1-4 Upshift (Skip Shift) Lamp Control Circuit Malfunction
The PCM has detected a malfunction in the 1-4 skip shift solenoid lamp control circuit. The 1-4 skip shift solenoid control circuit is sometimes called the upshift control circuit because it prevents the driver from shifting through second and third gears. This function is designed to improve fuel efficiency by eliminating the higher engine RPM levels associated with shifting through lower gears. This code pertains to the skip shift indicator lamp that is illuminated (in the instrument panel) to make the driver aware that the 1-4 skip shift system has been activated. This code applies only to vehicles equipped with a manual transmission. If this code is exhibited in a vehicle with an automatic transmission, suspect a defective PCM of programming error.
Code Set Parameters
If the PCM detects an abnormal voltage or resistance reading from the skip shift lamp control circuit, a code will be stored and a malfunction indicator lamp may be illuminated. Some vehicles may require multiple failure cycles in order for a service engine soon lamp to be illuminated. Other vehicle applications do not illuminate a service engine soon lamp when this code is stored.
Symptoms may include the noted absence of the skip shift indicator lamp, a stored code, and an illuminated service engine soon lamp.
The most common causes of this type of malfunction code being stored is due to a defective skip shift indicator lamp bulb, skip shift relay, skip shift solenoid, fuse, or actuator. Other possible causes include shorted, open, or corroded wiring and/or electrical connectors. The installation of an aftermarket skip shift eliminator may also cause this code to be stored.
Technicians report that this code often goes undetected because drivers tend to prefer shifting through al of the gears. Certain applications do not illuminate the service engine soon lamp when this code is stored making it virtually impossible to detect until fuel efficiency decreases are noted.
- A scanner (or code reader) and a digital volt/ohmmeter will be helpful in successfully diagnosing the potential causes of this code.
While the skip shift solenoid may vary by design from one manufacturer to another, it is usually of the “on/off” protrusion variety and is stationary mounted in the transmission housing near the shifter assembly
- Constant voltage is typically present on one side of the switch and the PCM provides a ground signal which energizes the solenoid and prevents shifting into second and third gears
Begin your diagnosis 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
- After the codes are cleared, operate 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
If the code immediately returns, check for battery voltage on the input side of the 1-4 skip shift solenoid using the digital volt/ohmmeter
- If there is no voltage present on the input side of the solenoid, disconnect any related control modules and check for continuity between the system fuses and/or relays and the input circuit at the 1-4 skip shift solenoid
- Repair open or shorted circuits as required
If voltage is present on the input side of the 1-4 skip shift solenoid, use the scanner to command the solenoid “ON” and check for voltage on the output side
- If you have voltage on the input side and none on the output side (when the solenoid is commanded on) suspect a faulty or misadjusted 1-4 skip shift solenoid
- Adjust or replace the solenoid as needed and retest the system afterward to ensure a successful repair.
If there is voltage present on the output circuit of the 1-4 skip shift solenoid, check circuit continuity and resistance between the switch and the PCM
- Again, refer to the vehicle wiring diagram to ensure that all circuits are tested
- Repair or replace system open or shorted circuits/connectors as required
If all circuits coincide with manufacturer’s specifications, suspect a defective PCM
- Remember that PCM failure is rare and replacement will require reprogramming