P1638: PCM Keep-Alive Memory Voltage Low
The P1638 code indicates that the voltage supplied to the Keep-Alive Memory (KAM) within the Powertrain Control Module (PCM) has dropped below the manufacturer's minimum threshold. KAM is a non-volatile memory that retains critical data such as adaptive learning values, calibration settings, pending fault codes, and module configurations, even when the ignition is off. It is powered directly by the battery through a dedicated circuit. When the voltage to this memory falls too low, the PCM detects the fault and sets the code. In Ford vehicles, this code specifically points to a problem in the constant power supply circuit to the PCM.
01. Most Common Causes
- Weak or discharged vehicle battery
- Blown or corroded KAM power fuse
- Loose, corroded, or oxidized electrical connections in the PCM power circuit
- Voltage drop in the constant power wiring to the PCM
- Internal PCM failure affecting the KAM voltage regulation circuit
- Parasitic battery drain reducing available voltage to the KAM
- Faulty alternator or voltage regulator causing insufficient charging
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Loss of adaptive memory (e.g., idle quality changes, transmission shift patterns reset)
03. Step-by-Step Diagnosis
Check battery voltage and condition; charge or replace if necessary.
Inspect and test the KAM fuse and replace if blown or corroded.
Visually inspect all PCM power and ground connections for corrosion or looseness; clean and tighten as needed.
Perform a voltage drop test on the constant power circuit to the PCM.
Check for parasitic drain; repair if excessive.
Test alternator output and voltage regulator function.
If all else fails, suspect internal PCM failure and consider reprogramming or replacement.
1. Frequently Asked Questions
Is it safe to drive?▼
Yes, but the vehicle may experience drivability issues such as rough idle or erratic transmission behavior due to loss of adaptive memory. It is recommended to diagnose and repair the underlying cause promptly to avoid potential starting problems or further electrical issues.
How much does it cost to fix?▼
The cost varies widely depending on the root cause. Simple fixes like replacing a fuse or cleaning a connection can cost $50-$150. Battery replacement ranges from $100-$250. If the PCM needs replacement, expect $500-$1,200 including programming.