Two technicians are checking the fluid level in an automatic transmission. Technician A says they should check the level while the vehicle is running, as some hydraulic circuits drain back to the sump when the engine is off. Technician B says checking the level while the engine is off and the transmission pump is not turning gives a more accurate reading on the transmission dipstick. Who is correct?

Two technicians are checking the fluid level in an automatic transmission. Technician A says they should check the level while the vehicle is running, as some hydraulic circuits drain back to the sump when the engine is off. Technician B says checking the level while the engine is off and the transmission pump is not turning gives a more accurate reading on the transmission dipstick. Who is correct?

• A. Technician A
• B. Technician B
• C. Both Technician A and Technician B
• D. Neither Technician A nor Technician B

Answer: (A)

The key idea is that the transmission fluid level is determined by fluid being circulated under pressure. When the engine is running, the transmission pump is actively moving fluid and hydraulic circuits are pressurized, which brings the fluid to the correct level on the dipstick. If you check with the engine off, the pump isn’t circulating and some fluid can drain back into the sump, giving a reading that doesn’t reflect the system’s normal operating state. That makes the reading unreliable. So checking the level with the engine running (and the transmission warmed to normal operating temperature, on level ground, with the vehicle in Park) provides the accurate measurement. Checking with the engine off would not give the correct reading, which is why Technician A’s approach is the proper one.

The key idea is that the transmission fluid level is determined by fluid being circulated under pressure. When the engine is running, the transmission pump is actively moving fluid and hydraulic circuits are pressurized, which brings the fluid to the correct level on the dipstick. If you check with the engine off, the pump isn’t circulating and some fluid can drain back into the sump, giving a reading that doesn’t reflect the system’s normal operating state. That makes the reading unreliable.

So checking the level with the engine running (and the transmission warmed to normal operating temperature, on level ground, with the vehicle in Park) provides the accurate measurement. Checking with the engine off would not give the correct reading, which is why Technician A’s approach is the proper one.