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Servicing Dual-Mass Flywheels an Expensive, but Viable Option
Sunday, June 1, 2008 - 00:00

Servicing Dual-Mass Flywheels an Expensive, but Viable Option

Servicing dual-mass flywheels is a challenge. Resurfacing or rebuilding a dual-mass flywheel (DMF) usually isn't an option. "Sticker shock" frequently results in replacement with a less expensive, solid flywheel. What about inspection and re-use? Here's what we learned from discussions with a number of DMF manufacturers and experts. First of all, why are so many auto manufacturers adopting DMFs?

Dual-mass flywheels offer benefits that manufacturers are convinced consumers want. Reduced noise, vibration and harshness (NVH) are foremost. Manufacturers of light diesel-powered trucks were the first to adopt DMFs. Today's light trucks offer gobs of torque, but consumers prefer they remain relatively smooth, quiet and car-like. A light diesel without a DMF would rattle its gears like a Class 8 truck.

Torque-limiting "slip" features were incorporated into some DMFs to protect less-than sufficiently robust transmissions. Car manufacturers discovered six-, seven-and even eight-speed transmissions in luxury vehicles will produce noticeable "body boom" when drivers loaf along in these gears at low rpms. The lower idle speeds of many of today's engines also produce more vibration. In each case, a DMF can eliminate or greatly reduce the noise, vibration and harshness manufacturers feel consumers would find objectionable.

For example

"The Corvette's ZF S6-40 transmission can produce higher levels of gear rattle than most. During development of this transmission, it was determined that conventional dampening springs in the clutch disc were not sufficient to eliminate this problem. To avoid customer satisfaction problems, GM worked with ZF to develop a DMF for this application," says a ZF spokesperson.

Customers are seldom happy when a dual-mass flywheel fails. Accustomed to solid flywheels with the longevity of anvils, they tend to view any DMF failure as "premature." Porsche owners were some of the first to suffer. The Freudenberg DMF costs $1,000 or more to replace when its elastomeric element turned "to bubble gum," in the words of one import tech. Ford PowerStroke diesel truck owners were next. Today, more than a few Duramax owners can be heard crying "foul" in various Internet forums.

Looking Into DMFs

What causes DMFs to fail? One of the nation's leading clutch experts, Peter Pyfer of South Bend Clutch, states, "If truck owners step out of bounds in any way, it's the dual-mass flywheel that's going to take the hit." This includes hauling more weight than the vehicle was designed for, lugging the vehicle up hills and slipping the clutch more than necessary during vehicle launches, especially on a hill or when hauling a trailer. A more insidious cause of premature DMF failure is a poor-running engine. Misfires can set up harmonics that continually exercise the springs in a DMF, which will subsequently overheat and lose their temper – or melt their plastic guides.

Replacing a DMF with a one-piece disk may make economic sense on older trucks, but there can be issues besides increased gear rattle. Engines on some high performance cars, including Porsches and Corvettes, reportedly will die while returning to idle because of the faster engine deceleration that occurs with a lighter, one-piece flywheel. Correcting this problem may require a chip change or reflash.

Can a DMF be turned if the friction surface is slightly scored or has hot spots? Actually, some can, including the Valeo unit. But other manufacturers nix turning DMFs. Why? An engineer at one firm replies, "Because that would require disassembly. Not only are some DMFs indexed to the crank, components may be internally indexed and the DMF is balanced as a unit."

A representative of yet another DMF manufacturer comments: "Most secondary flywheels in a DMF simply don't include sufficient additional material (meat) that can be removed during a reworking process (turning) because packaging for the DMF inside the transmission bell housing was too tight to begin with."

Engineers at LuK and ZF Sachs agree that replacement of a DMF just because a friction disk is being replaced is often unnecessary, unless there are indications of DMF damage. One comment offered was, "Techs are accustomed to replacing the disc, the pilot and release bearings, and turning the flywheel. When they're not allowed to turn the flywheel, many mistakenly feel they have no choice but to replace it."

Gary Coyle, of Perfection Clutch, cites several reasons to avoid trying to "resurrect" even a "rebuildable" DMF. They include the need to drill out heavy rivets, the difficulty or impossibility of renewing bonded-on friction or elastomeric materials, availability of internal parts and balancing issues. That's not counting individual needle or ball bearings — the Valeo DMF contains more than 128 parts.

Discussions with LuK's DMF engineers is resulting in the production of a document describing DMF inspection criteria, which should eventually appear on LuK's Web site. LuK says it would also offer a toolset necessary to measure the required dimensions of DMFs under inspection.

Inspection

Inspecting and testing DMFs is complicated by the wide variety of DMF designs. Damping can be performed with fluid, springs, elastomers and even air. The DMFs used in some trucks, both Ford and Chevy, have a torque-limiting feature. Using a torque wrench and a homemade adapter, a tech can at least determine if the torque-to-slip feature is working properly. The actual techniques can be found on the Web with a little searching.

Standard flywheel inspection techniques of looking for cracks, damage and overheating still apply. Yet DMFs contain internal parts – springs, and plastic guides – that can't be adequately inspected unless the unit is disassembled. These parts can and do wear out, and are particularly susceptible to damage from overtorque and overheat situations. The loss of fluid would be another sign of failure. Yet DMF damping fluid could easily be confused with engine oil or transmission fluid.

Bearing play can be measured, along with axial endplay, free rotation and runout of the friction-facing surface of the flywheel. Again, specs have proven extremely hard if not impossible to come by. Comparison to a new or replacement DMF is sometimes the only solution available to a tech trying to decide if re-use is justified.

Look, Listen, Feel

Aside from visual and mechanical inspection, DMF failure symptoms and customer complaints should be considered. A "death rattle" on shutdown may indicate a failed DMF. A certain amount of noise is expected as a DMF goes through a "critical rpm" during startup and shutdown. But a "rocks in the tranny" or "marbles in a can" sound is a dead giveaway of a failing DMF. When Ford truck owners allow a failing DMF to go too long, the result is usually a cracked transmission housing. "Chugging" at mid-rpm under light loads is a DMF failure symptom reported by some BMW owners.

Because a DMF is supposed to reduce transmission gear chatter at idle, if there is chatter, this is yet another indication of a possible failure. One Porsche Web site reports several owners having Malfunction Indicator Lamps (MILs) set with codes indicating a misfire, apparently due to a failing DMF. Misfires are determined by crank position sensor readings, and fluctuations can be caused by a bad DMF in addition to actual engine running problems.

"Anything which alters the timing and power delivery of a diesel engine can potentially injure the DMF. Owners who 'chip' their engines for added torque face increased risk of damaging their DMF," says Pyfer. "You need to use a manual gearbox's fifth gear or overdrive just like an automatic transmission would. If you come to a rise, shift down to fourth, or even third."

Too many truck owners simply put the pedal to the floor and overtorque the drivetrain — especially when towing. "Just because you've got gobs of torque doesn't mean the flywheel and clutch can handle all of it, especially after an engine's been chipped for added power," Pyfer adds.

While the Valeo DMF used in Ford 6.9L and 7.3L trucks is the one that has received the worst publicity, in Pyfer's mind, it was in some ways a superior design to what's being used in some newer applications. The Valeo unit, he says, could be disassembled and inspected. If found to be serviceable, it could be machined and reused. "Some dual-mass flywheels are inherently better built than others," he states. "Some use high quality bearings between the two masses, while others utilize cheaper bushings."

Pyfer told us about problems with the DMFs used in Duramax diesels starting to fail that can sometimes result in the clutch pedal sticking to the floor. This occurs through a nasty chain of events that includes flex, the secondary mass moving toward the clutch, pressure being applied to the release bearing and a one-way valve in the slave cylinder refusing to allow fluid to flow back to the master. The solution? DMF replacement.

Balance is another issue in DMF servicing. Nissans, for instance, index the DMF to the crankshaft. A "moaning" or vibration can result on some Fords if the DMF and clutch combination is unbalanced. TSB No. 03-21-19 describes adding washers to the clutch cover bolts to balance it. DMFs used on certain Corvettes include holes for balance weights. Matching the old and new DMF is the recommended procedure.

When replacing a DMF, many manufacturers recommend replacement of the flywheel bolts. Also, be sure to check for indexing before you remove the original unit. Some vehicles require sealant to avoid oil leaks caused by crankshaft bolt holes drilled too deep. Special tools are required to R&R a DMF on Nissans and certain other vehicles.

In the End

DMFs offer NVH reductions that auto and truck makers are convinced consumers want. They're no longer restricted to applications in light diesels. Unlike their bulletproof predecessors, DMFs don't last forever.

The high cost of replacement makes it critical to know when a DMF can be reused, reworked, or needs to be replaced. Manufacturers are just beginning to respond with the tools and information techs need to make good reuse/replace decisions on DMFs. In the meantime, looking and listening for DMF failure symptoms after a vehicle exceeds 50,000 miles may be the best approach.

Web Resources

A DIAGNOSTIC CHART FOR DMFS AND PREMATURE FAILURES: www.standardtransmission.com/dmf.html

A CARQUEST TECH TIP ON INSPECTING TRUCK DMFS www.carquest.com/common/downloads/partsTechDriveTC_2520.pdf

JASPER ENGINES TECH TIPS ON DMFS www.jasperengines.com/newsletter/nljan2005.pdf

DURAMAX DMF TALES OF WOE (SEARCH: DMF) www.dieselplace.com/forum/index.php

Wade Nelson is an electrical engineer who has worked for GM, Motorola and a mobility van conversion firm, and has taught Automotive Electronics at San Juan Community College. He has also written service and wrenched. Continually learning and writing about automotive electronics technology is his passion.

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