Transmission line pressure is the main hydraulic pressure an automatic transmission’s pump makes and its pressure control system regulates. It is what applies the clutches and bands. Low line pressure causes slipping, delayed engagement, soft shifts and overheating; high line pressure causes harsh shifts. You test it with a pressure gauge on the transmission’s line pressure port and compare the readings at idle and under load, in each gear range, with the chart in the service manual.
How line pressure is controlled
The pump, driven by the torque converter hub, makes pressure whenever the engine runs. A pressure regulator valve in the valve body sets the base level, and on electronic transmissions a pressure control solenoid raises it with throttle and load so the clutches hold under power. Some transmissions, such as Ram’s RFE family, also have a line pressure sensor that reports the actual pressure to the computer.
Low line pressure symptoms
- Slipping, especially under load; see transmission slipping.
- Delayed engagement into drive or reverse.
- Soft or flared shifts.
- High fluid temperatures.
- Codes such as P0868 (fluid pressure low) and pressure control solenoid codes like P0746.
Common causes
- Low fluid level or the wrong fluid.
- A clogged filter or a cracked filter neck drawing air; see how to change a transmission filter.
- A worn pump.
- A sticking pressure regulator valve or worn valve body bores; see valve body problems.
- A failing pressure control solenoid; see P0745 and transmission solenoid symptoms.
- Internal leaks from worn seals or rings.
How to test line pressure
- Check the fluid level and condition first.
- Connect a gauge rated for transmission pressures to the line pressure port shown in the service manual.
- With the engine at operating temperature, record pressure at idle in park, reverse, neutral and drive.
- Record it again under the load conditions the manual specifies.
- Compare every reading with the specification chart.
Low in every range points to a common cause such as fluid, filter, pump or regulator. Low in only one range points to a leak in that range’s circuit.
High line pressure
Pressure that is too high usually comes from a stuck regulator valve, a failed pressure control solenoid or a wiring fault, and makes every shift harsh; see P0869.
When the transmission is worn out
A worn pump or internal leaks mean the transmission needs rebuilding or replacing; see rebuilt vs used transmissions and the units on transmissions. For Ram trucks, see 68RFE problems.
More guides: what the metal on the pan magnet means · bad transmission pump symptoms.
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