Two technicians interpret stall test results: Technician A says a low stall speed is almost always caused by a slipping clutch inside the transmission. Technician B says an extremely high stall speed could indicate that the stator may be freewheeling. Who is correct?

Two technicians interpret stall test results: Technician A says a low stall speed is almost always caused by a slipping clutch inside the transmission. Technician B says an extremely high stall speed could indicate that the stator may be freewheeling. Who is correct?

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

Answer: (D)

Stall speed shows how hard the engine must work to move the drivetrain when a load is held in place. It reflects the engine’s available torque relative to what the transmission/drive system must overcome. If a clutch in a manual were slipping, you could rev the engine higher without the wheels grabbing, which actually tends to raise the stall speed rather than lower it. In an automatic with a torque converter, losing torque multiplication (for example, if the stator weren’t properly directing flow) would also tend to require a higher engine speed to achieve movement, not an obviously extreme reading on its own. Because stall speed is influenced by several factors and a stall test alone doesn’t pin down a slipping clutch or a freewheeling stator, neither interpretation is a reliable general rule. The proper approach is to compare the observed stall speed to spec and use additional tests and symptoms to diagnose the issue.

Stall speed shows how hard the engine must work to move the drivetrain when a load is held in place. It reflects the engine’s available torque relative to what the transmission/drive system must overcome. If a clutch in a manual were slipping, you could rev the engine higher without the wheels grabbing, which actually tends to raise the stall speed rather than lower it. In an automatic with a torque converter, losing torque multiplication (for example, if the stator weren’t properly directing flow) would also tend to require a higher engine speed to achieve movement, not an obviously extreme reading on its own. Because stall speed is influenced by several factors and a stall test alone doesn’t pin down a slipping clutch or a freewheeling stator, neither interpretation is a reliable general rule. The proper approach is to compare the observed stall speed to spec and use additional tests and symptoms to diagnose the issue.