A new or rebuilt transmission needs a short break-in so the new clutches, seals and converter bed in and the computer relearns its shift adaptations. Follow the builder’s instructions first. In general: check the fluid level hot after the first drive, look for leaks, drive normally with varied speeds and gentle to moderate acceleration, and avoid heavy towing and hard launches for the first several hundred miles or the distance the builder gives.
Before the first drive
- Flush the transmission cooler and lines, or replace the cooler, so old debris cannot reach the new unit; see transmission coolers.
- Fill with the correct fluid; see how much transmission fluid does it take? and the transmission fluid chart.
- Confirm the torque converter is fully seated, the cooler lines are tight and the electrical connectors are latched.
- Adjust any shift cable, throttle valve cable or range sensor to specification.
The first start
Start the engine and check the fluid level with the engine running, following the procedure for your transmission; many units must be checked hot, in park or neutral, on level ground. Shift slowly through each gear range with your foot on the brake, then recheck the level and look for leaks.
The first drives
- Drive in a mix of city and highway conditions, letting it shift through every gear.
- Use light to moderate throttle; avoid full-throttle launches.
- Avoid heavy towing or hauling until the builder’s break-in distance has passed.
- Recheck the fluid level hot after the first few drives.
Shift adaptations and relearn
Electronic transmissions adjust shift pressure over time. After installing a new unit, many vehicles benefit from clearing the old adaptive values, and some need a relearn procedure with a scan tool. Early shifts may feel firm or soft until the computer adapts; if they stay harsh or slip, have it checked rather than waiting.
First fluid change
Some builders recommend an early fluid and filter change to remove break-in material; others do not. Follow the instructions supplied with the unit; see how to check transmission fluid.
Warning signs during break-in
Stop and investigate slipping, a warning light, fluid leaks, burning smells or a gear that will not engage; see transmission slipping.
Keep the paperwork
Keep the builder’s instructions, your fluid receipts and notes of the break-in mileage. They help if you need to contact the builder about a problem.
Related
For installing the unit, see how to install a transmission; for engines, engine break-in procedure. Browse replacement units by code on transmissions.
More guides: how long a rebuilt transmission lasts · clutch break-in.
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