Inspecting shock absorbers correctly is actually very simple.
"See how low the car's sitting? And look," he says as he pushed once on the bumper. "It shouldn't bounce at all when you push down on the end of it. The shocks are gone. You need a new car. "
Shocking (no pun intended), isn't it that these myths are still out there? This advice may have been well intentioned, but that's certainly not the correct way of testing shock absorbers. Surprisingly, even though shocks and struts have been around forever, operating in much the same way and doing the same job, they're still misunderstood and misdiagnosed. And that's too bad, because they're really quite simple, and they're essential safety components that can and do wear out.
A typical shock absorber is basically an oil pump placed between the wheels and the frame of the vehicle to absorb and dissipate movement from the springs and keep the tires in contact with the road.Shock absorbers work both when they're being compressed and also when they're extending. During compression, the piston moves downward and compresses the hydraulic fluid in the chamber below the piston, and as the piston extends, it compresses the fluid in the chamber above the piston. The faster the suspension moves, the more resistance the shock absorber provides. There's usually more resistance during the extension cycle than the compression cycle. And incidentally, the parking lot diagnostician was also incorrect about the cause of low vehicle height. Shocks don't control ride height (usually it's the springs that do).
Getting a Good Look
Inspecting shock absorbers correctly is actually very simple. Start by looking for fluid leaks. While some "sweating" on the side of the tube doesn't indicate a problem, oil dripping out is a sign of failure.Also look at the mounts. Check for cracked or worn bushings where the shock absorber is bolted to the vehicle. And check that those mounts haven't broken off completely.
A general bounce test can also work well to inspect shocks, but not the one demonstrated in the parking lot. A typical bounce test involves pushing down hard on a corner of the vehicle, and as a very general rule, the vehicle should "bounce" no more than three times before stopping (always consult the repair manual for the exact procedures).
Look at the vehicle's mileage. Shock absorbers wear out gradually and often go unnoticed. Even though they usually don't have a catastrophic failure, shocks should be considered maintenance items and should be replaced at regular intervals, typically every 50,000 miles. Always look up the recommendation for the exact vehicle you're working on to avoid problems.
The best part about replacing shock absorbers? It's usually pretty easy. Just remove the old shock and install the new one. Just make sure you support the weight of the suspension and refer to any needed precautions by looking it up in your service information. It's usually easier to leave the new shock "compressed" until it's been installed, and there's often a strap on the new part to make this easier. Cutting the band too soon can make installing the new part a bit tricky since the shock's piston will extend abruptly.Of course, make sure the replacement fits through any passages in the body. If the replacement shock is much wider than the original, that may be a problem.
Struts
One additional problem with the parking lot diagnosis of bad shocks was that the vehicle in question has struts, not shocks, at all four wheels like many newer vehicles do.
Struts work in much the same way as shock absorbers – an oil pump dissipating movement and keeping the wheels firmly against the road. But the tube is mounted inside the coil spring, and the assembly provides support for the vehicle. Many front struts have bearings in the top plate that move when the wheels turn – bearings known to cause noise and steering concerns if they fail.
Common customer complaints often traced to problems in the struts include:
- Noise on turns
- Memory steer
- Noises over bumps
- Nose dive on braking.
Struts are checked in much the same way as shocks. The bounce test still applies, and the parking lot inspection was still incorrect – it's the general rule of fewer than three bounces. Also, the replacement interval is the same: typically every 50,000 miles.
Struts are a bit more work to service and replace, because so many components are packaged together. And that coil spring can pack a punch if not properly compressed, but the job can be simple and rewarding with the right tools.
Replacing a strut is usually a straightforward job. The old strut assembly is removed from the vehicle. Then the coil spring is caged or compressed so that it doesn't damage anyone or anything. The center fastener holding the unit together is removed, and the unit comes apart. The components being reused (often the bearing, mount, bushing and spring) are swapped over to the new strut. The new struts are then reassembled and installed in the vehicle, and the vehicle gets a wheel alignment if required.
Electronic Suspension
As with many other vehicle systems, suspension systems often have electronic controls, sensors and actuators to improve efficiency and performance. Some vehicles have crazy-advanced electronic suspension components that adjust the headlights, cameras, torsion bars and other components as the vehicle's height changes as it travels along the road. Other systems simply vary the firmness of the struts or shocks according to road and driving conditions.Servicing these electronic systems can seem intimidating, but surprisingly most electronically controlled suspension components are usually pretty simple to diagnose with the right tools and manuals.
Electronic suspension systems vary among vehicles, ranging from simple air suspensions with a level switch to shocks that electronically change to provide different handling characteristics, so it's important to look up service information specific to the vehicle's exact model and year. Electronic suspension systems often set DTCs that can be read through a scan tool or flashing bulb and diagnosed with service information and wiring diagrams.An important note about electronic systems: Air bags are often used in these systems, inflating and deflating according to a level sensor or selector switch. Some of these air systems are notorious for over-inflating the system when the vehicle is raised, permanently damaging the components. These systems must be disabled before the vehicle's raised. Switching the ignition OFF may not be enough – there's usually a master switch that disables the system.
Common complaints on these systems are that the vehicle is too low (check for leaks and check the height sensor), too high (check the sensor and check the air exhaust passages, or completely inoperative (check the fuse, electronic circuits and TSBs). Good service information will guide you through the complete diagnostic process.A thorough preliminary inspection on electronic suspension systems includes:
- Listening for leaks in the air lines and components.
- Looking for corroded components, wires and terminals (these components are usually exposed to road debris and elements, making them very vulnerable).
- Ride height (some high-end vehicles are incredibly sensitive to ride height; anything that affects this can cause problems in the rest of the system).
- Checking system voltage. If it's low, the system may not operate correctly and will set DTCs.
- Look for signs of damage and new components.
But, of course, always refer to the repair manual for specific directions.
Vanessa Attwell is a Master Technician for two major manufacturers and has also worked on the bench of an independent shop. She has developed and delivered training for both vehicle manufacturers and independents, and helped develop government training and regulations standards.
