Misdiagnosis and manual adjustment still a problem with auto slack adjusters

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Updated Jul 7, 2022
Bendix technician measures brake stroke in a wheel end
“If a wheel-end equipped with an automatic slack adjuster is out of adjustment, Bendix’s first – and most important – piece of advice is this: Do not manually adjust the adjuster. Instead, investigate why the brake is over the stroke limit,” says Keith McComsey, Bendix Spicer Foundation Brake director of marketing and customer solutions – wheel-end.
Bendix Spicer Foundation Brake

[This article was originally published in 2020 by Trucks, Parts, Service. It has been updated to include more timely information.]

They’re called automatic slack adjusters for a reason, and the manufacturers that make them want to keep it that way.

Still, despite years of warnings, automatic slack adjusters (ASA) continue to be manually adjusted by technicians who may not know any better or who may be eager to get a truck out of the shop and back out on the road.

“If a fleet is manually adjusting a slack adjuster regularly it can harm the longevity of the unit,” says Chris Christiansen, warranty/technical services coordinator, Accuride Corporation.

“The reasons behind adjusting an ASA outside of a brake reline should be investigated such as alignments, bushing wear, wheel conditions, etc.,” Christiansen says.

With out-of-service brake violations continuing to top the annual Commercial Vehicle Safety Alliance (CVSA) International Roadcheck, manually adjusting a slack adjuster can not only lead to more brake problems down the road, it can also lead to accidents.

“Despite being standard in the industry for almost two decades, there are still veteran technicians who will regularly put a wrench on an ASA to manually adjust it,” Accuride states in an August safety and performance report for Gunite ASAs. “Overriding the automatic adjustment method can cause premature wear on the internal components and eventually result in an early end-of-life condition. Accident investigations by the National Transportation Safety Board (NTSB) have shown worn and improperly adjusted ASA’s to be a contributing causal factor in some accidents.”

The push to end manual adjustments on ASAs is nothing new.

[RELATED: Accuride improves Gunite automatic slack adjuster warranty]

Following their mandated use on tractor-trailers in 1994 and trailers in 1995, the NTSB issued a scathing report in early 2006 condemning the practice of regularly adjusting ASAs. NTSB ruled an ASA adjustment had led to a runaway truck accident in Pennsylvania in 2003 that claimed the life of the driver and an 11-year-old child riding in a car that the dump truck had struck during its descent on a steep downgrade.

That report, in part, reads: “The drivers and mechanics who manually adjusted the automatic slack adjusters on the trucks involved in the Glen Rock and El Cerrito accidents did not look for underlying problems with the adjusters or related foundation brake components; consequently, they misdiagnosed the brake problems, probably because they were not properly educated on the function and care of automatic slack adjusters and how they relate to foundation brake systems.”

The NTSB went on to write that “warnings in existing materials available to owners, drivers, mechanics and inspectors of air-braked vehicles equipped with automatic slack adjusters have not been successful in communicating the inherent dangers of manually adjusting automatic slack adjusters to correct out-of-adjustment brakes.”

That ruling eventually led to a mandate.

automatic slack adjuster diagramAn automatic slack adjuster (ASA) is a lever between the air brake chamber and the s-cam brake camshaft used to actuate brakes. As its name implies, it is also a device, which takes up the slack or excess clearance caused by lining and drum wear. Its arm is fastened to the push rod of the air brake chamber utilizing a clevis and it is mounted on to the camshaft of the foundation brake by use of a splined gear.
Its primary purpose is to convert linear force of the air brake chamber push rod into a torque which rotates a camshaft forcing the brake shoes against the brake drum. (Source: Accuride)

“NTSB sent a directive to the manufacturers of automatic slack adjusters regarding ASA re-adjustment in August 2007,” says Jason Kraus, senior manager of braking components, Meritor.

According to Kraus, that directive requested a change to service literature on the topic of ASA adjustment.

“ASAs should not be manually adjusted to correct excess brake stroke, as doing so is a dangerous practice when a brake is only out of adjustment or over stroke limitations,” he says. “Excess stroke is an indication of component malfunction that manual adjustment cannot fix. Manual adjustment or de-adjustment shortens ASA life, except Meritor Stroke Sensing ASA due to its unique pull pawl design. A manual adjustment gives drivers false sense that everything is working correctly.”

Keith McComsey, Bendix Commercial Vehicle Systems brake director of marketing and customer solutions, says the name speaks for itself.

[RELATED: Bendix tips for measuring free play and brake stroke]

“Automatic slack adjusters are just that … they should act automatically,” McComsey says. “If a technician feels the need to adjust an automatic slack adjuster, it is because there is some other issue within the drum brake system that should be investigated.

“Automatic slack adjusters should not be manually adjusted in an effort to correct excessive pushrod stroke,” he adds, “because this condition indicates that a problem exists with the automatic adjuster, with the installation of the adjuster, or with related foundation brake components, which manual adjustment will not fix.”

There may be times, however, when manual adjustment is required — like during ASA installation and when brake repairs and overhauls, such as relines, are performed.

“Haldex does not recommend or approve manual adjusting of the automatic brake adjuster except for emergency purposes when the wheel-end brakes cannot be released by air pressure, and when routine brake/wheel-end maintenance is performed like a brake reline,” says Randy Petresh, vice-president of technical services, Haldex Brake Products. “In addition, manual adjustment will mislead diagnosis of wheel-end brake issues during troubleshooting investigations.”

When manual adjustments are necessary, “never use an air impact wrench to adjust an automatic slack adjuster as this can damage the internal mechanism/adjuster,” McComsey says. “Use of wrenches and sockets or ratchets is recommended.”

Keeping brakes in check

Of the 67,072 vehicles inspected in the U.S. and Canada During the 2019 CVSA International Roadcheck, 16,347 were parked because of out-of-service violations. Brake-related issues account for 45 percent of those OOS infractions — making them once again the top cause for inspection failures.

Some of those OOS violations stem from manual adjustments made on automatic slack adjusters, which can lead to ASA failures.

Wabco slack adjuster“With normal vehicle operation, and if the automatic slack adjuster is properly maintained, there is no need to perform manual adjustments outside of initial installment or during brake jobs,” explains Patrick Kealy, ZF’s North America OEM Trailer Business Leader, CVCS Division

“The most common failure of an automatic slack adjuster is when a mechanic or a driver continues to adjust them,” says Patrick Kealy, ZF North America OEM trailer business leader, CVCS division. “They are automatic and therefore typically only need to be adjusted when they are installed for the first time or every time a brake job is performed.”

However, ASAs may be targeted for OOS violations when other brake hardware is actually at fault.

“ASAs often get blamed during CVSA inspections for OOS conditions, but many times the other foundation components are worn down causing overstroke conditions,” says Jon Erickson, ASA product engineer, Gunite. “Changing the ASA in this instance may allow the system to reset enough to be compliant. Replacing an ASA due to an OOS may get your vehicle back on the road but the other foundation components should be studied at the earliest possible convenience to verify the braking system is in acceptable condition.”

ASAs have been in the market long before they were first mandated in 1994. For instance, Haldex rolled out its first U.S. version in 1980.

“The basic design has been very stable ever since that time except for continuous improvement initiated minor changes in material and processing like seal improvement,” Petresh says. “The only significant change occurred about 10 years ago with the design, development and production of a new model, the self-setting automatic brake adjuster.”

Like many other components that hit the market, ASAs were designed to improve safety and lower maintenance requirements for fleets.

“A truck owner had to make daily inspections or adjustments to the brakes before the automatic slack adjuster was implemented on trucks,” says Eric Iott, product specialist, Meritor. “The inspection expense was large and the potential for error in the adjustment was much greater with manual adjustment slacks.”

[RELATED: Meritor offers slack adjuster tech tips]

Regular maintenance and accurate trouble-shooting remains key to a long, safe service-life devoid of OOS infractions.

“Lubrication is a key element in maintaining an automatic slack adjuster to keep it working properly for years to come,” McComsey says. “Contaminants like moisture, etc., work their way into the slack adjuster and may damage them if not regularly greased.  So, cleaning the purge slot on the boot, and adding grease until the old grease is fully purged and you see new grease exiting the purge hole is a requirement.”

McComsey says to refer to “manufacturer’s recommendations for lubrication of the adjuster, noting the suggested mileage, frequency and lubricant grade.”

Meritor automatic slack adjuster“The first Meritor ASA did not have the current pull pawl design and required the technician to unscrew the pawl with a wrench to back off the brakes. Now we have the pull pawl design to dis-engage the mechanism without removing the mechanism,” says Meritor Product Technical Specialist Eric Lott.Meritor

A grade 2 lubricant endorsed by the National Lubricating Grease Institute is recommended for preventive maintenance inspections on Bendix ASAs approximately every three months or 30,000 miles.

And techs should be careful not to over-grease, which Kealy warns can compromise seals and allow contaminants to enter ASAs and increase wear.

Troubleshooting ASAs starts with a stroke test per “the CVSA to verify your brakes are out of adjustment,” Kealy says. “After confirming other components in the foundation brake are working correctly (such as brake shoes, drums, S-Cam, S-Cam bushing, etc.), look at the ASA.

“Back off the brakes to check operation of the ASA,” he adds. “Place a wrench on the adjustment screw and apply the service brakes. You should see the wrench rotate in a clockwise motion which indicates the ASA is taking up the clearance between the drum and shoe. Greasing the S-Cam spline and clevis pin should also be included as part of this maintenance and will help with ASA removal the next time.”

And technicians should always follow manufacturer service requirements and instructions. If the ASA is diagnosed as faulty, replace it with a new one.

“It is not recommended to ‘fix’ an automatic slack adjuster,” McComsey says. “If it is not adjusting properly, or has other damage to it, it should be replaced.”

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