ASE A4 Suspension and SteeringSuspension System Diagnosis and RepairHard
A vehicle with an active suspension system that uses magnetorheological (MR) fluid dampers is brought in for a diagnostic. The customer reports that the ride quality is intermittently harsh and then unusually soft, accompanied by a 'Service Suspension' light. A scan tool shows a DTC for 'Damper Control Valve Circuit Intermittent Open' for the right front damper. What is the MOST likely behavior of the right front MR damper when this intermittent open circuit occurs?
- ARemains in its last commanded setting until the circuit is restored.
- BDefaults to its firmest possible damping setting.
- CDefaults to its softest possible damping setting.
- DCycles rapidly between soft and firm settings.
Show answer & explanationAnswer & explanation
Correct answer: C. Defaults to its softest possible damping setting.
Magnetorheological (MR) dampers use an electromagnetic coil to stiffen the MR fluid. If an intermittent open circuit occurs in the damper control valve, the electromagnetic coil will lose power. Without the magnetic field, the MR fluid will return to its natural, un-stiffened state, resulting in the softest possible damping setting for that damper. This is often a fail-safe design to prevent a dangerously harsh ride.
Why the other options are wrong
- A. Without power, the MR fluid cannot maintain a commanded setting; it reverts to its default state.
- B. To achieve the firmest setting, the coil needs maximum current to generate a strong magnetic field; an open circuit prevents this.
- D. Rapid cycling would imply intermittent power delivery, but an 'intermittent open' suggests periods of no power, leading to the softest default.
MR Damper Fail-Safe (Open Circuit)
In an MR damper, an open circuit in the control valve or coil results in loss of magnetic field, causing the damper to default to its softest damping setting.
- MR fluid stiffens with magnetic field.
- No current = no magnetic field = softest setting.
- Often a fail-safe to maintain some ride comfort.
Memory trick: No power, no magnet, so the fluid gets quite soft, like a plush carpet.