P1633

What Does Code P1633 Mean?

The diagnostic trouble code P1633 indicates that the Powertrain Control Module (PCM), also commonly referred to as the Engine Control Module (ECM), has detected an abnormally low voltage supply to its Keep Alive Memory (KAM) circuit. The KAM is a dedicated, non-volatile memory bank within the PCM that retains critical adaptive learning parameters even when the ignition is turned off. These parameters include learned fuel trims (short-term and long-term), idle air control values, transmission shift strategies, knock sensor learned values, and other system adaptations that optimize engine and transmission performance based on real-world driving conditions and component wear. The PCM continuously monitors the voltage supplied to this KAM circuit. If the voltage drops below a calibrated threshold (typically around 9-10 volts, though this can vary by manufacturer and specific PCM design) for a specified duration, the PCM registers a P1633 code. This condition signifies that the PCM’s adaptive memory could be corrupted or reset to factory defaults, forcing the engine to operate on its base programming without the benefit of learned optimizations.

Common Symptoms

• Check Engine Light (CEL) illumination: The most direct and common symptom.
• Rough idle or stalling: Especially noticeable after a battery disconnect or an extended period of low voltage, as the engine reverts to base idle parameters.
• Poor fuel economy: The PCM operates without optimized fuel trims, potentially running rich or lean based on factory defaults.
• Reduced engine performance: Decreased power or acceleration due to the loss of adaptive ignition timing and fuel delivery strategies.
• Transmission shifting irregularities: If the KAM also stores transmission adaptive data, shifts may become harsh, delayed, or erratic.
• Hard starting or no-start condition: While less common for a simple KAM voltage issue, critically low system voltage that triggers P1633 could also impair other essential starting components.
• Emission test failure: Due to readiness monitors not being set or elevated exhaust emissions from unoptimized engine operation.

What Causes the Code P1633?

• Weak or faulty battery: A discharged, old, or internally damaged battery unable to maintain adequate voltage to the KAM circuit.
• Faulty charging system (alternator): An alternator not adequately charging the battery, leading to persistently low system voltage.
• Corrosion or poor connection at battery terminals: High resistance in the primary power path can lead to voltage drops throughout the electrical system, including the KAM circuit.
• Open circuit or high resistance in the KAM power supply wire: Damage, chafing, or corrosion in the specific wiring harness providing constant battery voltage to the PCM’s KAM input.
• Blown or corroded fuse: A compromised fuse (often labeled BAT, BCM, PCM, or constant power) in the circuit supplying constant power to the PCM. Corrosion within the fuse box terminals themselves can also cause high resistance.
• Parasitic draw: An electrical component staying active and drawing excessive current when the vehicle is off, draining the battery over time and leading to low KAM voltage.
• Faulty ignition switch: Internal wear or resistance in the ignition switch can sometimes interrupt constant power pathways, especially if the KAM circuit is routed through it or a related relay.
• Internal PCM failure: Less common, but an internal fault within the PCM itself preventing it from correctly receiving or retaining KAM voltage.

How to Diagnose and Troubleshoot

Diagnosing P1633 requires a systematic approach using a digital multimeter (DMM), an OBD-II scanner, and careful visual inspection.

1. Perform a Thorough Visual Inspection: Begin by checking the battery terminals for cleanliness, corrosion, and tightness. Inspect the entire battery cable path to the starter and fuse box. Examine the wiring harness leading to the PCM for any signs of damage, chafing, or rodent activity. Pay close attention to the fuse box for any signs of corrosion or burnt terminals.
2. Battery Voltage Test:
   • With the ignition off and no load, measure battery voltage using a DMM. A fully charged battery should read approximately 12.6 volts. If significantly lower, charge or replace the battery.
   • Start the engine and measure battery voltage again. It should read between 13.5 and 14.5 volts, indicating the alternator is charging correctly. If below 13.0V or above 15.0V, suspect an alternator or voltage regulator issue.
3. Check Relevant Fuses: Using a DMM set to continuity or by visual inspection, check all fuses related to the PCM, BCM (Body Control Module), and constant battery power. Refer to the vehicle’s fuse diagram for specific locations. Do not just visually inspect; physically test them or replace them if questionable. Also, inspect the fuse holder terminals for corrosion or poor contact.
4. Parasitic Draw Test:
   • Disconnect the negative battery cable.
   • Connect an ammeter in series between the negative battery post and the disconnected negative battery cable.
   • Ensure all doors are closed, lights off, and the vehicle is fully asleep (may take 10-20 minutes).
   • Monitor the current draw. Most vehicles should have a parasitic draw of less than 50 milliamps (mA). A draw significantly higher indicates an issue.
   • If a high draw is present, systematically remove fuses one by one (starting with non-essential circuits) until the draw drops to an acceptable level, thus isolating the faulty circuit.
5. PCM KAM Circuit Voltage Test:
   • Obtain the vehicle-specific wiring diagram to identify the dedicated “Keep Alive Memory” (KAM) or “Constant Battery Power” pin at the PCM connector. This circuit provides continuous 12V power to the PCM.
   • With the ignition off, back-probe the identified KAM wire at the PCM connector using your DMM (positive lead). Connect the DMM’s negative lead to a known good chassis ground.
   • The voltage reading should be within 0.5 volts of battery voltage. A significantly lower voltage indicates high resistance or an open circuit upstream from the PCM.
   • If voltage is low, trace the circuit back towards the battery, checking for voltage drops at connectors, splices, and the fuse box.
6. Check PCM Ground Circuit: While checking KAM power, also identify the PCM ground wires (refer to wiring diagrams). With the battery disconnected, measure the resistance between the PCM ground pin(s) and a known good chassis ground. Resistance should be very low, typically less than 0.5 ohms. High resistance indicates a poor ground connection for the PCM.
7. OBD-II Scanner Data Review: Use an OBD-II scanner to view “System Voltage” or “Battery Voltage” PIDs in live data, if available. While this might show overall system voltage, it may not specifically reflect the dedicated KAM circuit voltage if the issue is a localized wiring fault to the KAM input. Clear the code after diagnostics and retest.

Recommended Repairs and Solutions

Once the root cause of the P1633 code has been identified through proper diagnosis, the following repairs are typically recommended:

1. Battery Replacement or Maintenance: If diagnostic tests confirm a weak, old, or faulty battery, replace it with a new, correctly specified unit. Thoroughly clean and tighten battery terminals and cable connections. Apply dielectric grease to prevent future corrosion.
2. Alternator Replacement: If the alternator fails to produce adequate charging voltage (typically 13.5-14.5V with the engine running), replace it with a new or remanufactured unit. Ensure the serpentine belt tension is correct.
3. Wiring Repair: For damaged, corroded, or open circuits in the KAM power supply wire or ground path to the PCM, perform precise wiring repairs. Use appropriate gauge wire, solder connections (preferred method), and seal with heat-shrink tubing to prevent future corrosion and ensure durability. If the issue is within the fuse box, replace the affected fuse and clean or replace the fuse box itself if terminals are severely corroded.
4. Repair Parasitic Draw: Once the source of the parasitic draw is identified (e.g., a faulty interior light switch, an aftermarket alarm system, a stuck relay, or a failing module), repair or replace the offending component. This often requires careful electrical troubleshooting to avoid replacing non-faulty parts.
5. Ignition Switch Replacement: If the ignition switch is confirmed to have internal resistance or intermittent contacts affecting the constant power supply to the PCM, it should be replaced with an OEM-quality part.
6. PCM Replacement or Reprogramming: This should be considered a last resort, only after all other potential causes (battery, charging system, wiring, fuses, parasitic draw) have been rigorously tested and ruled out. If an internal PCM fault is suspected and confirmed, the PCM will need to be replaced and subsequently programmed to the vehicle’s specific VIN and options using specialized dealer-level diagnostic equipment.
7. Post-Repair Procedures: After any repair that involves disconnecting the battery or the PCM, the PCM’s adaptive memory will be reset. It is crucial to perform a comprehensive drive cycle to allow the PCM to relearn its adaptive values. This typically involves a combination of city and highway driving, varying engine loads, and idle periods. This relearning process helps to restore optimal engine performance, fuel economy, and transmission shift quality.