Auto Service World
Feature   February 1, 2008   by Jim Anderton, Technical Editor

Bump and Grind

Driveline noise is a common complaint with many causes...not always by the usual suspects

“It clunks when I put it into ‘Drive.'” That’s a common complaint among the many noises used by customers to describe driveline noises that range from annoying to downright scary. By definition, noise is a bad thing, but how do you know you’re chasing the right cause when its driveline related? Repair manuals often use weasel words like “inspect for worn or damaged components,” but a little brain power (plus a little interrogation of the motorist) can speed the process and make you look like a diagnostic genius.

Is it the engine?

Clunking and banging is a sure sign that a vehicle needs service, but what’s the origin of the noise? Internal engine racket like piston slap or preignition/ping are well known to experienced techs, but they’re not to the driver. The road test is essential to a quick diagnosis, but a good tech or service advisor can save a lot of time by framing a careful set of questions for the motorist. The idea is to get the owner to isolate the noises as engine RPM dependent or independent, and then to separate chassis from driveline sources. If the engine has thrown a rod, the task is easy, but what about a backfire that’s muffled by the intake plumbing? In the past, a solid backfire would kill the MAF sensor, creating a no-start or obvious code/driveability issue, so the needed service would automatically correct the noise problem. lem. Today, the sensor would likely survive and with modern adaptive electronics preserve enough driveabilty to keep all but the most sensitive owner from noticing the power drop. Intermittent coil pack failures (especially on waste-fire systems) or ECM issues can cause a hiccup as can issues as simple as fuel line icing or water contaminated gas. It’s important to find out whether the problem is consistent and repeatable or is truly random.

Mounting issues

While randomness strongly suggests a drivability-related issue, the consistent clunk drives most techs toward the rubber parts, specifically the engine/ transmission mounts and “dog bone.” Mount failures used to be catastrophic, with engines and transmissions literally ally falling out of and tearing up the chassis. It became enough of a safety issue that modern mount designs use interlocking mounting plates and strongly bonded rubber so post-failure the driveline stays in place. This also means that a mount can fail while leaving the vehicle drivable. Symptoms here are the classic torque-induced engine mount noise and at the transmission, driveline clunk when going into gear. For front-drivers, torque loads go through the dog bone strut, which is easy to replace.

Inspection works to an extent, but the combination of dirt/oil and rust can make it difficult to determine the condition of rubber bushings. The Laws of Physics still apply: for every action there is an equal and opposite reaction — so revving up the engine in “park” or neutral isn’t an adequate test. Why? Because the torque load delivered to the rubber is only the amount necessary to “push off” the flywheel and the converter’s inertia, which is extremely small. It’s necessary to put the vehicle in gear, both forward and reverse, to put realistic loads on suspect mounts. It goes without saying that this requires another individual to do the “driving” while the lead tech pokes around under the hood. It also goes without saying that the potential to pin a hapless wrench against the shop’s back wall like a dartboard is very real, so some means of restraining the vehicle other than the service or parking brakes is essential. Emissions dynos are safe, but not ideal as the key is to push against solid ground, not a roller. If you can’t chain the vehicle down, do the test outside and stand beside the engine compartment.

If the customer describes the noise as a “click” more than a “clunk,” an often overlooked procedure is the simplest: look inside the vehicle. Is there a ceramic coffee mug under a seat? What’s bouncing around in the trunk? Compact spare tires are usually much smaller than their mounting space, which is designed to take the full-size flat. If it’s not secured properly, especially in side-mount designs on wagons and some minivans, there is a possible “bump” that never goes away.

If the eyeball tests yield nothing, and the powertrain is solid with no codes, it’s time to look downstream. The exhaust system is an obvious place to start, followed by drive shafts and the ring and pinion in rear-wheel/four wheel drive machines. This is under car work, and while under the chassis, it makes sense to look for obvious damage, especially to drive shafts, hangar bearings and axle housings. Modern drive shafts are thin-wall and easily dented. The vibration from a damaged driveshaft can go unnoticed by the motorist, but pound the life out of a U-joint, resulting in a driveline clunk. Obviously, the driveshaft has to be serviced as well as the joint, or the issue will return. Outside of very high-mileage vehicles, differentials and transaxles are rarely the cause, with one exception: work trucks and tow vehicles. A Class 3 or 4 hitch and a brake controller in the cab is a dead giveaway and if your customer is in the trades, they’ve likely unloaded a considerable weight of tools and equipment before dropping off the vehicle. That steamfitter or bricklayer may simply need more than a half-ton for the loads they carry, prematurely wearing out driveline components.

Other Possibilities

If the driveline is solid, the likely culprit is in the suspension. Here the difference between front and rear-wheel drive also matters, again because of the physics. Engineers call it a “moment arm” and like an old bending- beam torque wrench, the drive train delivers torque over a “beam” that’s the length of the vehicle wheelbase. A front driver will compress the rear suspension under load, with minimal front end lift, while the rear driver lifts the front end with little rear squat. If the noise is an acceleration/deceleration phenomenon, then which end you scour first for worn bushings and joints depends on which end is driven: go to the rear for front-drivers and the front for rear/all-wheel drive vehicles. For hard-working, rear-wheel drive pickups and SUV’s with leaf spring rear suspensions, there’s another factor to consider: axle wrap. It’s again about action-reaction physics. Under load, the differential’s pinion attempts to “climb” the ring gear, and unless a tire slips or the vehicle moves forward, the reaction force drives the pinion snout downward, twisting the axle housing. If the leaf springs are long and soft, the OEM has to do something, usually staggering the shocks to counteract the twist. This means that damaged or worn out rear shocks, even a single shock, can also contribute to driveline noise if the leaf springs are torsionally loaded and unloaded with every launch from stopped. Coil spring designs do away with this problem by positively locating the axle by trailing arms and/or radius rods, but that introduces another set of bushings that can contribute to the problem. Add independent rear suspension, especially in a heavy SUV and there are also rear CV joints to consider.

If the driveline looks good, consider sub frames and on full-frame vehicles, body mounts. Today, sub frames are often rubber-isolated so dealing with a noise issue can sometimes be as simple as tightening bolts. Depending on how the vehicle is mounted on the lift, however, it’s possible to artificially load a sub frame with the vehicle weight, masking a loose or broken mount. Ideally, the vehicle’s weight should be on its tires, and the judicious use of a pry bar can be used to check for play, as well as the usual damage inspection. On full-frame vehicles, body mounts can occasionally be an issue.


There are also a couple of rare possibilities that techs won’t see often, but should keep in mind. Full-frame vehicles normally have a considerabl
e amount of frame flexibility and body mounts are normally placed at specially- chosen “nodal points” where flex is minimal for better noise isolation. How flexible? Off-road enthusiasts have been known to relocate pickup beds an inch further back to keep them from denting the rear of the cab. Add a trailer hitch that’s anchored to both the frame and body or box, for example, and the additional stiffness is counteracted by flexing loads trying to break the added connection. Something may break, with noisy consequences. If there’s evidence of frame welding, there are two possible modes of mount damage. One is by burning of the rubber from stray sparks, the easy one to spot, while the other is through secondary arcing across the mount rubber caused by improper grounding of arc or MIG equipment. The current can take multiple paths to ground, including some far from the repair site. Another rare-but-deadly noise maker is broken transmission cases and bell-housings. Anything that lets the engine move relative to the transmission is a sure noise source. Experienced techs will tell amazing stories of vehicles still running with nothing but an input shaft holding the driveline together. A favorite comes from the dreaded DIY customer: drum brakes. Self-energizing drum brakes need correct placement of the leading and trailing shoe, something every first year apprentice learns. If a wannabe tech tries a home job and switches the shoes, they may clamp up in reverse, then release with a bang going into “Drive.” It’s rare, but something to think about. There are many possible causes of driveline noise and as many weird stories about them as there are experienced technicians. Starting with simple “eyeball” checks and then moving systematically under the chassis will almost always deliver a quick, solid repair.

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