Introduction:
The P1664 diagnostic trouble code (DTC) indicates an issue within the 1-4 upshift lamp control circuit in a vehicle's automatic transmission system. This circuit is responsible for illuminating the indicator lamp, signaling the driver when the transmission is ready for a 1st to 4th gear upshift, typically for fuel economy purposes in older vehicles. A malfunctioning circuit can lead to the upshift lamp failing to illuminate or remaining on constantly, impacting the driver's awareness of optimal shift points and potentially affecting fuel efficiency.
| Topic | Description | Possible Causes |
|---|---|---|
| Circuit Function | The 1-4 upshift lamp control circuit's primary function is to activate the "upshift" indicator light on the dashboard when specific conditions are met. These conditions usually include engine temperature, vehicle speed, and throttle position. The Powertrain Control Module (PCM) monitors these inputs and, when all parameters are within the acceptable range, sends a signal to activate the upshift lamp. The circuit typically involves the PCM, wiring, connectors, a relay (if applicable), and the upshift lamp itself. | Faulty Upshift Lamp: A burned-out bulb in the upshift lamp prevents it from illuminating. Wiring Issues: Damaged, shorted, or open wiring in the upshift lamp circuit disrupts the signal flow. Connector Problems: Corroded or loose connectors in the circuit can cause intermittent or complete failures. Faulty PCM: A malfunctioning PCM may not send the correct signal to activate the upshift lamp. Faulty Sensors: Incorrect readings from sensors like the throttle position sensor (TPS) or vehicle speed sensor (VSS) can prevent the PCM from activating the lamp. Faulty Relay (if equipped): A malfunctioning relay in the circuit can prevent the upshift lamp from turning on or off. Software Issues: PCM software glitches can cause incorrect upshift lamp operation. Grounding Problems: Poor grounding in the circuit can lead to erratic behavior. Mechanical Transmission Issues: In some cases, internal transmission problems can indirectly affect the PCM's decision to activate the upshift lamp. Aftermarket Modifications: Modifications to the vehicle's electrical system can interfere with the upshift lamp circuit. |
| Symptoms | The most obvious symptom is the upshift lamp not illuminating when it should, or conversely, remaining illuminated constantly. Other symptoms may include a stored P1664 DTC, and potentially, a minor decrease in fuel economy if the driver is not shifting at optimal times. In some cases, other seemingly unrelated electrical issues might arise due to shared grounds or power circuits. | Open Circuit: A break in the wiring prevents the signal from reaching the upshift lamp. Short Circuit: A short to ground or power causes the lamp to illuminate constantly or not at all. High Resistance: Corrosion or loose connections increase resistance, weakening the signal to the lamp. |
| Diagnostic Steps | Diagnosing a P1664 DTC involves a systematic approach. Start by visually inspecting the wiring and connectors associated with the upshift lamp circuit for any signs of damage or corrosion. Use a multimeter to check for continuity and voltage at various points in the circuit, comparing readings to the vehicle's wiring diagram. Scan the PCM for other DTCs that may provide clues. If a relay is present, test its functionality. Finally, if all other components check out, the PCM itself may be the culprit. | Visual Inspection: Look for obvious signs of damage, such as frayed wires, corroded connectors, or burned-out bulbs. Continuity Test: Use a multimeter to check for breaks in the wiring. Voltage Test: Use a multimeter to check for proper voltage at various points in the circuit. Scan Tool Diagnosis: Use a scan tool to read DTCs and monitor sensor data. Component Testing: Individually test components like the upshift lamp and relay. Wiring Diagram: Consult a vehicle-specific wiring diagram to understand the circuit layout and test points. PCM Testing: If all other components check out, the PCM may need to be tested by a qualified technician. |
| Common Tools Needed | To effectively diagnose and repair a P1664 DTC, several tools are typically required. These include a diagnostic scan tool to read DTCs and monitor sensor data, a multimeter to check for voltage, continuity, and resistance, a wiring diagram specific to the vehicle, and basic hand tools such as sockets, wrenches, and screwdrivers. A test light can also be helpful for quickly checking for power and ground. | Diagnostic Scan Tool: Reads DTCs and monitors sensor data. Multimeter: Measures voltage, continuity, and resistance. Wiring Diagram: Shows the layout of the electrical circuit. Sockets, Wrenches, and Screwdrivers: Basic hand tools for disassembly and assembly. Test Light: Quickly checks for power and ground. Wire Strippers/Crimpers: For repairing or replacing wiring. Connector Cleaning Kit: Cleans corroded connectors. |
| Potential Solutions | The appropriate solution depends on the underlying cause of the P1664 DTC. Replacing a burned-out upshift lamp bulb is the simplest fix. Repairing or replacing damaged wiring or connectors is often necessary. A faulty relay should be replaced. If sensor readings are incorrect, the faulty sensor needs to be replaced. In rare cases, the PCM may require reprogramming or replacement. Ensure all repairs are performed according to the vehicle manufacturer's specifications. | Replace Burned-Out Bulb: If the upshift lamp is simply burned out. Repair or Replace Wiring: If there's damage to the wiring harness. Clean or Replace Connectors: If connectors are corroded or damaged. Replace Faulty Relay: If the relay is malfunctioning. Replace Faulty Sensor: If a sensor is providing incorrect readings to the PCM. Reprogram or Replace PCM: As a last resort, if the PCM is the source of the problem. |
Detailed Explanations
Circuit Function: The 1-4 upshift lamp control circuit acts as a notification system for the driver. The PCM constantly monitors various engine and vehicle parameters. When these parameters, such as engine temperature, vehicle speed, and throttle position, fall within a pre-defined range, indicating optimal conditions for a fuel-efficient 1st to 4th gear upshift, the PCM sends a signal. This signal activates the upshift lamp, alerting the driver to make the shift. The components within the circuit include the PCM, the wiring connecting the PCM to the lamp, any relevant sensors providing input to the PCM, a relay (in some vehicle models), and the upshift lamp itself.
Symptoms: The primary and most noticeable symptom of a P1664 DTC is a malfunctioning upshift lamp. This can manifest as the lamp failing to illuminate when the conditions for an upshift are met, or conversely, the lamp remaining illuminated constantly, regardless of the driving conditions. Beyond the lamp itself, a stored P1664 DTC will be present in the PCM's memory. In some instances, a slight decrease in fuel economy may be observed if the driver is not shifting gears at the optimal times. Rarely, other unrelated electrical issues could arise if the upshift lamp circuit shares a common ground or power source with other systems.
Diagnostic Steps: Diagnosing a P1664 DTC requires a methodical approach to pinpoint the root cause. Begin with a thorough visual inspection of the wiring and connectors associated with the upshift lamp circuit. Look for any signs of damage, such as frayed wires, corroded terminals, or loose connections. Next, utilize a multimeter to perform continuity and voltage tests at key points within the circuit, referencing the vehicle's wiring diagram for accurate test locations and expected values. Use a diagnostic scan tool to check for any other DTCs that might provide additional information. If a relay is part of the circuit, test its functionality to ensure it's switching properly. If all other components test within specification, the PCM itself could be the source of the problem and may require further testing or replacement.
Common Tools Needed: Effective diagnosis and repair of a P1664 DTC necessitate a specific set of tools. A diagnostic scan tool is essential for reading DTCs stored in the PCM and for monitoring real-time sensor data. A multimeter is crucial for measuring voltage, continuity, and resistance within the circuit. A vehicle-specific wiring diagram is indispensable for understanding the circuit layout and identifying test points. Basic hand tools, such as sockets, wrenches, and screwdrivers, are necessary for disassembly and assembly of components. A test light can be a quick and easy way to check for the presence of power and ground at various points in the circuit. Wire strippers and crimpers are needed for repairing or replacing damaged wiring. Finally, a connector cleaning kit can help remove corrosion from electrical connectors, ensuring good connections.
Potential Solutions: The appropriate solution for a P1664 DTC is directly related to the underlying cause of the problem. If the upshift lamp bulb is burned out, simply replacing it will resolve the issue. Damaged wiring or corroded connectors require repair or replacement to restore proper circuit function. A faulty relay should be replaced with a new one. If sensor readings are found to be inaccurate, the faulty sensor must be replaced to provide the PCM with correct input. In some instances, the PCM itself may be the cause of the problem, necessitating reprogramming or, in rare cases, replacement. Always ensure that any repairs are carried out according to the vehicle manufacturer's recommended procedures and specifications.
Frequently Asked Questions
What does the P1664 code mean? The P1664 code indicates a problem with the 1-4 upshift lamp control circuit in your vehicle's transmission system. This circuit controls the light that tells you when to shift from 1st to 4th gear.
Can I drive my car with a P1664 code? Yes, you can usually drive your car with a P1664 code, but the upshift lamp might not function correctly. It's best to address the issue promptly to ensure proper system operation and avoid potential fuel efficiency issues.
What are the common causes of a P1664 code? Common causes include a burned-out upshift lamp bulb, damaged wiring, corroded connectors, a faulty relay, or a malfunctioning PCM. Sensor issues can also contribute to the problem.
How do I fix a P1664 code? Fixing a P1664 code involves diagnosing the root cause and then repairing or replacing the faulty component. This might include replacing the bulb, repairing wiring, cleaning connectors, or replacing the relay or PCM.
Is a P1664 code a serious problem? While not immediately critical, a P1664 code should be addressed to ensure the upshift lamp functions correctly. Ignoring it could lead to decreased fuel economy and potentially mask other underlying issues.
Conclusion
The P1664 diagnostic trouble code points to a problem within the 1-4 upshift lamp control circuit. By following a systematic diagnostic approach, utilizing the appropriate tools, and understanding the circuit's function, you can effectively identify and resolve the underlying cause, restoring proper operation of the upshift lamp and ensuring optimal fuel efficiency.