Air Suspension Inoperative – Check these first


Driving a car with faulty air suspension out of an underground car park and a steep ramp is quite the challenge. My old girl lay down and simply refused to get up. As it turned out the fix was simple – blown fuse.

The top six reasons air suspension doesn’t work, include:

  1. Blown fuse
  2. Bad relay
  3. Faulty compressor
  4. Air leak
  5. Faulty wiring
  6. Faulty height sensor

In this post, you’ll learn the top causes of an inoperative air suspension message, how to diagnose your suspension problem, and also how to fix it.

Air Suspension Overview

Air suspension info-graphic

Your vehicle’s air suspension isn’t as complex as you might imagine. At its most basic the system comprises of:

  • Air bags (air-springs) – An air bag with integrated solenoid fitted at each wheel stores air to cushion suspension travel
  • Height adjustment sensors – Fitted between the chassis and suspension to measure ride height
  • Air reservoir and control valves – Central manifold where the pressurised air is stored and distributed to each air-spring as needed
  • Air compressor – Builds the pressure needed to run the whole show
  • Pipework – Plumbing to move the air to where it’s needed
  • Control module – The brains behind it all, without the control module (computer) nothing happens

More sophisticated high-end cars may have a few additional features such as auto headlight adjusting, accelerator sensors front and back but such features if faulty are not going to shut down your air system.

So how does it work?

This is only a basic description of the system but it does help to have a mental model of the flow of commands.

Car is started after the overnight park – the air suspension control module ((ASCM) Computer) wakes up and checks the air pressure in the reservoir. Seeing pressure lower than optimum, it powers the compressor until the sensor sees reservoir pressure at about 15bar (218 psi). The ride height sensors then input their positions to the ASCM, making a few calculations it seems that the air-springs need adjustment.

The ASCM commands airbags to inflate by opening their respective solenoids. The reservoir will continue to fill the bags until the height sensors indicate the desired height has been achieved. This process is continuous as long as the vehicle is running and of course, happens at light speed.

This is a simplified explanation but it’s enough information to form a mental model.

How To Diagnose

S class Mercedes

I begin my diagnosis by verifying the fault. I’ll pay special attention and listen carefully for compressor activation. I proceed to do a quick visual of the system, if possible (lots of latest-gen cars keep all the components concealed behind splash guards). I’ll then proceed to scan the computers for fault codes.

This really shortens the diagnostic process as the DTC fault codes may point you to the root cause of the fault. I say may because it doesn’t always, some detective work may be required.

If a fault code is present, I’ll check for recalls or know issues specific to this model vehicle and the fault code. This may show you the exact point of failure and allows you to direct your firepower.

A good scan tool does more than reading codes, it also has live data reading and activation. It allows you to see what the ASCM sees, you can read power supply voltage, system pressure solenoid activation, etc.

The tool will allow the tech to control functions such as run the compressors and open valves without removing any cover panels or referencing wiring diagrams. It’s a super-efficient way to fault find, but it’s not the only way.

I’ll assume you don’t have access to any of these tools and so we’ll tackle this old school, we’ll begin with the most likely causes.

1 Blown Fuse

CAR FUSE

I like to check all the low-hanging fruit first, it’s the best use of your time I’ll bet you’ve got a ton of better things to be taking care of. A blown fuse is the most common cause of inactive air suspension and could be a really simple fix.

I say could be because as you know a fuse is there to protect the circuit from the dangers of excessive amp draw. Excessive draw commonly happens when a wiring shorts out or a compressor pump fails or is starting to fail.

Finding a blown fuse and replacing it may only fix the problem for a short while if at all. But don’t worry we’ll get to that a little later.

Your system will likely have more than one fuse to run the system. A fuse for the compressor and a separate fuse for the ASCM. The compressor fuse is more likely to be the issue.

For now, go ahead and locate both fuses and check them. Their location can be found in the driver’s manual.

Fuse chart

2 Bad Relay

Relay

A relay is designed with a load side and a control side. The circuits are independent and allow the control side circuit (typically low amp) to control the load side (high amp) without risking interior switches or sensitive control module circuits.

The compressor is controlled by a relay, check your fuse box for a listing.  To check it go ahead and remove and check the pins for corrosion, damage, water, etc. Give it a shake and listen for a loose armature. Very often your fuse box will contain many identical relays. Try swapping out the relay for a similar relay, but be sure the pins match.

Follow the info-graphic above to hotwire the relay and check for continuity. Or use a relay tester which you can check out here on the Auto electrical repair tools page.

Relay testing

3 Faulty Compressor

The compressor builds all the air needed to run the system. It works hard you know that, and so you won’t be surprised to know that it’s one of the top failures on an air ride system. Checking the compressor to access can be difficult on some models, so I’ll show you a couple of shortcuts for testing. These tests aren’t foolproof but they are a valuable time saver when attempting to verify compressor health.

For this test, you’ll need a four-inch jumper wire. This test involves hot wiring the compressor, if it runs we call it good and if it doesn’t we’ve narrowed down the field of likely faults.

Hot wiring the pump

Car fuse box

Begin by locating and removing the compressor relay, the vehicle may need to be running for this test. THIS IS IMPORTANT – Jumping the “LOAD SIDE” of the relay should cause the pump to operate (CAUTION – jumping the control side will damage the control unit).

The relay pins are numbered, match them to the relay board pin receivers. The load side of the relay is 30 (3) and 87 (5), they are positioned opposite each other. A power probe makes this easy and you can check out the power probe I use on the Auto electrical repair tools page.

Hot wire starter motor

If the compressor is now running, your pump is likely good and your problem could be a fault with control module inputs, wiring, control module, or faulty relay connector. More on them below.

If the pump didn’t start, it is very likely the pump is faulty, but wow, pumps aren’t cheap so we’ll need to check it.

We could perform a resistance test on the relay load side and it may show us an open circuit or high resistance, but that information is only somewhat useful as we couldn’t be sure it wasn’t caused by bad wiring between the relay box and the pump.

We’ll need to access the pump for testing. More on that later.

4 Air Leak

Air suspension reservoir

Air pressure is obviously super important. The compressor pump works hard, if your system develops a leak (common) it’ll work a lot harder. If the pressure sensor detects an inability of the pump to build pressure, it will recognize it as a leak and shut the system down.

Common air suspension leaks are from pipe fittings, damaged lines, damaged reservoirs, and split bags. Check these areas first for signs of damage, repair any leaks and test the system again. If the system operates use soapy water to test for other leaks, older systems will likely have a few weak areas.

5 Faulty Wiring

Air suspension wiring info-graphic

As most of the wiring for the air suspension is located under the car, disconnection, corrosion, chaffing, loose connections, etc. are all common. Many times simply looking under the car and plugging in a loose connector fixes the problem. That’s why a visual before running any other tests may payoff.

Having run the above checks, you’ll now have some idea of which part of the air ride system your fault lies. Wiring issues are often the most difficult to find. By far a systematic approach is best, looking for random wiring is soul-destroying. 

Attack the big components first systematically, remove the block connectors and check for damage, water, bent pins, loose terminals, or fretting. Important block connectors use a cam-lock mechanism, be sure these are snapped closed.

All connectors should click when in place. External connectors use a weather pack seal to be sure it’s in place. Lubing the seal and the surface of the connector with dielectric grease helps keep moisture out and helps to seat the connector correctly.

CAN network info-graphic

Control modules are super sensitive to high resistance -loose/dirty/broken connections. All control communicate with each other by using precise high and low voltage it’s known as CAN communication (Controller Area Network) not unlike Morse code.

Any interference will cause them to go offline.

6 Height Sensors

The height sensors are important too, they relay the chassis height to the ASCM and so it knows where the chassis is and where it needs to be. Take this information away and the system is lost.

The height sensors live on the chassis close to the suspension and they measure suspension travel by using a drop link connected to a sensor. As the suspension height changes the lever simply moves the sensor arm and the ASCM is updated with the new height.

Check first that the arm and connector are in place, check the connector for damage water, corrosion, etc.

Faulty Pressure sensor

Pressure is mission-critical info for the ASCM, if it can’t read the air pressure in the reservoir, it won’t know to turn on or off the compressor pump. So the default position is to turn the whole system off. The sensor on some solenoids blocks is integral and if it is faulty you’ll need to replace the whole unit.

We can test the sensor by measuring its ref voltage feed with the engine running and reading it’s output, use an external household hobby type compressor to fill the reservoir through the Schrader valve service port. This will allow you to check for differences in the pressure sensor output.

Faulty Control Unit

Control Module

Left this until last because it’s the least likely and it’s expensive. Using the scan tool obviously will make life a ton easier. No communication with the controller is a symptom of failure, however, a CAN wiring issue or another fault controller could also be the root cause. Any Uxxxx code may cause other modules to go offline.

Check out this post, it covers no communication testing “OBD won’t clear codes”.

As a mechanic, I’ve been trained not to concern myself with the inner workings of a control module. Verifying, a good power source, ground, and solid inputs. The outputs can be measured, no outputs mean the controller is faulty.

Air suspension wiring diagram

But you do need to exercise caution. It’s not uncommon for even the pros to misunderstand how a system works. If a crucial input is missing and required, the system won’t work. That can lead to misdiagnoses of expensive components.

Related Questions

What’s the most common problem with air suspension? The most common issues with air ride suspension are air leaks followed by a faulty compressor.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty-five years, and I've worked for GM, Volvo, Volkswagen, Landrover, and Jaguar dealerships. My passion is cars. I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of car ownership, including buying advice, maintenance, and troubleshooting.

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