[RodeWorthy]

In the days when I dealt with such things torque converter lockup meant a sliding splined gear slipped over the torque converter output shaft to physically "lock" the output shaft to the input shaft thus removing any possibility of torque converter slippage affecting the load -- it, (the torque converter) is effectively by-passed or "locked out".

Tow-Haul mode with a Duramax/Allison does not lock up the torque converter as described above but the torque converter clutch (TCC) provides similar function and behaves differently in the two operating modes -- Normal or Tow-Haul.

In Tow-Haul Mode:
During deceleration, the torque converter clutch (TCC) remains applied at closed throttle to much lower vehicle speeds to significantly improve the effect of engine braking.

During acceleration, the TCC is applied in 2nd range and remains applied in 3rd, 4th and 5th. This improves the drivetrain efficiency and significantly lowers transmission sump temperature when towing heavy loads. In Normal mode, TCC generally applies only in higher ranges and is dependent on throttle position.

Shift Patterns:
Tow-Haul shift mode significantly changes the transmission's shift pattern to reduce shift cycling and to deliver better performance, control, and cooling when towing/hauling heavy loads.

Upshift points are raised at light to mid throttle position to use more of the engine's available power for accelerating. Downshift points are raised to enhance engine braking to help slow the vehicle.


Like others I routinely tow with Tow-Haul mode off. I will deploy it when I want extra control on hills and mountains or in traffic with frequent speed changes. There are times the transmission will not shift into sixth gear in Tow-Haul mode on but it always will in Normal mode. At my cruising speed (59 MPH) in sixth gear my engine is at its peak torque curve at 1600 RPM. I believe that is my optimum fuel efficiency point too.

I also use Tow-Haul mode under the same conditions when I am not towing to achieve the same control level on hills and in traffic.

If the transmission is used to retard speeds on a long grade it is most important to monitor the transmission temperature.