The rear air suspension is an air-operated, microprocessor-controlled, suspension system which replaces the conventional rear coil spring suspension and provides low spring rates for improved ride and automatic rear load leveling. It is standard equipment on the Lincoln Town Car.
Two air springs replace the conventional steel springs and support the vehicle load at the rear wheels. The air springs are mounted onto the axle spring seats and to the frame upper spring seats, similar to the conventional rear steel spring system.
The system is operational in the ignition RUN position and has limited operation for one hour after ignition is turned to OFF. The air suspension switch, located on the right side of the luggage compartment, must be turned off when the vehicle is on a hoist, being towed or jump started.
The Air Suspension warning indicator is located in the instrument panel, to the right of the speedometer. The warning indicator flashes five times and then stays on when service switch is turned OFF or a system malfunction is detected.
The rear leveling system operates by adding or removing air in the springs to maintain the level of the vehicle at a predetermined rear suspension "D" ride height dimension, and is controlled by a microcomputer module.
The rear air suspension system module also controls the electronic variable orifice (EVO) steering. Refer to Section 11-02A for Description and Diagnosis of the EVO steering system.
Air required for leveling the vehicle is distributed from the air compressor to the air springs by a nylon air line which runs from the compressor dryer through a Y-fitting to each individual air spring.
The Rear Air Suspension/EVO control module services both the rear air suspension system and the EVO steering system. The wiring harness connects to the module using two separate push button release connectors. The connectors are keyed so that they cannot be installed incorrectly.
In general the control module uses a 45 second averaging interval to determine when compress and vent operations are needed. However, door inputs can override the 45 second averaging interval so compress and vent operations can begin immediately, if needed.
The 45 second averaging interval is used to keep the module from making unneeded corrections. When a vehicle at the correct rear trim height hits a bump the height sensor output will read low and high in addition to trim until the oscillations die out. If the control module were to correct for these "bump induced readings", system duty cycle would increase unnecessarily. The 45 second averaging interval not only eliminates corrections due to bumps, but also eliminates unneeded corrections resulting from braking, accelerating, and turning. The module tabulates the height sensor readings, and does not begin a compress or vent operation until the height sensor reads low or high for 45 seconds continuously.
There are more restrictions on vent operations than there are on compress operations.
To eliminate the chance of catching a door on a curb as the vehicle vents down, the module will not allow any venting to occur when a door is open.
The module does not allow any vent operations for the first 45 seconds after the ignition has been turned ON. Even if a vehicle is extremely high in the rear, DO NOT expect it to vent until the ignition switch has been ON FOR 45 SECONDS.
The following flow charts describe the Ignition ON and Ignition OFF Air Suspension leveling logic.
The purpose of the air spring solenoid valve is to allow air to enter and exit the air spring during leveling corrections. The valve is electrically operated and controlled by the module. Because the spring solenoid valves are airtight, connecting air lines are not required to be completely airtight. The air lines only contain pressurized air during vent and compress operations.
The valve has a two-stage pressure relief system similar to a radiator cap. A clip is first removed and rotation of the solenoid out of the seat will release air from the spring before the solenoid can be removed.
When weight is added to the vehicle, the height sensor length is reduced from trim length, sending a "rear is low" signal to the control module. To restore rear of vehicle to the trim position, the control module turns the compressor on by grounding the compressor relay control Circuit 420. Note that the relay is ground-side switched. Battery voltage is provided to the relay coil by Circuit 414.
To allow pressurized air to enter the springs, the module opens the spring solenoid valves by switching Circuits 416 and 429 to ground. The spring solenoid valves are also ground-side switched by the control module. Battery voltage is provided to the air spring solenoid valves by Circuit 414.
Compressed air flows from the compressor, through the dryer assembly airlines and spring solenoid valves into the rear air springs. As the air springs raise the rear body height, the height sensor increases in length until the preset trim height is reached. The module then turns off the compressor (through the relay) and closes the air spring solenoid valves.
The compressor relay is needed because the control module cannot directly provide the high current needed to run the compressor motor.
The module grounds Circuit 420 to energize the relay coil. When energized, the coil creates a magnetic field which closes the relay contacts, connecting Circuit 414 to Circuit 417. High current then flows from battery to the compressor motor.
The relays can usually be swapped , all Ford relays operate through the same circuits within the relays.
The height sensor sends signals to the control module through Circuits 427 and 428. There are three possible conditions that the module interprets from signals from the height sensor:
The height sensor is connected to the frame crossmember at one end and at the left rear upper control arm at the other end. The sensor gets shorter when the rear of the vehicle goes down and longer when the rear of the vehicle goes up. Magnets mounted in the lower slide portion of the sensor move relative to the sensor housing, generating a signal that is sent to the control module through two small Hall effect switches that are attached to the sensor housing.
Movement of the magnets determines switch opening and closing. At trim height, the switches remain closed and the module receives a trim signal. Upward movement of the magnets will open one switch to indicate a high condition. Downward movement of the magnets will open the other switch to indicate a low condition.