Allison 1000

Allison's heavy-duty automatic — behind the Duramax diesel in the Chevrolet Silverado and GMC Sierra HD, and in the Hummer H1.

Allison builds commercial-vehicle transmissions, and this is one scaled down rather than a pickup transmission scaled up. That is the whole of its reputation: it is over-engineered for what a three-quarter-ton truck asks of it, and an enormous number of them are still working behind engines that have been substantially modified.

It came as a five-speed and later a six, and the same box is often described as either — GM enabled the sixth ratio in software, so the model year rather than the hardware decides. A truck advertised as a five-speed Allison and one advertised as a six may be the same transmission.

Its recurring complaint is not failure but the torque converter's lock-up clutch, felt as a shudder under light throttle when towing. On this transmission that is a fluid-condition symptom first, and it is far cheaper to address at that stage.

The genuine wear item is the C3 clutch pack, and it is well known enough to have a standard remedy; the symptom is a flare on one specific change under load, and — as everywhere else here — heat and fluid decide whether it arrives at 150,000 km or never.

It was a five-speed before it was a six. The early Allison 1000 behind the Duramax has five ratios and the later one six, on the same casing — so the model year decides which procedure applies, and a torque-converter or valve-body part follows the ratio count rather than the name.