The Obdii P1738 Code is a diagnostic trouble code (DTC) that indicates a problem within the 2nd Clutch Pressure Switch Circuit in your vehicle’s automatic transmission system. This code signals that the Powertrain Control Module (PCM) has detected an abnormality in the electrical circuit related to the pressure switch for the second clutch. Understanding this code is crucial for maintaining the health and performance of your vehicle’s transmission.
What Does OBDII Code P1738 Mean?
The P1738 code specifically points to “Problem in 2nd Clutch Pressure Switch Circuit.” In modern automatic transmissions, pressure switches play a vital role in monitoring hydraulic pressure at various points, including within the clutch packs. These switches are essentially sensors that inform the PCM about the fluid pressure, which is directly related to clutch engagement and overall transmission operation.
When the PCM detects an issue in the 2nd clutch pressure switch circuit, it means the electrical signal from this switch is not within the expected parameters. This could be due to a variety of electrical faults, ranging from a simple wiring issue to a faulty switch or even a PCM problem in rare cases. The “2nd clutch” refers to the clutch pack that is engaged when the transmission shifts into second gear.
Symptoms of OBDII P1738 Code
When the P1738 code is triggered, you may experience several symptoms, which can include:
- Check Engine Light (CEL) Illumination: This is the most common and immediate symptom. The malfunction indicator lamp on your dashboard will light up to alert you to a problem.
- Transmission Shifting Problems: You might notice erratic shifting, delayed engagement into second gear, harsh shifts, or the transmission getting stuck in a particular gear.
- Limp Mode: In some cases, the PCM might put the transmission into “limp mode” to prevent further damage. This mode usually limits the transmission to second or third gear, reducing vehicle speed and performance.
- Reduced Fuel Economy: Inefficient shifting due to the issue can lead to decreased fuel efficiency.
- Possible Transmission Slipping: Although less direct, issues with clutch pressure can contribute to transmission slippage.
It’s important to note that these symptoms can vary depending on the severity and specific cause of the P1738 code in your vehicle.
Potential Causes of OBDII P1738 Code
Several factors can lead to the P1738 code being set. Common causes include:
- Faulty 2nd Clutch Pressure Switch: The pressure switch itself might be defective, failing to send the correct signal to the PCM.
- Wiring Issues: This is a frequent culprit. Problems can include:
- Short circuit: The wiring to the pressure switch might be shorted to ground.
- Open circuit: A break in the wiring can interrupt the electrical signal.
- Corrosion or damage: Wires or connectors can become corroded or physically damaged, leading to poor connections.
- Connector Problems: Loose or corroded connectors at the pressure switch or PCM can disrupt the circuit.
- PCM Issues (Less Common): While less likely, a fault within the PCM itself could be misinterpreting signals or incorrectly diagnosing the pressure switch circuit. This is usually considered after ruling out other possibilities.
- Low Transmission Fluid Level or Pressure: Although not directly electrical, extremely low fluid or pressure issues within the transmission could theoretically affect pressure switch readings, though this is less direct and other codes would likely accompany P1738 in such cases.
Diagnosing the OBDII P1738 Code
Diagnosing the P1738 code requires a systematic approach, often involving electrical testing to pinpoint the fault. Here’s a diagnostic process based on the provided troubleshooting steps, expanded for clarity:
-
Initial Voltage Check:
- Objective: Verify if voltage is reaching the 2nd clutch pressure switch circuit.
- Procedure: With the ignition ON, measure voltage at specified terminals (B14 and B20 or B22 – refer to vehicle-specific wiring diagrams for exact terminal locations on your PCM). You should expect to see battery voltage.
- Interpretation:
- Battery Voltage Present: Indicates power is reaching the circuit. Proceed to step 7 (further voltage checks under specific conditions).
- No Battery Voltage: Indicates a break in power supply to the circuit. Proceed to step 3 to investigate upstream.
-
Ground Short Check (If No Battery Voltage in Step 1):
- Objective: Check for a short circuit to ground in the wiring before the pressure switch.
- Procedure:
- Turn ignition OFF.
- Disconnect PCM connector B and the 2nd clutch pressure switch connector.
- Check for continuity between terminal B14 (PCM side of the circuit) and body ground using a multimeter set to measure continuity.
- Interpretation:
- Continuity Present: A short to ground exists in the wiring between the PCM and the 2nd clutch pressure switch. Repair this short.
- No Continuity: Short to ground is not the issue at this point. Check for loose terminals at the PCM connectors (step 5) or consider PCM fault (as a last resort).
-
Voltage Check Under Load (Simulated Operation):
- Objective: Simulate transmission operation and check if the voltage drops as expected when the 2nd clutch should be engaged (simulated by shifting to 2nd position).
- Procedure:
- Safely raise the front of the vehicle and secure it. Engage parking brake and block rear wheels for safety.
- Start the engine and shift the transmission to the “2” position. Drive (wheels rotating) for more than five seconds to simulate second gear engagement.
- Measure voltage again between terminals B14 and B20 or B22.
- Interpretation:
- Approximately 0V: This might indicate the pressure switch is functioning and the circuit is responding. However, the next steps are crucial to confirm. Check for loose terminals at PCM connectors (step 9) or consider PCM fault (as a last resort).
- Voltage Remains High (Battery Voltage): Indicates the circuit is not responding as expected when 2nd clutch should be engaged. Proceed to step 11 to investigate the pressure switch and its wiring more directly.
-
Pressure Switch Connector Voltage Check (Direct Switch Input):
- Objective: Check if voltage is reaching the pressure switch connector itself.
- Procedure:
- Turn ignition OFF and disconnect the 2nd clutch pressure switch connector.
- Turn ignition ON.
- Measure voltage between the pressure switch connector terminal and body ground.
- Interpretation:
- Battery Voltage Present: Power is reaching the switch connector. The issue is likely with the switch itself or the wiring after this point. Proceed to step 15 (resistance check of the switch).
- No Battery Voltage: There’s an open circuit (break in the wire) before the pressure switch connector. Repair the open circuit in the wire between the pressure switch and the PCM.
-
Pressure Switch Resistance Check (Switch Functionality):
- Objective: Evaluate the internal resistance of the pressure switch to determine if it is functioning within specifications.
- Procedure:
- Measure resistance between the 2nd clutch pressure switch connector terminal and body ground.
- Interpretation:
- Resistance 10 M ohms or More: High resistance could indicate a faulty switch (internally open or malfunctioning). Check for loose terminals at PCM connectors (step 16) or consider PCM fault (as a last resort).
- Resistance Less Than 10 M ohms: The pressure switch may be shorted or have incorrect internal resistance. Replace the 2nd clutch pressure switch (step 17).
Important Notes During Diagnosis:
- Wiring Diagrams: Always refer to the vehicle-specific wiring diagrams for your make and model to accurately identify terminals and wire colors.
- Terminal Fit: Throughout the process, check for loose or corroded terminals at all connectors (PCM and pressure switch). Poor terminal connections are a common cause of circuit problems.
- PCM as Last Resort: Replacing the PCM should be considered only after thoroughly ruling out all other possibilities, as it is a complex and expensive component. Try substituting with a “known-good” PCM to test if you suspect PCM issues, but this should be done by a qualified technician.
Solutions for OBDII P1738 Code
The appropriate solution depends entirely on the root cause identified during diagnosis. Potential solutions include:
- Wiring Repair: Repairing shorts, open circuits, or damaged wiring in the 2nd clutch pressure switch circuit. This might involve splicing wires, replacing sections of wiring harness, or cleaning corroded terminals.
- Connector Repair/Replacement: Cleaning or replacing corroded or loose connectors at the pressure switch or PCM.
- 2nd Clutch Pressure Switch Replacement: Replacing a faulty 2nd clutch pressure switch.
- PCM Replacement (Rare): In very rare cases, if the PCM is definitively diagnosed as faulty after all other possibilities are eliminated, PCM replacement or reprogramming might be necessary.
- Transmission Fluid Service: While less direct, ensure proper transmission fluid level and condition as a preventative measure and to rule out fluid-related issues contributing to pressure anomalies (though this is less likely to directly cause a P1738 code).
Recommendation:
Due to the complexity of automatic transmission systems and electrical diagnostics, it is highly recommended to have the OBDII P1738 code diagnosed and repaired by a qualified automotive technician. Attempting to diagnose and repair transmission issues without proper training and equipment can lead to further damage and safety concerns. A professional technician will have the necessary tools, knowledge, and experience to accurately pinpoint the problem and implement the correct repair, ensuring your vehicle’s transmission operates reliably.
