Automatic Level Control

Automatic leveling, first used on 1965 model year cars, is a pneumatic system and is standard on some models and optional on others. See a list by year of standard and optional applications.

The system consists of an air compressor with integrated storage tank and pressure regulator; a control valve; and two Superlift shock absorbers in the rear of the car. The components are connected by 1/8th inch flexible air lines.

When enough load is put into the car to lower the body, the control valve detects the load by the change in distance between the axle and body. After a time delay of about 4 to 18 seconds the control valve begins to inflate the shock absorbers bringing the body back to the original height. The delay is required to prevent the valve from trying to control the height during normal spring action while driving.

After the load has been removed and the time delay has passed the control valve vents the Superlift shocks and the body to axle distance is reduced. The control valve also has a retainer valve to prevent the shocks from complete deflation. If the shocks are completely deflated the air bag will rub against itself and will wear out prematurely.

First available from 1965 and used through 1977. in 1978 the vacuum powered system was replaced by the Electronic Level Control? (ELC) system.

Table of contents

How it works

Theory of ALC compressor operation.

In the following description please refer to figure 1, Vacuum compressor schematic.

The distributor valve covers and connects the vacuum supply port and either port 1 or port 2. Let's start with the distributor valve connecting port 1. Vacuum is sent to chamber 1 pulling the air out of the chamber and lowering the pressure. Meanwhile port 2 is open to the atmosphere and allowing air to flow into chamber 2. The difference in pressure on the diaphragm moves it and the connected piston compressing the air in cylinder 1. Check valve 1 is closed by a spring. The spring for check valve 2 is over come by the pressure of the air being compressed and opens to allow the air to flow into cylinder 2.

As the piston nears the end of its stroke a mechanical link toggles the distributor valve. It now covers and connects port 2 to the vacuum supply while port 1 is now open to the atmosphere. The pressure differential now moves the diaphragm and piston in the opposite direction, compressing the air in cylinder 2. Cylinder 2 is smaller than cylinder 1 and the air in cylinder 2 is above atmospheric pressure even at the start of the stroke.

As the piston moves to further compress the air in cylinder 2 check valve 2 closes from spring action and check valve 3 opens to allow the air to exit to the reservoir.

Meanwhile check valve 1 opens and more air is pulled into cylinder 1. As the piston nears the end of its travel the mechanical linkage again toggles the distributor valve to cover and connect port 1 to the vacuum supply and vent port 2 to the atmosphere. This cycle repeats until the piston can no longer push air into the reservoir against the pressure. The compressor stalls until the vacuum increases or the reservoir pressure decreases. Stall pressure will vary depending on the engine intake manifold vacuum and the atmospheric pressure at the time will be between 150 psi and 275 psi1

Theory of regulator operation

Theory of Height Control Valve operation

Theory of shock absorber operation

Repair and maintenance



The normal mode of failure for the regulator is a piece of dirt gets stuck in the valve and the regulator can not regulate the outlet pressure. The air pressure under the piston then goes to reservoir pressure and the piston and associated parts are blown out of the cylinder.

It is important to not manually pressurize the reservoir with an air hose before you do this procedure and clean the regulator. If you have already pressurized the reservoir then do not let the compressor pressurize the reservoir to high pressure.

The following is a description of how to clean the regulator and put it back together after it has self destructed.

If your regulator has a yellow cap on it, it is still intact but you should clean only the reservoir side of the regulator.

In Fig.1 shows the inside of a regulator with the piston and associated parts missing. If yours is intact do not try to take it apart. Just clean what you can.

Fig. 1 Photo by Planet Eldorado

Fig.2 shows what is under the yellow cap. This is also intact and usable. Do not try to take it apart.

Fig. 2 Photo by Planet Eldorado

Fig.3 shows the parts that reside in the bore. The retaining ring is not shown. Your O ring will most likely be red.

Fig. 3 Photo by Planet Eldorado

The first step in cleaning is to remove the screen. You can pry the screen off using a thin bladed screwdriver as shown in Fig.4.

Fig. 4 Photo by Planet Eldorado

Fig.5 shows what you may find inside the regulator. This corrosion can become dislodged when you manually fill the reservoir and can get stuck in the valve. The thing in the center of the bore is the bottom of the Schrader valve.

Fig. 5 Photo by Planet Eldorado

When the corrosion is light clean the inside of this bore with a bamboo skewer or other hard wood. If the corrosion is heavy as in Fig. 5, use sandpaper rolled up to clean the sides. If the piston is out you can remove the Schrader valve from the bottom of the piston bore. Blow out all the dust from the screen bore. Also clean the screen itself. If you don't have a compressor use one of those dusting cans for cleaning keyboards etc. If the piston is still in the regulator blow air through the valve to be sure there is no dust in it. Also clean the passage from the manual fill valve with a 1/16 inch drill. Fig. 8 Blow it out too.

Fig. 6 Photo by Planet Eldorado

Fig. 7 Photo by Planet Eldorado

Fig. 8 Photo by Planet Eldorado

After the inlet bore is clean put some paint on the walls to prevent more corrosion. Apply the paint with a tooth pick or other small stick. Make sure no paint gets into the valve.

Fig. 9 Photo by Planet Eldorado

After the paint dries put it all back together. Us a leather mallet or other soft hammer to install the screen. If the piston etc. is out us a small gear puller to press it into the bore. Fig. 10 shows a gear puller with a small socket to push the retaining ring in. A 1/4" bolt to fill the hole in the back of the socket so the puller screw would not fall into the hole is helpful. Do not push the parts in too deep. You can tap it in with a hammer but again be careful not to press it in too deep. The deeper the retainer is pushed in the higher the regulated air pressure out is. You do not want to exceed 125 PSI.

Fig. 10 Photo by Planet Eldorado

Fig. 11 shows the parts ready to go in. The retainer ring is not shown

Fig. 11 Photo by Planet Eldorado

Height Control Valve

Shock Absorbers

Leak detection

The best way to detect leaks is with the children’s bubble solution Miracle Bubble. Or you can mix up a batch of liquid dish washing soap and water.

Apply with an artist paint brush to all the seams of the compressor and hose connections. You will probably have some bubbles from the application but what you are looking for is new bubbles forming. The smaller the bubbles the smaller the leak.

This detects external leaks. The compressor can also have internal leaks.

To detect an internal leak; with the reservoir filled; disconnect the vacuum hose from the intake manifold and the air supply hose from the air cleaner and put both of them in a glass of water. Keep the ends of the hoses as shallow as possible. Watch for bubbles rising from the hose ends. If there are bubbles rising from the hose ends then you have an internal leak. Usually this leak is caused by the final check valve leaking and a good cleaning will fix that.


1. 1968 Cadillac Shop Manual page 4-3

Created by admin. Last Modification: Monday 03 of February, 2014 23:39:25 MST by Glen995 points .

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