying close attention to the area around the transmission housing. Look for signs of chafing, pinching, corrosion, or damage to the wiring leading to the second gear switch.
   • Locate the second gear pressure switch (refer to the vehicle’s service manual for its exact position, which may be on the valve body internally or on the exterior transmission case). Inspect its electrical connector for damage, bent pins, corrosion, or improper seating.
3. Check Transmission Fluid Level and Condition: While not a direct cause of a circuit malfunction, ensure the transmission fluid is at the correct level and its condition is acceptable (not burnt or excessively contaminated). Low fluid can affect hydraulic pressure, which might indirectly influence pressure switch operation, although P1736 typically points to an electrical issue.
4. Electrical Circuit Testing (using a DMM):
   • Disconnect Battery: For safety and accurate resistance/continuity tests, disconnect the vehicle’s battery.
   • Isolate Circuits: Disconnect the electrical connector from the second gear switch and, if accessible, from the PCM/TCM.
   • Continuity Check: Using a DMM, check for continuity in the signal wire(s) between the second gear switch connector and the PCM/TCM connector. An open circuit indicates a broken wire.
   • Short to Ground Test: With the signal wire(s) isolated, test for continuity between the signal wire(s) and a known good chassis ground. Any continuity indicates a short to ground.
   • Short to Voltage Test: Reconnect the battery (ensure switch and TCM are still disconnected). Using the DMM, check for unwanted voltage on the signal wire(s) at the disconnected switch connector. The presence of voltage indicates a short to a power source.
   • Reference Voltage and Ground Check (at TCM/PCM connector): With the TCM/PCM connector disconnected, and the battery reconnected, verify that the PCM/TCM is providing the correct reference voltage (typically 5V or 12V, per service manual) to the switch’s circuit and that the ground circuit is solid.
   • Second Gear Switch Resistance Test: With the switch disconnected, test the resistance of the second gear switch itself. Refer to the factory service manual for the expected resistance values in its various states (e.g., open/closed circuit depending on pressure applied). Some switches are simple ON/OFF, while others are variable resistance.
5. Live Data Monitoring (Advanced Scan Tool): If your scan tool supports it, monitor the second gear switch status or raw voltage signal in live data. With the engine off and key on, manually shift through the gears (if applicable), or perform a controlled road test, observing how the switch status changes. Look for readings that are stuck, erratic, or inconsistent with actual gear engagement.
6. Wiggle Test: While monitoring live data with a scan tool or a DMM connected to the suspect circuit, gently wiggle sections of the wiring harness and connectors associated with the second gear switch and PCM/TCM. Any intermittent fluctuations in readings indicate a loose connection or chafed wire.

Recommended Repairs and Solutions

Once the root cause of P1736 has been accurately diagnosed, implement the following repairs:

1. Repair or Replace Damaged Wiring/Connectors: If electrical testing reveals an open circuit, short circuit, or compromised connections, perform professional wiring repair. Utilize OEM-grade repair techniques, including solder and heat-shrink tubing for splices, or replace damaged sections of the harness with appropriate automotive-grade wire. Replace corroded or broken connector shells and pins as necessary.
2. Replace the Second Gear Pressure Switch/Sensor: This is often the most common repair. If the switch fails its internal resistance test or exhibits incorrect electrical behavior during live data monitoring, replace it with a new OEM or high-quality aftermarket unit. Be aware that some pressure switches are external and easily accessible, while others may be located inside the transmission valve body, requiring transmission pan removal and fluid drainage. Always use fresh, approved transmission fluid when refilling.
3. Address Low Transmission Fluid (If Applicable): If low or contaminated transmission fluid was noted during the initial inspection, correct the fluid level and perform a fluid and filter change if the fluid is burnt or severely degraded. While not directly resolving the circuit issue, it ensures overall transmission health.
4. PCM/TCM Replacement (Last Resort): Only consider replacing the PCM or TCM after meticulously verifying that the second gear switch, its wiring, and all related connectors are in perfect working order. This is a costly repair and often requires specialized programming or ‘flashing’ to synchronize the new module with the vehicle’s other control units. Ensure all power and ground circuits to the PCM/TCM are also verified as per the service manual before condemning the module.