P077F: Reverse 2 Incorrect Ratio

What it means

Some transmissions provide a second reverse ratio (typically a slower crawler reverse for trailer manoeuvring or hill descent). The TCM detects that the input/output ratio in this gear does not match its calibrated value — slipping clutch or wrong gear actually engaged.

At a glance

• Usually lights the check-engine light

How it works

On transmissions with a second reverse range, the transmission control module (TCM) compares input- and output-shaft speeds while Reverse 2 is engaged. This code sets when the measured ratio does not match the expected Reverse 2 ratio, which can indicate that the gear is slipping or the transmission has not engaged it correctly.

Common causes

Most likely first.

1. Worn or damaged reverse 2 clutch or other friction element
2. Low, incorrect, or degraded transmission fluid
3. Hydraulic pressure loss or internal leakage
4. Valve body or control issue preventing the expected gear engagement

Symptoms

• Check engine or transmission warning light on
• Harsh, delayed, or missing shifts
• Possible limp mode (locked in one gear)
• Slipping or shuddering during gear changes

How to diagnose it

1. Use a scan tool to record freeze-frame data and check which range was commanded when the ratio fault occurred.
2. Check the transmission fluid level and condition using the manufacturer’s procedure; note low, contaminated, or incorrect fluid.
3. Inspect the transmission wiring and connectors for damage, corrosion, loose pins, or fluid intrusion, especially around the transmission and valve body.
4. With a scan tool, compare input- and output-speed readings in Reverse 2 and confirm whether the TCM reports that range as engaged.
5. If the ratio remains incorrect, test the Reverse 2 solenoid controls and hydraulic pressure against the manufacturer’s specifications to locate an electrical or pressure fault.
6. If external checks pass, inspect the valve body and Reverse 2 friction elements for sticking, leakage, or wear.

Common fixes

• Correct the fluid level or replace incorrect or degraded fluid using the specified fluid and service procedure.
• Repair damaged wiring or clean and secure corroded or loose transmission connectors.
• Repair a faulty Reverse 2 solenoid or valve body control fault identified by testing.
• Repair internal hydraulic leakage or replace worn or damaged Reverse 2 friction components.

Can I keep driving?

Avoid using Reverse 2 for manoeuvring or on hills if it slips, shudders, or fails to engage; continued use can worsen internal damage. If the transmission loses reverse or shifts unpredictably, stop driving and arrange recovery.