AL4 / DP0

The four-speed automatic PSA and Renault built together and fitted to almost everything front-wheel-drive they sold with two pedals for more than a decade — the 206, 307 and 407, the C3, C4 and C5, and their Renault equivalents. PSA calls it the AL4 and Renault the DP0, and it is one gearbox: the difference is calibration rather than hardware, so a repair described for one applies to the other.

Almost everything that goes wrong with it comes back to heat. It is a physically small gearbox asked to sit in a crowded engine bay, and it was sold for most of its life with no cooler worth the name and no service schedule. Fluid that gets hot thins out; thin fluid changes what the solenoids in the valve body actually do; and the valve body is unusually sensitive to that, so the first symptom is not a noise or a leak but shifting that gets worse on a long run and improves after the car has stood overnight. A gearbox that misbehaves only when hot is describing this problem and not a worn one.

The fluid is the whole argument, and "sealed for life" was never true of it. These went out with no stated interval, which owners and some dealers read as never — and the fluid is the coolant here as much as the lubricant. Changing it on a schedule of roughly 60,000 km is the single thing that separates the ones that reach high mileage from the ones that do not. It also has to be the right fluid: the specification is an ATF of the LT 71141 type, and the unit is intolerant of substitutes in a way that a bigger, better-cooled gearbox would forgive.

What then fails, in the order a workshop meets it:

• The solenoids in the valve body, which are the most frequently replaced parts on the whole unit and are a consequence of the heat and the fluid rather than a fault of their own.
• The electronic control unit, which has a poor reputation independently of everything above and produces faults that read as mechanical.
• The torque converter and its lock-up clutch, whose shudder is often the first thing an owner actually notices.
• Needle bearings and the brake bands, the internal wear that arrives last and is what makes the difference between a repair and a replacement.
• Leaks from worn bushings, which are cheap in themselves and expensive in what running low on fluid then does.

The honest summary for an owner deciding whether to keep the car: this gearbox has a bad reputation it partly earned and partly inherited from being fitted to cheap cars that were never serviced. Kept cool and given fresh fluid it is unremarkable. Neglected it fails in a way that costs more than the car is worth, and by the time it shifts badly when cold the decision has usually already been made for you.