Signs of AYC Pump Failure

On a Lancer Evolution fitted with Active Yaw Control, a single electric pump assembly supplies hydraulic pressure to both the AYC and ACD circuits. When that pump can no longer build or hold pressure, the control unit disables the affected functions and warns you. Here is how to recognise a failing pump, and how to separate a genuine pump fault from a sensor or fluid problem.

What are the first signs of a failing ACD/AYC pump?

The earliest symptom is usually an intermittent warning: the ACD/AYC light appears on the dash, sometimes with a service message, then clears on a later start. A light that comes and goes is a classic sign of a pump that is still just managing to reach the required pressure, but not reliably. Owners also report the pump running noticeably longer than it used to before it shuts off, and a change in the sound it makes while charging.

On the Evolution X the same underlying fault often presents as a flashing S-AWC warning. Handling changes — reduced turn-in, the car feeling less willing to rotate — are real but usually noticed after the warning light, because the system disables itself as soon as pressure is out of specification rather than degrading quietly.

What do the warning lights actually mean?

The number and behaviour of the lights is diagnostic information, not just an alert:

• Light appears and later clears on its own. Pressure is marginal. The pump is likely on its way out, but still working intermittently.
• All three lights illuminated. Community diagnosis is that no pressure is being built at all, which points to a mechanical fault inside the pump — an overhaul rather than a sensor swap.
• Light stays off after the system charges. The pressure switch is functioning and the accumulator is holding some pressure.
• Light returns while driving. Accumulator pressure should be measured; it is dropping under demand.

How can I test the pump myself?

There is a widely used self-test that tells you a great deal before you spend anything. With the car safely stationary: switch the ignition on without starting the engine, wait around five seconds, then floor the throttle for about three seconds. This triggers the pump.

On a healthy system the pump runs, the note changes after roughly four to five seconds as pressure builds, and it switches off after about ten to twelve seconds with no ACD or AYC lights showing. If the pump keeps running for around twenty-five seconds and then the lights come on, the fault more often lies with the pressure sensor than with the pump's hydraulics. If all three lights come on, the pump is not building pressure and the assembly needs to come off.

These figures come from the Evolution owner community rather than from a factory manual, so treat the test as a strong indicator, not a verdict. A definitive diagnosis needs Mitsubishi's MUT-III tool, which can read the system's live data and run its own routines.

What actually fails inside the pump?

The pump assembly is a hydraulic unit driven by an electric motor, and both halves wear:

• Internal corrosion. The most common root cause. Corrosion damages seals and opens up the clearances between components that must stay within fine tolerances for the pump to generate pressure.
• Seals. Once seals are compromised, the pump loses pressure internally and can no longer charge the accumulator.
• Bearings. Wear here changes running clearances and is usually audible.
• Electric motor. The best-known weak point on Evolution X pumps.
• Solenoids and pressure sensor. Electrical faults that mimic a hydraulic failure and are worth ruling out first, because they are far cheaper to fix.

One point worth understanding: a failed motor is often a symptom, not the root cause. If the hydraulic side has degraded and the pump has to work harder and longer to reach pressure, the motor sees far more duty than it was designed for and wears out early. Replacing only the motor on a pump with a tired hydraulic section tends to buy months rather than years.

Is it the pump, the sensor, or the fluid?

Rule out the cheap causes first. Check the ACD/AYC pump circuit reservoir — on the Evolution it lives in the boot, and it uses ATF (Diamond/Diaqueen SP III specification), which is separate from the gear oil in the differentials themselves. A low or contaminated reservoir, or air introduced after work on the system, can produce pressure faults without any pump damage.

Next, consider the pressure sensor, using the run-time behaviour described above as a guide. Only once fluid and sensor are eliminated does a pump overhaul become the answer. Note that the system normally needs bleeding with the MUT-III tool after any work that opens the hydraulic circuit.

Can a failing pump be rebuilt instead of replaced?

Yes, and for most owners it is the sensible route. New assemblies are expensive and increasingly hard to source, while the failure modes above — corrosion, seals, bearings, motor — are all addressable on a bench. A proper rebuild replaces the worn hydraulic components and seals rather than just cleaning the unit, and the rebuilt pump should be pressure-calibrated and load-tested before it goes back on the car.

If your car is showing ACD or AYC warnings and you have ruled out fluid and sensor faults, a restored pump is normally the difference between a car that works as Mitsubishi intended and one driving around with its drivetrain electronics switched off.