Suspension system issues can be difficult to diagnose when the scan tool doesn’t want to interact with the Air Suspension Module (ASM), but a bit of knowledge and understanding of system function can go a long way in identifying the cause of a system problem.
The 1997-2002 Ford Expedition and Lincoln Navigator models use a three-wire potentiometer-style height sensor. The sensor is supplied a 5-volt reference signal and a ground circuit from the module. The third wire (tan at the front sensor or orange/black at the rear sensor) is the sensor’s signal circuit which feeds back to the ASM. The module will attempt to maintain a target voltage of 2.3 to 2.7 volts from the sensor.
When the vehicle’s ride height is low, sensor voltage will be higher than normal. The module will react by activating the compressor to raise the vehicle, activate the air spring solenoids and activate the fill and/or gate solenoids to bring the sensor voltage back to within the normal range.
If the vehicle ride height is high, the sensor voltage will drop to a range lower than normal. The module should then react by activating the vent solenoid at the compressor, activating the air spring solenoids, and activating the fill and/or gate solenoids to release air from the struts and raise the voltage to within the normal range so the vehicle “trim” height is achieved.
The 2003-2006 Expedition/Navigator models also use three-wire sensors, but the main difference is that this system uses dual front height sensors, does not use the rear air-fill solenoid, and some of the base suspension components are designed differently. The 2007-2008 models use only rear air struts with a height sensor on both sides but there is no front pneumatic function.
The most common symptom reported on the 1997-2002 system is that the rear suspension is sitting low and the compressor does not activate when the ignition switch is turned to the ‘Run’ position to raise the rear of the vehicle. To check the compressor and the compressor relay function, access the compressor relay near the passenger side headlight. backprobe and ground the dark blue/yellow wire with a jumper. This should activate the compressor relay and send power to the compressor on a grey/red wire. If the compressor does not run, verify power and ground to the relay before condemning it.
Compressor relay failure is usually the result of a compressor that intermittently draws too much amperage and damages the relay. The most common cause of premature compressor failure is a leaking air strut(s) that causes the compressor to run too often attempting to maintain normal vehicle height.
Other problems on the system may include intermittent dropping of the rear suspension when the vehicle is driven. Generally this will include illumination of the “Check Suspension” message or light. Diagnosis of this symptom should again start by verifying that the compressor will run in an attempt to raise the rear suspension. If the compressor will run when the ignition is turned on, it suggests that the input of the height sensor is OK; otherwise the ASM would not know that it needs to engage the compressor to raise the suspension. If the compressor runs, either the suspension compressor is weak or there is leakage of the air struts. Again, over time, leakage of the air struts will cause the compressor to weaken from excessive usage.
To test for a weak compressor, use a rubber-tipped pressure gauge to check the output air pressure of the compressor with the air line removed at the assembly. The compressor should be capable of producing at least 120 psi.
Sometimes the compressor can supply sufficient output pressure when it’s initially activated, but if it is activated repeatedly from strut leakage, its performance can drop off as it gets warm. This can result in a symptom where the suspension height is fine initially but then after “x” miles of driving, the suspension drops when the compressor is activated to maintain vehicle height. The air pressure of the strut overcomes the weak output of the compressor, and the vehicle slowly drops to the ground.
If the main issue of the system is intermittent leak-down after sitting overnight, a leak test is in order. To perform the leak-test of the struts:
1. place the vehicle on a level surface after the suspension is at normal height;
2. turn the suspension switch, located behind the passenger side kick panel, to the “Off” position;
3. measure the ride height from the corners of the bumpers front and rear;
4. road test the vehicle, leaving the suspension switch in the “Off” position, and make some “evasive cornering” maneuvers such as a cloverleaf-style turn to roll the body on the suspension from one side to the other;
5. recheck the suspension height measurements, if necessary, to determine if the suspension lost height.
The vehicle can also be allowed to sit overnight or over a weekend. Since the suspension switch is off, the module will not have the capacity to affect system function.
When rechecking the height, if the suspension is down, the air strut(s) is/are leaking. To exercise the system for general functionality, verify that the suspension switch is on and turn the ignition switch to the “Run” position. While monitoring the input of the height sensor(s), have a 200-plus pound assistant stand on the rear bumper. This should result in the height sensor providing a low input to the ASM. The ASM should respond by activating the system components to raise the rear of the vehicle back to trim height and then deactivate the system components.
At this point, have the assistant step down from the rear bumper. This should cause the ASM to detect a high input from the sensor. The ASM should then respond by activating the compressor vent solenoid, rear strut solenoids and the rear air-fill solenoid to vent off the rear to again obtain trim height.
