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Scottdagoalie, Senior Master Tech.
Category: Ford
Satisfied Customers: 1940
Experience:  30 years with Ford dealers
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I have a 1991 Lincoln town car with a failed air bag system... on an island in the Bahamas with few parts and no “Star Tester”. The trunk switch is on... fuses were checked under hood and dash... the dash light shows failed after 10 min ... (so the height sensor at the axel is working or the system would not know it has failed to pump itself up)... circuits have been reset (battery disconnected ignition on and reconnected)... airbags hold car up for weeks... no sounds of system leaks (so the physical lines and bags are OK)... But the pump fails to electrically activate on when the axel is loaded... I removed and disassembled the pump and checked for seizures (internal vent solenoid appearance was like new, O ring at solenoid in good condition and refit was snug and lubricated, pump sleeve and piston were free and unscarred, the two rubber piston rings were in good condition, the pump ran free for 20 min when hotwired and the compressor was not overheating... (it cannot be the valve assembly at the head of the compressor or the fit of the compressor’s piston rings... since low or no compressor output would generate a continuously running or seized compressor after the system reset that was performed). I re-installed the pump to the air tank and hoses then hotwired the air compressor... it ran free under apparent back pressure with no sounds of leaks at the pump or under the car... I hotwired the vent solenoid and it clicked open freely and depressurized the pump head... I removed the vent solenoid with the pump running and the pump ran freely without backpressure and vented physical air... However with the vent solenoid reinstalled and the compressor hotwired and running (even with the air ride computer is de-energized) the car still fails to rise the missing inch or so, and the pump still fails to automatically energize when the trunk was loaded with weight... At first it looks like two problems. First the pump output when manually hotwired fails to lift the car and secondly the circuit is not electrically responding to a low axel. So the question: What are the fault conditions and sensor locations for the “Air SYS” fault light on the dash. Can I get a schematic? I.e. This one question might solve the other two; how can a compressor with known working output, with a confirmed close vent solenoid, not raise the car when the airbags are not leaking even when isolated from the control module and with the module de-energized... and secondly why does the system not electrically activate and still generate a dash board fault indication. Maybe the control module holds system pressure “normally closed” both ways when de-energized and/or failed? How do I check the control module without a “Star Tester”?
What a great question! My source for information only goes back as far as 1992 ,pretty sure the systems are the same. I'll attach info from the manual that may help you with your problem.It sounds as though you are quite knowledgeable and can read this and understand it better than I can tell you what to look for. Let me know if you need more or different info and good luck!
1992 Town Car/Crown Victoria/Grand Marquis

Suspension, Computer Controlled

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.graphic

Item Part Number Description Item Part Number Description
1 Compressor Relay Power Junction Block 9 From Compressor
2 Air Line (Attached to Brake/Fuel Bundle) 10 Y-Fitting
3 3B484 Air Compressor With Regenerative Air Dryer and Vent Solenoid 11 To RH Air Spring
4 Air Line To Compressor 12 Heat Shield
5 System On/Off Switch 13 Spring Retainer Clip
6 System Diagnostic Connector 14 Air Springs With Integral Solenoid
7 Quick Connect 15 Rear Height Sensor
8 To LH Air Spring

1992 Town Car/Crown Victoria/Grand Marquis

Control Module

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.


Customer: replied 8 years ago.
Ah standby
Customer: replied 8 years ago.
Thx Scott for the electrical schematic. Can you think of any physical valves in the system that might have the ability to hold the system pressure "normally closed" in both directions (even when I'm charging the system by manually hotwiring the compressor and when the control module is electrically de-energized)?
Customer: replied 8 years ago.
Also... which of the two relays in the power distribution box under the hood is for the air bag system. I've got "3C18-1822" and "0L09-1229" (can they be swamped for trouble shooting?)
graphicThe air solenoids on the springs are normally closed and would have to be energized to get air to pass through to the springs.

1992 Town Car/Crown Victoria/Grand Marquis

Air Spring Solenoid Valve

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.

1992 Town Car/Crown Victoria/Grand Marquis

Weight Added

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.

1992 Town Car/Crown Victoria/Grand Marquis

Compressor Relay

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.

Customer: replied 8 years ago.
Ok of the spring solenoid valves have to be electrically energized to allow compressed air to enter the airspring and since there are two spring solenoid valves and since driving the compressor manually fails to inflate either air spring, would you concur that I'm looking at a simple control module failure? I'm going to have to FedEx it off the island here... where is the control module?
It certainly could be a module failure given the facts. The module is located in the trunk near the service switch.
Scottdagoalie and 6 other Ford Specialists are ready to help you
Customer: replied 8 years ago.
Ok Scott: Unless you have any other thoughts on what to trouble shoot, I'm grateful for the info.
Thanks for the accept on my help! The only other possibility may be a height sensor concern. The module reacts to signals from one height sensor. If the signal is not enough for the module to warrant suspension activation,then it may give you the same symptoms.
Glad I could help and let me know if there is anything else I can do!
1992 Town Car/Crown Victoria/Grand Marquis

Height Sensor

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:

  • Vehicle is at trim height—204-209 mm (8-8.2 inch) when measured from ball stud to ball stud.
  • Vehicle is below trim height.
  • Vehicle is above trim height.

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.