AfterMarketNews AfterMarketNews Auto Care Pro AutoCareCareerHub Brake&Frontend BodyShopBusiness Counterman EngineBuilder Fleet Equipment ImportCar Motorcycle & Powersports News Servicio Automotriz Shop Owner Tire Review Tech Shop Tomorrow's Tech Underhood Service Speedville

Are All Cars ‘Supercars’ Now?

I attended an open house at Smokey’s Dyno in Akron, Ohio, last month. The shop was filled with Lamborghinis, Jaguars and other high-end cars. It was a great chance to look under the hoods of some supercars. The shop even had a rare McLaren P1 sitting...

Read more...

Documenting Inspections: Are You Leaving Maintenance Dollars on the Table?

How do you translate scribbles on a ­repair order into sales? There is no magic trick involved — the key is to document the vehicle ­inspection process. The more you know about your customers’ vehicles, and the more you are able to document...

Read more...

The Invisible Killer Of Brake Systems

Brake fluid maintenance services can be the toughest item to sell. Oil can become dark and transmission fluid can smell funny, but brake fluid in the reservoir can look clear and still be in need of replacement. Brake fluid can become contaminated...

Read more...

Import Automatic Transmission Diagnostics

Don’t be alarmed if you pull an automatic transmission trouble code when diagnosing a “check engine” warning light! Since the automatic transmission operation has a major effect on grams-per-mile exhaust emissions, you’re going to see the...

Read more...

Honda: Vehicle Won’t Move or Barely Moves

A customer brings in a vehicle that won’t move forward, ­­backward or both. Check first to see if it grinds or clicks. And does the speedometer read a lot higher than you’re actually going? Chances are the driveshaft is disengaged. This can...

Read more...

When Timing Is Everything: A Look At The 2.5L, 2.0L Powerplants

Last month, we looked at the timing chain setup on the Audi 3.2L engine and saw how a lack of oil changes can adversely affect the tensioners and cause engine failure. We’ve also seen similar issues with the timing chain setups on the 2.5L 5-cylinder...

Read more...

iATN Exceeds 2 Million Forum Messages

The number of messages in the professional automotive discussion forums of the International Automotive Technicians Network (iATN) exceeded 2 million in early December 2014, with the Shop Management and Technical Discussion forums being the most popular...

Read more...

Diagnosing Starter Misses

Contributing writer Gary Goms was called to a friend’s shop to help with a no-cranking condition on a 2006 Chevy Tahoe. After diagnosing a faulty PCM ground, locating the missing ground proved to be problematic. Find out how Gary solves The Case...

Read more...

Snap-on Adds Diagnostic Calculator To Website

Snap-on announces a new diagnostic calculator feature has been added to its website at http://diagnostics.snapon.com to help automotive repair technicians and shop owners determine how much profit they could be making by using a Snap-on diagnostic platform,...

Read more...

Harmonic Dampeners: Why They Fail, What to Look For

Every time a cylinder fires, the crankshaft speeds up a very small amount for a short period of time. It is then slowed down as the next cylinder in the firing order compresses air and fuel for the next combustion event. The speeding up and slowing...

Read more...

Be Careful When Replacing That Diesel Air Filter

Modern passenger and light-duty diesels with ECMs have mass airflow sensors and precisely control fuel ­delivery. Key to the efficiency and lifespan of the diesel engine is the diesel air filter. A restricted air filter will not affect fuel economy but...

Read more...

Replacing a Faulty Ignition Coil

After a faulty ignition coil is diagnosed and replacement is determined, best practices are essential. Here is a list of tech tips when performing ignition coil replacements. • Coils shouldn’t have any visible damage and the boot should be...

Read more...

Home Brakes Causes & Cures for Disc Brake Problems

Print Print Email Email

There are as many causes as there are cures for disc brake problems. Brake pedal pulsation tops the list, while disc brake noise and brake pedal sensitivity follow as second and third place contenders. Most of these problems can be avoided if technicians have the right parts and follow the correct procedures. Let’s begin by looking at two leading causes of brake pedal pulsation: rotor warpage and uneven rotor wear.

ROTOR WARPAGE
By now, most technicians know that overtorquing lug nuts causes brake pedal pulsation by distorting or warping the brake rotors. Of course, if overtorqued lug nuts were the whole story of disc brake diagnosis, I could close this column and take the rest of the day off! Unfortunately, less obvious faults can cause pedal pulsation, especially on that fresh new brake job. Although some vehicles are very sensitive to over-torquing, other vehicles are relatively immune.

SETTING UP THE LATHE
Inaccurate lathe setups play a major role in creating uneven rotor wear. If you can visualize a brake rotor “wobbling” between the pads mounted in a brake caliper, it’s not hard to also visualize that, each time the high point of the rotor contacts the pad, some metal is worn away from the rotor.

Eventually, the high point of the rotor’s friction surface is worn off, which ruins the parallelism between the inner and outer rotor surfaces. It is important to make sure you are using the correct settings.

WHEEL BEARING ADJUSTMENT
Loose wheel bearings will cause the rotor to tilt in the caliper when a load or side thrust is placed on the bearings. Disc brake pistons require lots of fluid volume and pressure to push the pad against the rotor. If loosely adjusted wheel bearings force the pistons into the caliper, the result will be a low or spongy brake pedal.

Conventional wheel bearings must be adjusted with a slight amount of preload in the bearings. Excessive preload, however, will cause the bearings to overheat. It’s best to follow specified adjustment procedures from an applicable shop manual.

MOUNTING THE BRAKE PADS
As mentioned above, disc brake squeal occurs due to a high-frequency vibration between the brake pad and rotor. Following correct pad mounting procedures can prevent the majority of brake squeal.

When installing the pads, it’s important to use new mounting hardware. Most mounting hardware is made from soft materials like rubber or spring steel, which is designed to acoustically insulate the caliper from the mounting bracket and wheel spindle. These parts lose their soundproofing qualities as they age.

Second, the caliper guide pins or slides must be cleaned and lubricated with a recommended high-temperature brake part lubricant to prevent corrosion or sticking. Free-floating calipers relieve the tendency of pads to rub or bind against the brake rotor and, thus, reduce noise.

Pad shims must be reinstalled if they were originally used. Too often, these shims are discarded with the old pads while the brake tech unwittingly installs the new pads – minus the required shims.

To prevent shim loss, many manufacturers are now attaching adhesive shims to the backing plates of the new pads. Other pads require clip-on shims. It’s important not to attach clip-on shims to the backing plate with an adhesive noise-silencing compound. Adhesive destroys the noise-insulating properties of the shim. In many cases, manufacturers actually recommend that these shims be lubricated with high-temperature silicone grease. Last, many pads are made with mounting tabs designed to hold the pad tightly in the caliper. Taking the time to crimp these tabs into the caliper will reduce the possibility of pad vibration and brake squeal.

DISC BRAKE NOISE
Disc brake squeal is perhaps the most difficult problem to eliminate on most vehicles because it’s aggravated by uncontrollable factors including weather changes and the design of the brake itself.

The squeal is caused by the pad vibrating at a high frequency as it engages the rotor. The type of material used in the pad and the rotor may further aggravate the squeal. High-performance racing-type pads, for example, are designed for maximum stopping power and durability, with noise factors being the least part of the design criteria. Many early European vehicles are particularly offensive in this regard because a premium is placed upon pad performance rather than noise suppression.

ROTOR FINISH
The issue of applying a non-directional or swirl finish to a freshly turned rotor is complicated by the fact that, while brake pad manufacturers recommend a sanded swirl finish, most deliver their new rotors with a smooth, cut finish. There’s a reason for this apparent dichotomy: Brake manufacturers know that many technicians operate rotor lathes at maximum cutting and feed rates, which cause the metal to be folded or peeled over instead of cleanly cut away.

Microscopically, the surface of the rotor looks like a freshly plowed field. These metal furrows are then torn away by the new brake pad and become imbedded in the lining. The result is a metal-to-metal contact effect between pad and rotor that theoretically causes brake squeal.

Although several types of sanding and buffing pads are now being marketed to help remove this folded metal, no amount of sanding or honing can remedy the damage caused by a dull lathe bit or by fast, deep cutting. Dull bits leave a pitted finish on the rotor. Likewise, excessively fast feed speeds or cutting depths tend to tear away, rather than smoothly cut.

The other problem caused by using a fast cutting feed for the finish cut is cutting a “record player” groove in the rotor. This spiral groove causes the pad to vibrate against the caliper, which, according to theory, aggravates disc brake squeal.

To summarize, the rotor should have a smooth, non-directional finish. Some technicians test the finish by scribing a line on the rotor with a ballpoint pen. If the ink line is smooth and continuous, the rotor is finished correctly.

Last, it’s always a good idea to scrub loose particles from the rotor by using brake parts cleaner and a clean cloth. Before mounting the brake rotor, make sure that the reluctor or tone ring for the anti-lock brakes is free of grease and metal chips. Dirty tone rings can cause faults in the anti-lock system.

BRAKE PEDAL SENSITIVITY
Occasionally, a customer may complain that “the brakes just don’t feel the same as before.” In other words, the new brakes may not feel as sensitive as the old ones. Keep in mind that loose wheel bearings will cause sponginess in disc brakes.

Second, many rear disc brakes incorporate a park brake mechanism that adjusts itself each time the park brake is applied. If the driver doesn’t occasionally use the park brake, excess clearance develops between the rear disc pads and the rotors, which causes a low brake pedal. Some of these mechanisms can also become corroded through lack of use, which complicates the repair.

Last, anti-lock braking systems require special brake bleeding procedures. In some cases, an interactive computer scanner is required to flush air out of the system. If the new brakes “don’t feel the same,” one or more of these factors may be responsible.

CORRECTLY RESURFACING ROTORS
Unfortunately, many repair shops don’t use the correct mounting fixtures or use the correct procedures to correctly resurface rotors. Many technicians use brake drum fixtures, for example, to mount rotors on the brake lathe. These old-style fixtures simply lack the accuracy to align the rotor at a perfect perpendicular angle with the brake lathe arbor.

Other times, brake techs don’t make sure the rotor is mounted correctly on the lathe. Before mounting the rotor, the chips and grease should be cleaned from the mounting fixtures and arbor. The fixtures should also be free of nicks and burrs. If a hub-type rotor, on which the fixtures bear against the wheel bearing races is being turned, the bearing races must be pressed securely into the hub, and be free of chips and debris.

If a hubless rotor is being turned, the rust should be buffed from the lug bolt surfaces of the rotor with an abrasive buffing pad. It also pays to use mounting fixtures specially designed to mount hubless or stamped-steel hub composite rotors. These fixtures offer optimum accuracy and have enough mass to significantly reduce lathe vibration.

The last step after mounting a rotor to a lathe is to check the accuracy of the set-up. The quickest procedure is to start the lathe, take a light scratch cut on the rotor with the cutting bit, and then shut the lathe off. The arbor nut should then be loosened and the rotor or rotor/fixture assembly rotated one-half turn.

After the arbor nut is retightened, the lathe should be started and another scratch cut taken. If the lathe and set-up are accurate, the scratch cuts should occur at the same place on the rotor. If not, the set-up or fixtures are incorrect for the application.

The following two tabs change content below.

Gary Goms

Gary Goms is a former educator and shop owner who remains active in the aftermarket service industry. Gary is an ASE-certified Master Automobile Technician (CMAT) and has earned the L1 advanced engine performance certification. He also belongs to the Automotive Service Association (ASA) and the Society of Automotive Engineers (SAE).
Latest articles from our other sites:

Brake Parts Inc Raises Over $150,000 For United Way

Brake Parts Inc (BPI) raised more than $150,000 for the United Way of Greater McHenry County during its annual campaign drive that ran from early October through the end of November, it was announced recently...More

TPMS Service Tip: Ask the Right Questions

If there is one piece of major advice for any tire tech facing a TPMS issue, it would be this: Test before you touch, and document the answers you get. Understanding the potential TPMS land mines can...More

Import Automatic Transmission Diagnostics

Don’t be alarmed if you pull an automatic transmission trouble code when diagnosing a “check engine” warning light! Since the automatic transmission operation has a major effect on grams-per-mile...More

Infiniti: Removal of a Seized Exhaust Sensor

If an exhaust sensor is seized in the exhaust manifold/catalyst/front tube, perform the service procedure to remove the sensor and prevent unnecessary replacement of the exhaust manifold/catalyst/front...More

iATN Exceeds 2 Million Forum Messages

The number of messages in the professional automotive discussion forums of the International Automotive Technicians Network (iATN) exceeded 2 million in early December 2014, with the Shop Management and...More

Loosen Seized Fasteners with Lisle’s Small Fastener Remover

Use Lisle’s Small Fastener Remover (60530) with a pneumatic impact tool to loosen rusted or seized fasteners. A 3/4" open end wrench can be used to turn the socket while impacting the fastener. A...More

Harmonic Dampeners: Why They Fail, What to Look For

Every time a cylinder fires, the crankshaft speeds up a very small amount for a short period of time. It is then slowed down as the next cylinder in the firing order compresses air and fuel for the next...More

Be Careful When Replacing That Diesel Air Filter

Modern passenger and light-duty diesels with ECMs have mass airflow sensors and precisely control fuel ­delivery. Key to the efficiency and lifespan of the diesel engine is the diesel air filter. A restricted...More