74 Transmission Input Speed Error (4L80-E)
How to read it
Code 74 reads as 7 flashes, pause, 4 flashes on the check-engine light. Jump terminals A and B on the ALDL connector with the key off, then turn the key on with the engine off and count the check-engine flashes.
New to flash codes? The How to Read GM OBD-I Codes with a Paperclip walks through the test and what the numbers mean. The general OBD-I guide covers the rest.
What does GM code 74 mean?
74. Transmission Input Speed Error (4L80-E). This code sets when your vehicle computer detects transmission input speed error (4l80-e).
Common causes of 74
- Failed transmission input speed sensor
- Open or shorted input speed sensor wiring
- Damaged reluctor or tone ring
- Internal 4L80-E fault or low fluid
How to fix 74
- Retrieve codes with a paperclip across ALDL pins A and B and count the Check Engine flashes
- Check the transmission fluid level and condition
- Test the input speed sensor and its wiring at the transmission connector
- Compare input and output speed readings to confirm the fault
- Clear codes after the repair and recheck after a road test