System Voltage Unstable
The PCM has detected an abnormal voltage reading from the vehicle battery, starting, or charging system.
Code Set Parameters
If the PCM detects a voltage reading from the starting/charging system that is lower or higher than expected, a code will be stored and a service engine soon lamp will be illuminated.
Symptoms may include an engine stall at idle, the transmission may not shift properly (automatic), the red battery lamp may illuminate, a decrease in fuel efficiency may exhibit itself over a prolonged period of time, and the service engine soon lamp will be illuminated. In other cases there are no symptoms other than illumination of the service engine soon lamp.
The most common cause of this code can be traced to a faulty alternator. Other causes could include a bad battery or starter, open or shorted wiring or battery cables, insufficient battery primary or secondary grounds, defective voltage regulator, corroded battery terminal ends, a faulty PCM (rare), a large battery drain, or a defective battery.
Technicians report that batteries and starters are often replaced in error when the alternator is either not charging or overcharging. If the battery is discharged, charge it and test the vehicle charging system to make sure that it is operating properly.
- The starting/charging system is monitored by the PCM whenever the key is placed in the “ON” position
- When the engine is running, the PCM is programmed to anticipate between 13.8 and 14.4 volts from the charging system
- If a voltage reading that either exceeds or underachieves, a code will be stored and a service engine soon lamp illuminated
- Some vehicle models (especially Chrysler) also use the PCM to regulate charging system voltage
- System voltage malfunctions related to the PCM are much more common in vehicles equipped with this type of system than in other systems.
Before beginning your diagnosis, make sure that the battery is fully charged
- If the battery fails to take or hold a charge, it should be replaced.
Several tools will be instrumental in successfully diagnosing this code
- A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
Continue with a visual inspection of the battery and battery cables
- Should you find excessive corrosion on the battery terminals, suspect a poor battery connection
- If this is the case, disconnect the battery cables from the battery (always disconnect the ground cable first and see manufacturer’s recommendations for disconnecting/reconnecting the battery) and remove the corrosion from the battery posts and terminals
- Reconnect the battery and retest the charging system
Connect the starting/charging system tester to the battery and perform a load test
- If the battery fails the load test, replace it
- If the battery passes the load test, leave the tester connected and start the engine
- Observe the voltage readings on the tester and compare your findings with manufacturer’s specifications
- Replace starting/charging system components as required
- Typical alternator output should read approximately 14 volts with the engine started and no accessories or lighting turned on
- Leave the tester connected and use the amp probe to test the starter current draw
- Compare your findings to manufacturer’s specs and repair or replace starting system cables or other components as needed.
If the starting/charging system are in proper working order, you will need to test secondary wiring
- Disconnect the alternator connector and test all circuits using the digital volt/ohmmeter
- If alternator wiring values are in line with acceptable specifications, disconnect the PCM connector and perform a continuity test on all system circuits
- Compare actual findings with manufacturer’s specifications and repair open or shorted electrical wiring as necessary.
If the malfunction has still not been rectified, suspect a defective PCM