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Commercial vehicle clutch care

June 18, 2021
Spec’ing, service, and repair considerations to keep manual and AMT clutches in optimal condition.

Before the early 2000s, a heavy duty commercial vehicle with anything other than a manual transmission was almost unheard of. Since then, demand for automated manual transmissions (AMTs) has increased; gradually at first, then more quickly as the technology became smoother and more reliable.

Fleets began to see the benefits of using AMTs: they could be more efficient than a manual, plus new, less experienced drivers could operate them, widening the pool of viable candidates – a relief fleets are happy to embrace considering the driver shortage.

Today, AMTs are spec’d on the majority of new commercial vehicles rolling off the factory line. Despite this trend, however, manual transmission trucks are still desirable to many fleets.

“We still see demand for manual transmissions from fleets with experienced driver pools and in specific applications like heavy haul or severe service,” said Becky Parsons, global clutch product strategy manager at Eaton Vehicle Group.

To determine whether manual transmission or AMT trucks are the best fit, fleets need to take a variety of factors into consideration.

Transmission spec’ing basics

Manual transmissions may still make more sense for some fleets. This can be the case for fleets with a large pool of experienced drivers who prefer to drive a truck equipped with a clutch pedal rather than an AMT, or fleets whose vehicles are used in more extreme duty cycles.

Applications the truck will be used in is one of the most important considerations when spec’ing the transmission. This can be especially critical since some AMTs are limited in how much torque they can handle.

“Customers need to look at what application the trucks will go into,” said Chad Seth, director of truck sales at Rihm Kenworth. “Specifically, a truck that pulls an end-dump or side-dump may need a specific type of AMT or go to a fully [automatic] Allison transmission due to the low gearing needed. Some AMTs only have limited torque capacities.”

Where an AMT may only be able to accommodate a maximum torque rating of 1,850 ft-lbs, an 18-speed manual transmission could handle up to 2,050 ft-lbs, Seth said.

The majority of fleets without extreme torque needs, however, can expect to see improvements in almost every efficiency metric by making the switch from manual transmissions to AMTs. This is due mostly to the fact that drivers – experienced or not – can more easily take advantage of the features AMTs offer to improve efficiency.

“The biggest difference between AMTs and manuals is that with AMTs, more features and functions can be provided by the truck OEM in critical areas of response, performance, and fuel efficiency,” noted Johan Agebrand, product marketing director at Volvo Trucks North America.

With proper training, fleets can expect more of their drivers to utilize the AMT features and, therefore, improve performance, he added.

“The top 5 to 10 percent of manual drivers might not improve drastically with an AMT, but the other 90 to 95 percent certainly will,” Agebrand said.

He also noted that Volvo offers a remote programming service which allows fleets to change parameter settings on Volvo AMTs to optimize the truck for a specific application.

“This makes sense if a fleet’s assignments vary in weight, topography, and other factors,” Agebrand explained. “Each truck can be tailored to deliver optimum fuel efficiency, productivity, and drivability based on its assignment.”

In cases where a truck will maintain a set application for the majority of its life, the fleet can choose to spec the transmission for that specific duty cycle.

“If your vehicle is being run OTR (over the road), make sure your AMT is spec'd for the road speeds it will encounter,” said Len Copeland, Detroit product marketing manager. “Likewise, if you are a heavy haul application make sure the transmission selected is designed to handle the weights and operating conditions it will encounter. Selecting the right AMT is important for fleet operations as well as driver satisfaction.”

Some vocational AMTs, for example, offer unique features specific to the vocational market such as off-road mode and rock-free mode.

While the majority of new commercial vehicles being delivered are spec’d with AMTs, there are still many trucks with manual transmissions on the road today. Plus, it is important to account for older truck models still being operated by fleets. That means fleet technicians need to be trained on maintaining and servicing both AMTs and manual transmissions for the foreseeable future.

While there are apparent differences between the two, AMTs and manuals are more similar than they may first appear. One of the most important similarities that should be a key focus for fleet maintenance departments is the clutch.

Clutch operation

For a driver operating a truck with a manual transmission or an AMT, the difference is obvious: one has a clutch pedal and the other does not. Similar to driving a truck with a fully automatic transmission, an AMT requires only the use of the accelerator and brake pedals. That is where the similarity ends, however; AMTs are more similar to a manual than an automatic transmission in how they operate mechanically.

According to Allison Transmission, an automatic transmission utilizes a hydraulic torque converter to couple the engine to a planetary-geared transmission. An AMT, on the other hand, uses a standard manual gearbox with either electrical or pneumatic clutch actuation.

In other words, an automatic transmission in a truck, like an automatic transmission in a passenger car, provides steady, near-constant acceleration. An AMT, on the other hand, shifts like a standard manual transmission truck where acceleration pauses while it shifts.

“Mechanically, there is no difference [between manuals and AMTs],” Volvo’s Agebrand explained. “However, in an AMT, the vehicle’s computers operate the clutch versus the human driver in a manual. The algorithms in today’s AMTs are so good that when it comes to the actual shift and impact on the clutch, the AMT will perform a more technically perfect shift 100 percent of the time.”

AMTs utilize a control module along with a mechanical or pneumatic switch to actuate the clutch itself, Detroit’s Copeland said. This allows the transmission to shift gears as efficiently as possible. Some AMTs, he added, offer multiple driving modes to help the driver stay in control of how the transmission shifts.

Shifting more efficiently to always be in the correct gear as required means AMTs shift more frequently. Manufacturers have taken this into consideration by using different clutch materials than what would traditionally be used for a manual transmission clutch.

“Manual clutches are made of a semi-metallic composition while AMTs use an organic fiber material because they shift more frequently based on the different shifting strategies, which require a smoother clutch engagement and disengagement than a metallic clutch can offer,” Copeland said.

In order to further protect itself, an AMT system also includes a clutch temperature estimator, according to Eaton’s Parsons, which it uses to avoid excessive wear on the clutch and related components.

Manual transmissions tend to have fewer sensors and rely more on the driver to avoid overheating the clutch and creating excessive wear.

Avoiding a clutch catastrophe

Misuse and improper maintenance are two of the leading causes of clutch failure on a manual transmission. AMTs remove direct human interaction, so misuse is less of an issue. Improper maintenance, however, can still lead to clutch failure even on an AMT.

Not properly maintaining a vehicle’s clutch can lead to slippage, hard pedal actuation (for manual transmissions), excess heat, and premature failure, noted Steve Toninato, director of service at Rihm Kenworth.

Clutch failure due to misuse or improper maintenance is, however, somewhat by design. The clutch in both manual transmissions and AMTs is designed to be a weak point in the system that will wear out or break first in order to protect the transmission itself.

“Improper maintenance or vehicle operation can result in the clutch wearing down and breaking,” said Volvo’s Agebrand. “In prolonged or extreme cases, the clutch is designed to fail; otherwise, the actual gears and gearing shafts inside the transmission will break. This is the same for both AMTs and manuals.”

Detroit’s Copeland noted that maintenance is critical since transmission issues can lead to wider spread vehicle problems.

“Improperly maintained transmissions can lead to cascading failures throughout the powertrain,” Copeland noted. “Fleets can experience decreased service life, [decreased] operating performance, increased clutch failures, and severe overheating. Always refer to your OEM maintenance manual to make sure you are maintaining your equipment as recommended to help prevent these issues.”

Warning signs

The first step to avoiding a clutch failure is to be on the lookout for signs of imminent trouble. Drivers and technicians alike should watch for clues that the transmission is stressed. Manual transmissions and AMTs provide different warnings, so it is important to be well versed in both.

“With manual transmissions, drivers experience a loss of ‘feel’ in the clutch pedal or may hear grinding and experience problems completing the gear shift,” Agebrand said.

Toninato noted slippage can be another obvious indicator of a clutch issue, while Copeland said drivers should be aware of the clutch’s heat – a factor that can often be made apparent by a burning smell, which applies to both manual transmissions and AMTs.

Copeland also noted that most AMTs are equipped with a clutch temperature sensor that will notify the driver when the clutch is too hot for safe operation. Additionally, if the transmission fails to shift into a gear smoothly when driving, it is a likely indicator of a clutch issue.

“AMTs provide warnings through fault codes and alerts to the driver to indicate the need to stop and inspect the clutch,” Parsons added.

She noted if the driver receives a warning or alert while operating the vehicle, the driver should stop the vehicle to inspect the clutch.

Diagnosis and repair

Diagnosing clutch issues can also vary somewhat between manual transmissions and AMTs. For manual transmissions, clutch pedal feel and visual inspection are the main diagnostic methods. AMTs, with their additional sensors, will typically offer alerts and fault codes and can be more readily diagnosed with a diagnostic scan tool and software.

When it comes to repair, it is imperative to use high quality replacement parts for both types of transmissions. While manufacturers tend to recommend OEM brand parts, fleets are strongly advised to at least use OEM-approved parts.

“Detroit strongly recommends using factory-approved parts for all service and maintenance on our products,” Copeland said. “Factory-approved parts are typically the same parts that were installed in the initial build of the component. This means the parts have the same stringent testing and quality assurance controls as our factory built parts.”

“You get what you pay for,” Toninato added. Higher priced parts tend to provide better materials and better warranty coverage, making it worth spending extra up front to avoid potential issues in the future.

For AMTs, it is extremely important to use a clutch that has been integrated with the system to ensure performance, Parsons said. This can be seen in terms of driveability, noise/vibration/harshness (NVH), comfort, and life of the clutch.

She added that for manual transmissions, using the correct clutch for the application ensures long life, optimal driveability and performance, and smooth engagement.

“For both applications, if the correct clutch is not used the overall driveline is at risk due to excessive torsional vibrations,” Parsons said. “This means the overall life of the driveline would be at risk.”

Best practices for clutch maintenance

Clutch issues can be avoided, for the most part, by implementing proper preventive maintenance practices. In all cases, it is crucial to follow manufacturer recommended maintenance intervals.

“First and foremost, always follow all published maintenance intervals for your transmission,” Copeland stressed. “Periodically review all operating parameters of a few units within your fleet to help tailor your maintenance intervals to your vehicle operations. Clutches need adjustments as they wear; some clutches require manual adjustments as they wear, and others have self-adjusting clutches.”

The self-adjusting function – in transmissions equipped with that feature – allows the clutch to always remain in prime adjustment, he added.

In addition to regular adjustment, clutches require regular lubrication.

“For both types of clutches, fleets need to maintain proper lubrication of the release system to avoid release bearing wear,” Parsons explained.

Finally, both types of transmissions require the transmission fluid to be changed at regular intervals. Again, fleets should refer to the transmission manual for recommended intervals.

While the component is designed to wear and may eventually require replacement, following these best practices can help fleets get the maximum life from their manual transmission and AMT clutches.

About the Author

David Brierley | Editor | Fleet Maintenance

David Brierley is a former editor of Fleet Maintenance magazine.

Brierley’s education and career have been based in the publishing industry. He is an award-winning writer and comes from a background in automotive, trucking, and heavy equipment. Brierley joined the Endeavor Business Media vehicle repair group in 2017 as managing editor for Fleet Maintenance, PTEN, and Professional Distributor magazines, as well as VehicleServicePros.com. In his current role, he writes for and oversees production of Fleet Maintenance magazine. He has worked in the publishing industry since 2011.

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