A slipping transmission isn't always a bad transmission. Sometimes it's a bad input telling the computer the wrong thing. The trick is narrowing it down before anybody pulls it out of the car.
Start with the fluid level and condition, then use these steps.
Use the apply chart
Every automatic has a chart showing which clutches and bands are applied in each gear. If it slips in some gears and not others, find the part that's applied in every slipping gear and not used in any gear that works fine.
Example: it slips in 2nd and 4th, but 1st, 3rd, and reverse are fine. If the 2-4 band is the only thing applied in both 2nd and 4th, that's your suspect.
Check line pressure
Put a gauge on the line pressure tap. With the scan tool, change the pressure control command and watch the gauge. If the solenoid gets the changing command but the pressure doesn't follow, the solenoid or the valve it controls is sticking, often from debris.
Normally-high solenoids
On a lot of transmissions, the pressure control solenoid is normally high. With no current, line pressure goes to max. More current lowers it.
That's a fail-safe. If the solenoid circuit opens, pressure goes to max, so you get harsh engagements instead of slipping and burning up the clutches.
Bad inputs cause slipping too
The computer sets line pressure and shift timing based on engine load, using throttle position or calculated torque. If the throttle position input reads low at full throttle, the computer thinks the load is light. It shifts early and commands low pressure, so the clutches slip under load.
Firm shifts after a reflash
Reprogramming the transmission module can reset the learned shift adjustments. The shifts feel firm until it relearns. Service information gives the relearn procedure or the driving it needs.
Cheat sheet
- Check fluid level and condition first
- Apply chart: find the part used in every slipping gear
- Pressure doesn't follow the command = sticking solenoid or valve
- Normally-high solenoid: no current = max pressure
- A low throttle position input can cause early, soft, slipping shifts
Common questions
Can a sensor cause a transmission to slip?
Yes. If the throttle position input reads low, the computer thinks the load is light and commands low pressure, which can let the clutches slip.
Why are my shifts firm after the transmission module was reprogrammed?
Reprogramming can reset the learned shift values. It needs to relearn, using the procedure in service information.
How do you find which clutch is slipping?
Use the apply chart. Find the part that's applied in every gear that slips and not in the gears that work fine.
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