Like many technicians, I was trained to inspect the suspension system, particularly the shock absorbers, by performing a “bounce” test. Is this a relevant test and if so, how should it be properly performed and what should the technician be observing?
Bacarella: Due to the relatively low velocity generated during a “bounce,” or “jounce,” test, it is not an effective way to determine the amount of control a shock or strut is capable of producing. Jouncing the four corners of a vehicle is, however, a good way to try to pinpoint noises originating from suspension components. It also can help a tech locate binding suspension parts and can be used on some electronic suspensions to identify changes in damping characteristics/control under different system settings.
When servicing a MacPherson strut assembly, it is still common practice to disassemble the unit and replace the damper alone. Which other components should be serviced at the same time and why?
Bacarella: Due to the labor and special tools involved in disassembling a strut assembly, it makes sense to inspect and replace any related, worn components. Starting at the top of the assembly is the upper mounting plate, which is designed to isolate road noise and vibration. If it is worn, it might enable noise to be transmitted into the passenger compartment. Closely inspect the rubber section of the mount for wear, cracks and/or signs of excessive movement. Also inspect the metal part of the mount for excessive rust or damaged mounting studs.
A bearing is typically located directly below the mount. The bearing is designed to facilitate smooth steering motion and prevent “memory steer.” If it is damaged, binding or feels “gritty,” it should be replaced. Next, inspect the upper spring seat and isolators. Corrosion and deterioration of the rubber are the major concerns with these items. The coil spring should be checked for significant nicks, kinks and corrosion.
It also is a good idea to check vehicle ride height prior to removing the assembly. Keep in mind that on some lighter vehicles, dynamic ride height issues are more prominent than static ride height issues. A worn compression bumper or boot assembly is a good indicator of a dynamic spring concern. This raises one final issue: If the jounce bumper and boot assembly is excessively worn, cracked or missing, it should be replaced. These items protect the new strut, mount and spring from damage that can result when the suspension bottoms out.
From time to time I have noticed oil stains on the bodies of shock absorbers. Some sources tell us that a little staining is normal, while others say any sign of oil loss is reason for replacement. Which is correct?
Bacarella: If the shocks or struts are functioning correctly, a light oil film covering the top half of the shock body does not usually warrant replacement. This film results when oil used to lubricate the rod gets wiped from the rod as it travels into the shock or strut. Tenneco adds an extra amount of oil to shocks and struts during manufacturing to compensate for this slight loss. On the other hand, fluid actively leaking down the side of a ride control unit indicates a worn or damaged seal, and the unit should be replaced.
It is not uncommon to encounter a worn or broken damper mounting bushing. What is the proper service procedure for this instance if the damper appears otherwise serviceable?
Bacarella: In many cases, new cushions for stem mounts or new bushings for loop mounts are available to replace worn items. To improve durability, many modern dampers feature non-serviceable, high-compression, pressed-in sleeves or bonded assemblies. In these cases a worn bushing would require damper replacement. It is good practice to identify the root cause of bushing failure prior to replacement. Often, a missing suspension stop (jounce bumper) or weak springs can cause premature bushing failure. Other potential causes include shock absorbers that are the incorrect length for the application and contamination of the bushing by engine oil or other chemicals.
