P279A: Transfer Case Gear High Incorrect Ratio

What it means

When the transfer case is in High range the measured input/output speed ratio does not match the expected value. Worn chain or planetary set.

At a glance

• Usually lights the check-engine light

How it works

The transfer case's high range sends drive through a specific gear ratio for normal road use. The control module compares input and output speed signals while high range is selected; this code sets when their ratio differs from the expected high-range ratio, which can point to a stretched chain or worn planetary gears.

Common causes

Most likely first.

1. Stretched transfer case chain
2. Worn planetary gear set

Symptoms

• Check engine or 4WD warning light may be on
• High-range operation may feel abnormal or produce unusual noise
• 4WD system may limit operation

How to diagnose it

1. Record freeze-frame data and review transfer-case range commands, input and output speeds, and related 4WD codes to confirm the fault occurred in high range.
2. Check that the displayed and commanded range is high and inspect the transfer case and driveline for leaks, damage, or abnormal looseness.
3. Check transfer-case fluid level and condition using the manufacturer's procedure, noting contamination or signs of internal wear.
4. With a scan tool, compare transfer-case input and output speed readings in high range against the expected ratio and manufacturer's specifications.
5. If the speed signals and range command are credible, inspect the transfer case internally for chain stretch or planetary gear wear.

Common fixes

• Correct low or unsuitable transfer-case fluid according to the manufacturer's procedure.
• Repair a wiring or connection fault affecting transfer-case speed signals, if found.
• Replace a stretched transfer-case chain.
• Repair or replace worn planetary gears or the transfer case assembly as appropriate.

Can I keep driving?

Avoid using high range under heavy load if it produces unusual noise, vibration, or abnormal operation, and limit driving if the 4WD system restricts operation. Continued use with a worn chain or gears can worsen internal damage.