The AL4 automatic: how it works and what goes wrong with it

The AL4 — sold as the DP0 by Renault — is in a very large number of cars: the automatic 206 and Pars, the 307 and 407, the Xsara and Xantia, the Mégane and Scénic. It is a competent four-speed design, and it has a poor reputation, and the gap between those two facts is almost entirely about heat and fluid.

The original of this article is the best technical source in this archive: a genuine walk through the torque converter, the epicyclic sets, the hydraulic unit and the sensors, clearly translated from service documentation. It is worth reading for that.

It also contains the two most costly sentences in the whole collection, and they need dealing with first.

The two claims to disregard

"One of the most reliable automatic gearboxes." It is not, and stating it first sets a reader up to buy one without knowing what to check. The AL4's failure modes are specific, well documented and largely preventable — but they are common enough that this gearbox is the reason a lot of people distrust automatics.

"The AL4's oil is permanent. Only check its level every 50,000 km." This one is worse, because it is an instruction rather than an opinion, and it is the manufacturer's own line repeated in good faith. PSA and Renault marketed the gearbox as sealed for life. Its defining failure is degraded fluid, and the trade consensus — arrived at by rebuilding a great many of them — is to change the fluid at somewhere around 60,000 km and keep doing it.

Those two sentences reinforce each other into the exact behaviour that kills the gearbox: trust it, and never touch the fluid.

How it works

This is the original's material, compressed. It is accurate and worth having.

The torque converter replaces the clutch. Three elements sitting in oil: a pump driven by the engine, a turbine facing it and connected to the gearbox, and a stator between them. Pulling away, the pump flings oil at the turbine, the turbine begins to move, and the stator redirects the returning oil so that it helps rather than fights — which is what multiplies torque at low speed and is the whole reason an automatic pulls away smoothly. That is the conversion phase.

As the turbine catches up with the pump, the stator is allowed to freewheel and the unit simply couples the two together — the coupling phase. There is still a little slip, and slip is wasted fuel, so a clutch inside the converter finally locks pump to turbine mechanically. That is lock-up, and it is the subject of P0740 and P0741 when it stops working.

The gearing is two epicyclic sets, controlled by two multi-plate clutches and three brakes — one multi-plate and two band brakes wrapped around a drum and pulled tight by hydraulic pistons. A gear is not selected so much as composed: each ratio is a particular combination of which members are driven, which are held still and which are free.

The hydraulic unit is what does the choosing. A pump behind the converter feeds a valve body with eight solenoid-controlled spool valves. One electrical valve produces a signal pressure of 0 to 3 bar, which commands a regulator that sets the line pressure between 3 and 21 bar, and that pressure is what applies the clutches and bands. Get the line pressure wrong and every clutch either slips or slams.

The module decides from seven inputs: throttle position, engine speed, turbine speed (the gearbox input speed), vehicle speed, oil temperature, the multi-function switch reporting the lever position, and the brake switch. Throttle and road speed do most of the work.

One detail worth knowing as a driver: in D the car normally pulls away in second, not first, for smoothness. First is available by selecting position 2, and there is a winter mode that reduces torque at the wheels on a slippery surface and a sport mode that holds gears longer.

Why heat is the whole story

Everything the AL4 is known for traces back to one loop.

The fluid is cooled by a small water-to-oil heat exchanger, with a thermostatic valve to keep it warm in cold weather. That exchanger is modest for the job, and it silts up. Fluid temperature climbs.

Hot fluid loses its film strength and its friction characteristics change, so the clutches and bands begin to slip fractionally as they engage. Slipping friction material generates heat, which degrades the fluid further, which makes the slip worse. Meanwhile the heat attacks the rubber and plastic inside the unit — piston seals, solenoid components — so line pressure becomes erratic, which makes the clutches slip for a second reason.

At some point the module notices and drops the gearbox into limp mode, usually locked in third, and stores P0730 for an incorrect gear ratio or P0218 for over-temperature. By then the damage is not in the electronics.

The practical consequence: an AL4 that has started shifting harshly in traffic on a hot day is telling you something, and the answer is not to wait and see. Get the fluid changed, and have the heat exchanger looked at.

What actually fails, in order

• The solenoids — the pressure regulation (EPC) and lock-up (TCC) ones above all. They are the most replaced parts on this gearbox, and they fail partly because of everything above: worn bushings drop the lubrication pressure, the pump runs at maximum to compensate, and the solenoids live at their limit.
• The heat exchanger, silted or restricted.
• Bushings and the valve body. Wear here loses pressure internally and a warped valve body gives jerky shifts that no solenoid replacement will cure.
• The turbine speed sensor — P0715 and P0717. The module needs input speed to know whether the ratio it commanded actually happened.
• Low line pressure as a symptom of most of the above, which is P0868.

And note that P0700 on its own tells you nothing except that the transmission module has something to say. Read the transmission's own codes rather than treating P0700 as a diagnosis.

The fluid, done properly

This is the part the original gets wrong, so here is what to do instead.

Change it. Around 60,000 km is the interval the people who rebuild these settle on, and more often if the car sits in traffic or tows or lives somewhere hot.

Accept that it is a partial change. Roughly three litres come out; the torque converter cannot be drained and holds a good share of the total. That is not a reason to skip it — it is a reason to do it more than once rather than to do it never.

Use the specified fluid. This is not interchangeable and it is not a place to save money: transmission fluid is a hydraulic medium, a coolant and a friction modifier at the same time, and the friction figure is what decides how the clutches take up. Transmission fluid explains why the wrong one produces shift quality problems that look mechanical.

And the level check is a procedure, not a dipstick. There is no dipstick. The level is set through a plug in the sump with the fluid at a specified temperature, because the volume changes measurably as it warms — check it cold and you will overfill it, check it hot off a motorway run and you will underfill it. Overfilled fluid aerates, and aerated fluid does not carry pressure. Look the procedure up for the car in front of you rather than improvising it.

The park pawl, which the original explains unusually well

A small thing, kept because it answers a question most drivers have and few ask. Park works by dropping a pawl into square notches on the output shaft. Two details make it civilised: the pawl's tip is coned, so the weight of a car parked on a hill pushes it back out rather than jamming it; and if the pawl does not happen to line up with a notch when you select P, a spring holds it against the shaft until the car rolls a few centimetres and it drops in.

That is why the car often moves slightly as you release the brake in Park. It is the pawl finding its notch, and it is normal.

What to check before buying one

Given how many of these cars are on the road, this is the useful ending the original does not have.

• Ask when the fluid was last changed. "Never, it is sealed for life" is the answer that should set the price.
• Drive it from cold and then in traffic until it is thoroughly hot. The faults here are heat faults, and a ten-minute test drive proves nothing.
• Feel for the 1–2 and 2–3 changes. Flare — engine speed rising between gears — is slip, and slip is the fluid or the clutches.
• Check for a stored P0730 or P0218, and be suspicious of a module whose codes have just been cleared.
• Watch the lock-up. A converter clutch that judders at steady light throttle around 60–80 km/h is a classic P0741, and on this gearbox it is often the fluid rather than the converter.

None of that makes an AL4 a bad car to own. It makes it a gearbox that needs the one thing its own handbook told everybody it did not need.

Video guides

Video, for the parts of this that are easier watched than read. The English-language ones come first.