What is the ACD System and How Does It Work
By ACD/AYC PUMPS · UPDATED SEP 7, 2026
How Mitsubishi's Active Center Differential works, which Lancer Evolution models actually have it, and why US-market cars differ.
The Active Center Differential (ACD) is Mitsubishi's electronically controlled centre differential, fitted to the Lancer Evolution to manage how engine torque is split between the front and rear axles. Rather than a fixed mechanical bias, it varies the differential's locking state continuously as you drive.
What does the Active Center Differential actually do?
The ACD controls the limited-slip locking state of the centre differential across a full range — from completely open, through every intermediate state, to fully locked. Torque can be split up to 50:50 between front and rear. An open centre differential lets the axles rotate at different speeds, which helps the car turn; a locked one ties them together, which helps it put power down. The ACD's job is to pick the right point on that scale, many times a second, based on throttle, steering, wheel speed and the driving mode you have selected.
How does the ACD control the differential?
Mechanically, the ACD is an electronically controlled hydraulic multi-plate clutch. The clutch pack's preload is proportional to the fluid pressure applied to it, and that pressure is set by regulating current through a control solenoid. More current means more pressure, more clutch preload, and a more tightly locked centre differential.
The hydraulic pressure itself comes from an electric pump assembly, which also serves the Active Yaw Control circuit on cars that have it. The pump does not run constantly: it charges an accumulator that stores pressure, and the system draws on that reserve as the solenoids demand it. This is why a pump that can no longer build or hold pressure disables the whole system rather than just degrading it.
What do Tarmac, Gravel and Snow modes change?
The mode selector does not switch between unrelated programs — it changes how aggressively the ACD locks and how quickly it releases. Each setting determines how much locking force is applied to the clutch pack, and how long the ACD delays before freeing the centre differential after a steering input. Snow mode uses the most lock-up force, on the logic that a low-grip surface benefits from the axles being tied together more of the time. Tarmac allows the differential to free off sooner so the car rotates more readily on high grip.
Which Lancer Evolution models have ACD?
ACD arrived with the Evolution VII (CT9A chassis) and continued through the VIII and IX. Earlier cars used different hardware: the Evolution I to III (CD9A and CE9A) ran a viscous centre differential, and the Evolution IV introduced Active Yaw Control at the rear rather than an active centre diff. The Evolution X (CZ4A) integrates the centre differential into S-AWC.
Market matters here, and this is a common point of confusion for owners in the United States. USDM Evolution VIII cars from 2003–2004 were built with a viscous centre differential and a mechanical clutch-type rear LSD — no ACD. ACD reached the US market with the 2005 Evolution VIII MR and the 2006 Evolution IX. Neither of those generations was sold in the US with AYC; American cars did not get Active Yaw Control until the Evolution X. If you are diagnosing a drivetrain fault on a US car, confirm which system it actually has before ordering parts.
| Generation | Centre | Rear |
|---|---|---|
| Evo I–III (CD9A / CE9A) | Viscous centre diff | — |
| Evo IV–VI (CN9A / CP9A) | — | AYC — introduced Evo IV, 1996 |
| Evo VII–IX (CT9A) | ACD | AYC (JDM — not US) |
| Evo X (CZ4A) | ACD within S-AWC | AYC within S-AWC |
- 2003–2004 Evo VIII — viscous centre diff and a mechanical clutch-type rear LSD. No ACD, no AYC.
- 2005 Evo VIII MR and 2006 Evo IX — ACD, but still no AYC.
- Evo X — the first US car with AYC.
If you are chasing an “AYC fault” on a USDM Evo VIII or IX, check what the car actually has before buying parts.
How does ACD relate to AYC and to S-AWC?
ACD and AYC solve different halves of the same problem. ACD works front-to-rear, deciding how much the axles are tied together. AYC works side-to-side across the rear axle, shifting torque between the left and right rear wheels to influence the car's yaw — how readily it rotates into a corner. On Evolution VII to IX these were controlled independently. On the Evolution X, S-AWC integrates the centre differential, yaw control, ASC and ABS under one control strategy, which Mitsubishi describes as giving better overall stability than managing the systems separately.
The practical consequence for owners is that on cars fitted with both, one pump assembly feeds both circuits. A pump fault therefore shows up as an ACD and AYC problem at the same time, which often misleads people into suspecting the differentials themselves.
A pressure sensor watches the circuit. If pressure falls outside specification the control unit disables the functions and warns you — which is why a single failing pump takes out both ACD and AYC at once, and why the differentials themselves are rarely the culprit.
What typically goes wrong with the ACD system?
The differentials themselves are generally robust. The failure point on high-mileage cars is usually the hydraulic side: the pump assembly that supplies pressure to both circuits. Internal corrosion damages seals and opens up clearances between components that need to stay within fine tolerances for the pump to build pressure at all. Bearings wear. Solenoids develop faults. On the Evolution X in particular, the electric motor is a well-known weak point.
Because the whole system is pressure-dependent, the first symptom is rarely a subtle change in handling — it is a warning light, because the control unit detects that the required pressure is not there and disables the function. If you are seeing ACD or AYC warnings, the pump assembly is the first thing to investigate.
Diagnostic figures and self-test procedures circulated in the Evolution community are useful for narrowing down a fault, but they are not a substitute for a proper diagnosis with Mitsubishi's MUT-III tool or by a specialist. Always confirm against the factory service data for your specific chassis and market.
