ACD Pump Restoration Process

An ACD/AYC pump that has stopped building pressure does not have to be scrapped. Almost everything that fails in these assemblies — corroded internals, tired seals, worn bearings, a burnt-out motor — is repairable on a bench, provided the unit is rebuilt properly and then proven under load before it goes back on the car. This is what a full restoration involves and why each stage matters.

Why rebuild rather than replace?

New pump assemblies are expensive and increasingly difficult to source for cars that are now twenty to thirty years old. More importantly, replacement does not tell you anything about why the original failed. A rebuild puts the unit on a bench where the actual failure mode is visible: whether the hydraulic section had corroded, whether the motor died because it was compensating for a leaking pump, or whether the fault was electrical all along. That diagnosis is what stops the same failure repeating.

What does a full restoration involve?

Our process runs in six stages, and each one exists to catch something the previous stage cannot:

1. Diagnostics. Full system bench diagnostics and fault isolation, before anything is taken apart. The aim is to know what failed while the evidence is still intact.
2. Teardown. Complete disassembly and ultrasonic cleaning. Corrosion and varnish hide wear; a unit cannot be measured honestly until it is clean.
3. Restoration. Worn components replaced with genuine parts — the seals, bearings and hydraulic components that have moved outside tolerance.
4. Calibration. Pressure, flow and solenoid calibration on the bench, so the rebuilt unit behaves the way the car's control unit expects.
5. Testing. Final quality control under simulated drivetrain load, rather than a brief no-load spin.
6. Warranty. A 12-month warranty applied, and the unit is ready to ship.

What gets replaced during a rebuild?

The core of the job is the hydraulic section. Seals are renewed, bearings replaced, and the plate and sealing surfaces that let the pump generate pressure are restored to specification. This is the part that distinguishes a rebuild from a clean-and-refit: the clearances that corrosion opened up are what stopped the pump building pressure in the first place, so simply reassembling a cleaned unit does not fix it.

The electric motor is assessed separately. On Evolution X pumps in particular the motor is a known failure point, and a motor that has been driving a leaking hydraulic section has usually had a hard life even if it still turns.

What is a Stage 2 build, and when is it worth it?

A Stage 1 restoration returns the pump to correct function: rebuilt hydraulics, restored performance and faster pressure build-up compared with a tired original. A Stage 2 build goes further and replaces the electric motor with a more reliable unit.

Stage 2 makes most sense where the motor is the documented weak point or where the car has already been through a motor failure. If your pump's hydraulic section was badly degraded, the motor has been working overtime for a long time, and renewing it at the same time avoids a second job later. Where a motor is healthy and the hydraulics were caught early, a Stage 1 rebuild is often all the unit needs.

How is a rebuilt pump tested before it ships?

Bench testing is the part owners cannot do at home and the part that matters most. A pump can be reassembled cleanly and still fail to hold pressure under sustained demand. Calibrating pressure, flow and solenoid response, then running the unit under simulated drivetrain load, is what confirms the rebuild actually worked — rather than discovering it on the car after the system has been bled.

This matters because refitting is not trivial. The hydraulic circuit normally needs bleeding with Mitsubishi's MUT-III tool after any work that opens it, so a pump that turns out to be faulty after installation costs a great deal of extra labour.

What happens after the pump is refitted?

Once the restored pump is on the car, the system has to be bled and the warning lights cleared. It is worth checking the ACD/AYC circuit reservoir — located in the boot and filled with ATF to Diamond/Diaqueen SP III specification, separate from the differential gear oil — and confirming the pump completes its charge cycle correctly on the ignition-on self-test.

A correctly restored and properly bled system should charge, reach pressure, shut off and stay quiet, with no ACD, AYC or S-AWC warnings. At that point the centre and rear differentials are being commanded as Mitsubishi designed, rather than sitting disabled because the control unit cannot see the pressure it needs.

Sending us a core

If your pump is showing the symptoms described above and you have already ruled out fluid and sensor faults, the unit can be restored rather than replaced. We also run a trade-in programme, so a failed core can go towards a restored assembly. Every unit we ship is bench-tested and covered by a 12-month warranty.