Description and Operation
The rear air suspension:
- Is an air-operated, microprocessor-controlled, suspension system.
- Replaces the conventional rear coil spring suspension.
- Provides low spring rates for improved ride and automatic rear load leveling.
- Is standard equipment on the Town Car.
- Is available as optional equipment on Crown Victoria and Grand Marquis.
The rear air suspension system has the following features:
Suspension, Computer Controlled—Town Car
- The system is operational with the ignition switch in the RUN position.
- Automatic rear load leveling has limited operation for one hour after ignition switch 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 control module (5A919) .
- The rear air suspension control module also controls the electronic variable orifice (EVO) steering.
- Refer to Section 11-02 for Description of the EVO steering system.
- Air required for leveling the vehicle is distributed from the air compressor to the rear air springs by a nylon air line which runs from the compressor air dryer (5346) through a Y-fitting to each individual air spring.
Compressor Relay Power Junction Block
Air Line (Attached to Brake/Fuel Bundle)
Air Compressor With Regenerative Air Dryer and Vent Solenoid
Air Line to Compressor
Air Suspension Service Switch
Data Link Connector
To LH Air Spring
Y-Fitting (Part of 5A911)
To RH Air Spring
Spring Retainer Clip
RearAir Suspension Height Sensor
Air Spring Solenoid
Control Module Power and Ground
The control module (5A919) has the following features:
- Power is provided by Circuit 418 and is controlled by the air suspension switch.
- The air suspension switch is powered through a 15 amp fuse in Circuit 296 in Town Car.
- AIR SUSPN PUMP (30A MAXI) fuse provides this power through Circuit 414 in the Crown Victoria/Grand Marquis.
- Control module ground is provided through control module wire harness Pins 6 and 21, to the RH inner quarter panel sheet metal, near the control module .
When weight is added to the vehicle:
- The air suspension height sensor (5359) length is reduced from trim length, sending a "rear is low" signal to the control module (5A919).
- The control module then turns the compressor on by grounding the compressor relay control Circuit 420. This restores the rear of the vehicle to trim position.
- Battery voltage is provided to the relay coil by Circuit 414.
- The control module opens the spring solenoid valves (5311) by switching Circuits 416 and 429 to ground. This allows pressurized air to enter the springs.
- Battery voltage is provided to the air spring solenoid valves by Circuit 414.
- Compressed air flows from the compressor, through the compressor air dryer (5346) airlines and spring solenoid valves into the rear springs (5560).
- As the rear springs raise the rear body height, the air suspension height sensor increases in length until the preset trim height is reached.
- The control module then turns off the compressor (through the relay) and closes the air spring solenoid valves.
When weight is removed:
- The air suspension height sensor (5359) length is increased from the trim length, sending a "rear is high" signal to the control module (5A919).
- The control module then opens the vent solenoid valve (located in the compressor assembly) by switching Circuit 421 to ground and opens the solenoid valves (5311) by switching Circuits 416 and 429 to ground. This restores the rear of the vehicle to trim position.
- Compressed air flows from rear springs (5560), through the air spring solenoid valves, air lines, compressor air dryer (5346), and out the vent solenoid valve.
- As the body lowers, the air suspension height sensor length decreases until the preset trim height is reached.
- The control module then closes the vent and solenoid valves.
Control module operates as follows:
- In general the control module uses a 45-second averaging interval to determine when compress and vent operations are needed.
- However, door courtesy lamp switch (13713) 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 control module from making unneeded corrections.
- When a vehicle at the correct rear trim height hits a bump, the air suspension 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 control module tabulates the air suspension height sensor readings, and does not begin a compress or vent operation until the air suspension height sensor (5359) reads low or high for 45 seconds consistently.
- 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 control module will not allow any venting to occur when a door is open.
- The control module does not allow any vent operations for the first 45 seconds after the ignition switch (11572) has been turned to RUN.
- Even if a vehicle is extremely high in the rear, DO NOT expect it to vent until the ignition switch has been turned to RUN for 45 seconds.