Automatic Level Control (ALC)
The Automatic Level Control (ALC) system automatically adjusts the rear height of the vehicle in response to changes in vehicle loading.
The ALC system consists of a height sensor, an air compressor assembly, an ALC compressor relay, an
intake hose and filter, an air tube, and two rear shock absorbers. The air compressor assembly consists of an air compressor and an air dryer mounted on a bracket. The air compressor head is a replaceable part of the air compressor. The exhaust solenoid is a non-replaceable part of the air compressor head.
The compressor is activated when the ignition is on, and weight is added to the vehicle.
The exhaust solenoid is connected directly to the battery (+), enabling the system to exhaust with the ignition off when excess weight is removed.
Raising the Vehicle
When a load is added to the vehicle, the vehicle body moves down causing the suspension position sensor actuating arm to rotate upward.
An arm movement of about 25 mm (1 in) amounts to a height change of approximately 12 mm (0.5 in) at the rear bumper. The upward arm movement activates an internal timing circuit inside the position sensor, and after an initial fixed delay of 17–27 seconds, the sensor provides a ground to complete the compressor relay circuit. The 12 V (+) circuit to the compressor is then complete and the compressor runs, sending pressurized air to the shock absorbers through the air tubes.
As the shock absorbers inflate, the vehicle body moves upward rotating the actuating arm towards its original position.
Once the body reaches its original height, +/− 25 mm (+/− 1 in), the position sensor opens the compressor relay circuit, and the compressor is turned off.
Air Compressor Head Relief Sequence
In order to reduce current draw during air compressor starting, the position sensor performs an air compressor head relief sequence before air compressor operation. This sequence reduces the air pressure in the air compressor cylinder during start-up. The air compressor head relief sequence occurs as follows:
- The exhaust solenoid is energized by the position sensor.
- The air compressor is activated 1.3 seconds after the exhaust solenoid is energized.
- The exhaust solenoid is de-energized 0.5 seconds after the air compressor is activated.
Lowering the Vehicle
When a load is removed from the rear of the vehicle, the body rises, causing the position sensor actuating arm to rotate downward. This again activates the internal timing circuit. After the initial fixed delay, the sensor provides a ground to complete the exhaust solenoid circuit, energizing the solenoid. Now, air starts exhausting out of the shock absorbers, back through the air dryer and exhaust solenoid valve, and into the atmosphere.
As the vehicle body lowers, the actuating arm rotates to its original position. When the vehicle body reaches its original height, +/− 25 mm (+/− 1 in), the position sensor opens the exhaust solenoid circuit, which closes the exhaust solenoid and prevents air from escaping.
Air Replenishment Cycle
The position sensor actuating arm position is checked when the ignition is turned on. If the sensor indicates that no height adjustment is needed, an internal timer circuit is activated.
After about 35–55 seconds, the compressor is turned on for 3–5 seconds. This ensures that the shocks are filled with the proper residual pressure. If weight is added to or removed from the vehicle during the time delay, the air replenishment cycle is overridden, and the vehicle rises or lowers after the normal delay.
Air Compressor Description
The air compressor is a positive displacement air pump, powered by a 12 volt DC permanent-magnet motor. The air compressor head contains
intake and exhaust ports plus a solenoid-operated exhaust valve that releases air from the ALC system when energized. Air compressor
intake air is drawn through an
intake air hose and filter, mounted in the right side body rail. The air compressor is a serviceable part and is mounted on a bracket. The bracket is mounted to the underbody, right side, rear of the rear suspension.
Air Dryer Description
The air dryer is attached to the air compressor head assembly at the pressure outlet and provides a dual function:
- It contains a moisture-absorbing chemical that absorbs moisture from the compressed air before it is delivered to the rear shocks. Moisture is removed from the air dryer and returned to the atmosphere when air is exhausted out of the shocks during lowering.
- It contains valving that maintains a system residual pressure of 48–97 kPa (7–14 psi). This system pressure is maintained for improved ride characteristics and improved air sleeve reliability. (THIS IS THE IMPORTANT PART THE TSB FROM GM FOR ADJUSTING THE PRESSURE FOR AFTERMARKET AC DELCO(PRESUMABLY ALL SHOCKS OTHER THAN OEM FACTORY UNITS) IS FOR
Exhaust Solenoid Description
The exhaust solenoid is a non-serviceable part of the air compressor head and provides a dual function:
- When energized, it exhausts air from the rear shocks.
- It acts as a relief valve to limit the maximum air pressure output of the compressor.
Air Compressor Relay Description
The relay, located in the relay center under the rear seat, completes the 12 V (+) circuit to the compressor motor when it is energized. The sensor controls the relay.
Height Sensor Description
The solid-state sensor detects height changes at the rear of the vehicle and, in turn, controls the functions of the compressor and exhaust solenoid. To prevent energizing the compressor relay or exhaust solenoid during normal ride motions, the sensor circuit provides a fixed delay before ground is completed to either circuit.
In addition, the sensor limits compressor run time and exhaust solenoid energized time to a range of 4.5–7.5 minutes. This prevents continuous compressor operation in case of a severe system leak or continuous exhaust solenoid operation. The timer circuitry is reset whenever the ignition is cycled OFF and ON, or the sensor exhaust or compress output signal changes.
The sensor is mounted to the left front side of the rear suspension support assembly. The actuation arm is attached to the left rear control arm by a short link.
Air Shock Absorber or Strut Description
The rear air adjustable shock absorbers are constructed with a rubber-like air sleeve attached to the dust tube and reservoir. This sleeve forms a flexible chamber which extends the shock absorber when pressure in the chamber is increased.
When air pressure is reduced, the weight of the vehicle collapses the shock absorber. In order to maintain proper operation and reliability of the air adjustable shock absorbers, a system pressure from 48–97 kPa (7–14 psi) (see previous underline/italic/bold portion)must be maintained at all times.
Air Lines and Connectors Description
The air line is a non-serviceable item consisting of the following:
- One connector at the ALC air dryer
- Air lines between the ALC air dryer and the 2 rear shock absorbers
- Two connectors with spring clips at each rear shock absorber
At the rear shock absorbers, the air line connectors are held on with spring clips which snap into the grooves of the shock absorber air fittings. Air lineconnectors are sealed using 2 O-rings.
Although the lines are flexible, care should be taken not to kink them and to keep them from contacting the exhaust system.
I Just provided the Description and operation for a 2004 buick park avenue ALC (Automatic Level Control) system which I verified to match a 1999 Buick Park Avenue ALC for the operational qualities which, unless I am missing something, should match most gm ALC systems from 1996 ish to 2005 across Pontiac, Buick, Oldsmobile, and Cadillac units. Presumably if any Chevy cars like a monte carlo or impala had the system at all, they likely did too. But I dont know much about the Chevy cars from this time as it relates to ALC. Bonneville, Aurora, Park Avenue, Lesabre, Deville, Eldorado(I think), and 88, 98 are the models, I believe, that would have used this same system. If I am wrong please correct me, but the GM compressor and ride height sensor for the most part was used interchangeably to 1999, then 2000-2005 between models. The 2000+ compressor works on the previous 1999 and older cars(down to 1995 or 1996 when first utilized) it just lacks the tech 2 interfacing and also potentially requires a pinout change on the connector, POSSIBLY. 2000-2005 biggest difference was a revised compressor I believe that was smaller and slightly more reliable and had tech 2 height calibration and stored codes under C____ so as to be easier diagnosed. the 1999 and couple year earlier units were adjusted by manipulating the height sensor and the diagnostics were old school with DVOM and service data.
Being the 1999 Data (for Riviera, Aurora, and Park Avenue) matches the 2004 Park Avenue data, I would suspect the behavior of all cars with the same suspension and sensor/compressor had the same behavior.
If you are using anything but the OEM part number shocks, so even AC delco kit replacements or monroes or etc, I would suspect the GM tsb on increasing the residual pressure is a necessary adjustment for proper shock operation. But beside that point, all the information I provided in the first part should help you understand how it should normally behave.
When I had my old 2001 aurora, which would be like a lesabre and had the same system etc, When I turned the key on, I usually heard the compressor after a short while and it would make a couple noises but it was short lived. I believe that was the behavior described above, but I did not have the car in a "perfect" test state either, with no items in side, seat back, jouncing, etc. So I cant go back in time to test properly, but if that helps understand then good 🙂
When the car is adjusting height, the compressor will be noisier and probably run for more than 5 seconds continuous if it has to adjust. If you suspect leak, you would notice the amount of time of noises after Key on to be longer with the longer the car was unused. Worth a try sometime if you dont end up going anywhere for a while. Colder temperatures theoretically should allow it to leak faster sitting, if the sealing is an issue.
Finally, @Quist22 , if you want a visual way to check, could you jack up your car with your control arm, close to the wheel as possible, then doing the same on the other side after, and pull off tire. The idea is to load the suspension arm just enough to pull the wheel, take a picture or measure your spring on both sides, pictures would be best of course, and show us? if your spring is compressed more to one side under the same weight, the thudding could be a side-to-side imbalance or the shock is having to compensate more to one side causing the components to be slightly unloaded or the mount to be maybe unbound or binding on the opposite. Also if you ever touched your sway bar links, they could be preloaded or something. 
