General Motors 4L80-E / 4L85-E

General Motors' heavy-duty longitudinal four-speed — behind the three-quarter-ton Silverados and Sierras, the Express and Savana vans, the Suburban, Tahoe and Yukon, and the Hummer H1.

And behind the Jaguar XJ, the XJS and the Bentley Continental R, which is the part worth knowing: those cars carry an American truck gearbox, nothing on them says so, and a great deal of the specialist advice written for them is really advice about a GM transmission with a very large service literature of its own.

It is a descendant of the Turbo-Hydramatic 400 with electronic control added, and its reputation follows from that: it is among the most durable automatics GM ever built, and it is the unit people fit when something lighter has failed twice.

Its failures are almost all electrical or hydraulic rather than structural. The pressure control solenoid and the shift solenoids account for most of them, producing harsh changes or a limp mode locked in a single gear; the input and output speed sensors produce the same picture. A 4L80-E that has stopped changing gear is far more likely to have a solenoid fault than a broken clutch pack, which is the opposite of the assumption a lighter transmission earns.

Heat is what kills the ones that die. Towing without an auxiliary cooler cooks the fluid, and the fluid is what protects everything else.

It is not the 4L60-E. The two are not interchangeable in either direction, and the 4L80-E is physically much larger — the tell is the pan, which has seventeen bolts against the 4L60-E's sixteen and a quite different shape.