Home, Automotive Repair Library, Auto Parts, Accessories, Tools & Equipment, Manuals & Books, Car BLOG, Links, Index
Pro-Cut International sponsored a gathering of brake experts from around the industry to talk about a variety of brake service and repair issues. Attendees include brake engineers from DaimlerChrysler, Ford and General Motors, several suppliers of brake service tools, representatives from a variety of different types of service outlets (retailers like PepBoys, tire chains, muffler shops, etc.), training instructors, people from AAA and the Motorist Assurance Program (MAP), plus members of the media.
The purpose of the symposium was to share knowledge and real world experience with our readers so you can better understand some of the brake repair issues on late model vehicles.
The hottest topics for discussion were:
* Brake rotors -- Should they or shouldn't they be resurfaced when doing a brake job? If so, what's the best way to resurface rotors?
* Brake fluid -- What do the OEMs say about inspecting and changing brake fluid for preventive maintenance?
* Calipers -- Should calipers be rebuilt or replaced when doing a brake job?
* Changes in OEM brake technology that are extending the service life of the brake system.
ABS is currently available on all Chrysler/Dodge/Jeep products, but is standard only on the Jeep Grand Cherokee. On everything else, it's an extra cost option. Traction control is available on Chrysler cars only, but is coming to Dodge and Jeep trucks and SUVs. Stability Control (ESP) is standard on Mercedes only, but will be added to future Chrysler products as the OEM supplier (Bosch) develops the system for these applications.
Electro-Hydraulic Braking (EHB) is a "brake-by-wire" system that eliminates the physical connection between the brake pedal and brake hydraulics. The brake booster and master cylinder are replaced with a "pedal simulator" and failsafe hydraulic manifold to actuate the brakes through the EHB unit. Chrysler began offering brake-by-wire back in 2002 on certain luxury models. Further down the road, electro-mechanical braking that uses no hydraulics at all may find its way onto vehicles.
Mercedes has been using brake rotors made of ceramics on some of its models (such as the Porsche 91). The ceramic rotor is very lightweight and does not conduct heat into the hub, but it's also extremely expensive ($900 each!). It wears very little, but if it is damaged it can't be resurfaced and must be replaced. It also requires a special type of high temperature friction material to withstand the heat.
Mercedes is also using drilled and slotted rotors on some of its high performance models. The Chrysler engineers said they don't offer a significant improvement in cooling compared to a standard rotor, and are used primarily to enhance the "racing image" of the vehicle.
On the issue of brake life, premature pad wear and rotor wear are major customer issues with all the vehicle manufacturers. The latest approach is to make the design of the brake system more robust so the brakes last longer and run quieter. Chrysler tests for wear using city traffic driving runs. They load the vehicle with 400 lbs. of extra weight to simulate a heavy load, then drive the vehicle for 4,000 to 5,000 miles and measure the pads and rotor to see how much they're worn. The results are then extrapolated to estimate the service life of the brake components.
One engineer said a performance target for Jeep products is to have the front brakes last at least 25,000 miles and the rear brakes 50,000 miles in the city traffic driving test. Because the test assumes a "worst case" situation, most vehicle owners will probably get a lot more miles out of their brakes.
The Chrysler engineers said their design goal is to make the rear brakes last twice as long as the ones up front so the rear brake linings only have to be replaced at every other pad change. Right now, that isn't necessarily the case. On many Jeep Grand Cherokee models, the rear brakes use an aggressive friction material that wears the rear rotors and may require the rear rotors to be replaced when the pads are changed.
The current design life for OEM brake calipers is now 150,000 to 180,000 miles, says Chrysler. Some Jeep Cherokee owners have reported getting 200,000 to 300,000 miles out of their calipers! Since the early 1990s, Chrysler has been using premium EPDM seals as well as phenolic pistons. So Chrysler no longer recommends rebuilding or replacing the calipers when the brake pads are changed -- unless, of course, an inspection reveals the caliper is leaking, worn or damaged. Chrysler also says the caliper slide pins are lubed for life and should last life of vehicle. But these too should be carefully inspected and relubricated if necessary when servicing the brakes.
As for brake hoses, there is no recommended replacement interval. Today's brake systems can develop up to 2,500 PSI or more, which requires top quality hose materials. The newer EPDM hoses not only withstand higher pressures but are also more resistant to moisture permeation, which means there's less water contamination of brake fluid over time. Therefore, Chrysler sees no need to replace brake hoses unless a hose is damaged.
With the exception of Viper, all current domestic Chrysler products use high temperature DOW 1000 DOT 3 brake fluid, which has a boiling temperature of 550 degrees F and surpasses the government's minimum performance requirements for DOT 3 brake fluid. Chrysler says their fluid can absorb up to 3 percent water and still meet the DOT 3 specifications for boiling temperature. Consequently, they see no reason to change the fluid for preventive maintenance.
In the past, European brake systems used vented master cylinder caps and lower quality hose and seal materials which allowed moisture to contaminate the fluid relatively quickly. Because of this, the brake fluid had to be changed periodically. But this is no longer true. Most European and domestic vehicles use sealed master cylinders with EPDM hoses and seals that resist moisture penetration. So fluid changes are a thing of the past, Chrysler says.
Fluid contamination by wear particles from seals and internal corrosion can sometimes cause problems, and particles as small as 5 microns in size ( a human hair is 100 microns in diameter) may be large enough to affect the operation of an ABS modulator. Even so, Chrysler makes no recommendation to replace the fluid.
Chrysler's position on rotor refinishing is to resurface or replace as needed based on a careful inspection of the rotors. Reasons for resurfacing would include grooving or variations in thickness (too much runout). Some vehicles are more sensitive to runout than others. The rotors on Jeep Grand Cherokee may cause vibration or pedal judder with only 18 microns of runout. Ram trucks, on the other hand, can handle up to 180 microns of runout with little or no feedback.
Another reason to resurface rotors is to remove the transfer layer of material from the rotor's surface. Friction material from the old pads becomes embedded in the metal and may interact with the new pads if not removed.
A rotor should be replaced if it is worn down to the discard thickness or cannot be resurfaced without exceeding the minimum "machine to" thickness.
* TSB 05-03-97 -- Covers a brake pull problem on 1994-'97 Dodge Ram trucks. The fix is to inspect the brakes, check alignment and install a front wheel shim if needed to eliminate the pull.
* TSB 05-04-97 -- Deals with reduced rear brake lining life on 2500/3500 Ram trucks. The cause, in most cases, is operating the truck in a heavily loaded condition. This shifts more of the brake load to the rear brakes, which accelerated lining wear.
* TSB 05-02-98 -- Clicking noise from the brakes in 1998 Jeep Cherokee. The cure is to install new rubber backed pad insulators.
* "Creep Groan" Brake Noise on 1998 Jeep Cherokee. The brakes are noisy when the brakes are cold. The problem seems to be moisture sensitive and is worse in cool, damp conditions. The cure is to replace the linings.
Ford says it has no official policy on resurfacing rotors. As long as no shake, vibration or brake pedal judder is present and the rotors are not badly grooved or rusted, there should be no need to resurface them. Ford says it hasn't seen any problems caused by film transfer from the friction material.
If rotors do need to be resurfaced, they encourage their dealers to use an on-car lathe rather than a bench lathe to minimize runout.
One item that seems to cause some confusion when it comes to resurfacing Ford rotors is Ford's use of a single "machine to" spec on their rotors. GM and Chrysler include both a "machine to" thickness and a discard thickness on their rotors.
The "machine to" spec on a Ford rotor is the minimum thickness to which the rotor can be turned and safely returned to service. The number includes enough margin of safety so that the rotor should last as long as the pads. Once the rotor is worn below the "machine to" thickness, it should not be resurfaced any more or returned to service when the pads are replaced. It must be replaced.
The problem is Ford does not put a "discard" thickness on their rotors so technicians sometimes think the "machine to" thickness is the discard thickness and replace the rotors unnecessarily.
Ford says the main issue with rotor thickness is not rotor warping, mechanical failure or heat management, but the physical operation of the brakes. The rotor must be thick enough to prevent the caliper pistons from popping out if the rotor and/or pads are worn. The "machine to" spec they use allows for this and leaves enough thickness to assure safe operation of the brakes.
When resurfacing a rotor, the minimum recommended depth of cut .002 in. The maximum is .008 in. If you take too deep a cut, it will leave a rough surface finish above 100 microns that may cause problems.
As for rotor runout, sensitivity depends on the vehicle application. On Ford 4x4 trucks, the limit is .001 in of runout.
If rotor runout is excessive, it will lead to uneven wear that causes the thickness of the rotor to vary. This, in turn, will cause a vibration or pedal shudder when braking. Problems such as rust between the rotor and hub, hub runout and uneven torquing of the lug nuts can all cause runout that leads to uneven wear and pedal vibration. Always clean the hub/rotor interface when servicing the rotors, and apply a thin coating of antiseize to the mating surface to inhibit corrosion.
Ford says aluminum alloy wheels are much stiffer than steel wheels, and are much more likely to cause a rotor runout problem if the lug nuts are not tightened evenly.
|
WARNING: DO NOT USE OIL, GREASE, ANTI-SEIZE OR LUBRICANTS OF ANY KIND WHEN TIGHTENING LUG NUTS! Proper torque on lug nuts is very important for three reasons. One is to keep the lug nuts from loosening up and the wheel coming loose, another is to prevent distortion of the brake rotor behind the wheel, and a third is to prevent broken studs. A torque wrench should be used for final tightening of the lug nuts, and the nuts should always be torqued to the recommended specifications. CAUTION: Torque specifications for lug nuts are always for CLEANand DRY studs and lug nuts. That means no oil, no grease, no anti-seize and no lubricants of any kind. Any of these products will reduce the friction between the threads. This may seem like a good thing to prevent rust and frozen lug nuts, but the reduction in friction means a much higher percentage of the applied torque (up to 25% or more) will go toward loading the lug nuts. The end result may be brake rotor distortion or broken studs! Wheel studs should be cleaned with a wire brush to remove rust and dirt BEFORE the wheels are mounted. If the lug nuts are heavily rusted or have damaged threads and won't turn easily on the studs, replace the lug nuts. The same goes for any wheel studs with damaged or badly corroded threads. And remember to mount the wheels DRY with nothing on the threads. |
BRAKE FLUID -- Ford does not recommend changing DOT 3 fluid or flushing the system. Ford says their recommendation is a "risk versus benefit" assessment because there's a potential for doing more damage than good. If sediment or contamination from the master cylinder is pushed through the ABS unit, it can cause big problems. Flushing the system also increases the risk of air entrapment and leaks, says Ford.
Ford says their brake systems are engineered to tolerate brake fluid contamination of up to 3 percent moisture without fluid boil (which is close to the saturation point for DOT 3 brake fluid). If the fluid has more moisture, it probably has some liquid water in it and should be changed.
Ford said they measured the water content in 7 to 10 year old Ford vehicles and found that the water content was actually quite low: only 1 to 1-1/2 percent. So based on their findings and the design target of 3 percent water, they see no need to change the fluid for preventive maintenance.
Does pushing the caliper pistons in when changing the pads push contaminants from the calipers back toward the ABS unit or master cylinder? Ford says no. There's not enough movement or migration to cause a problem.
LONGER BRAKE LIFE -- Ford says increasing brake life is a major goal for future vehicles. Ford wants the brakes to be "unseen and unheard" by the vehicle owner for 150,000 miles. Nobody is there yet with friction, but Ford says the rest of the brake system is now being built to go 150,000 miles plus. They're doing it by using larger, cooler running rotors, improving airflow to the brakes, installing thicker brake linings and revamping their durability validation tests for brake parts. The key to brake longevity is better heat management.
Ford says older ABS systems used 8-bit processors that could provide only limited brake control functions. The newer ABS systems have 32-bit processors and are much faster, so they can handle a wider range of control functions. The newer systems pulse the ABS control valves in small ,quick movements rather than simply switch them on and off, which allows them to provide more precise ABS, traction control and stability control functions. This feature has allowed the ABS system to take over the job of providing rear brake proportioning on some vehicles, eliminating the need for a rear brake proportioning valve.
TSBs -- The alloy wheels on Expedition and F-series trucks were originally equipped with flat washers under the lug nuts. The lugs nuts had a tendency to work loose, so Ford had a recall to replace the washers with ones that had a special coating to increase torque retention. Back in 2002, Ford increased the size of the studs to 12 mm and upped the torque spec for the lug nuts from 100 ft. lbs to 150 ft. lbs. on the 2002 models only. Do not increase the torque on the earlier model lug nuts as overtightening can stretch and break the studs.
Older Ford Focus models have had a rear drum squeal problem. The fix includes a redesigned drum (changing the mass of the drum dampens the harmonic frequency).
There's no bulletin from Ford, but on 1997 and older models that have Teves Mark 4 ABS (Taurus and others), some failures of the ABS modulator have been traced to sediment and plugged filters inside the unit. The modulator has a lot of dead end passages and sits low near the battery.
GM's policy on resurfacing rotors is to do so on an as needed basis. "We are not in a replace only mode except for Malibus." Heavily corroded rotors cause about 30 percent of brake pedal pulsations. The rest are due to distorted or wobbling rotors, which causes uneven wear and variations in rotor thickness. This can develop in 3,000 to 7,000 miles following a brake job. The cure, says GM, is to resurface the rotors on the car using an on-car lathe, or to measure rotor runout on the hub and correct as needed by installing a tapered shim behind the rotor and/or indexing the rotor to minimize runout. TSB 01-05-23-001 (Feb 2001) covers the Brake Align system for correcting rotor runout using a tapered shim behind the rotor. More than .003 in. of rotor runout will usually cause problems on most cars.
Rotor thickness must be greater than the discard dimension on the rotor. The "machine to" spec includes a built-in safety margin of .030 to .060 in. to allow for wear when the rotor is returned to service.
TSB 00-05-22-002 (issued Feb 2000) covers GM's brake rotor warranty service procedures on 1995 to 2000 cars and light trucks. GM says turning the rotors will not solve a noise problem. Also, new rotors should not be resurfaced prior to installation.
The general specification for maximum rotor runout on all GM applications is now .003 in. (.060 mm), and may be decreased to .002 in. in the future.
To minimize rotor runout, GM recommends using the Pro-Cut on-car lathe. GM also approves the use of a bench lathe provided it can produce less than .001 in lateral runout on the lathe, less than .0003 in. thickness variation, is capable of single pass finishing, can leave a surface finish of 40 RA or less, and can be used to do a nondirectional finish.
GM recommends using torque-limiting sockets when tightening all lug nuts,and to use a two-step procedure. First snug the lug nuts up to approximately half the specified torque value, then final tighten to specification using torque-limiting sockets or a torque wrench.
GM said it has tested the accuracy of using torque-limiting sockets and found that the torque load on the lug nuts depends on line pressure and the setting on the impact wrench. Regardless of the settings, though, the sockets produced consistent results: consistently high, consistently low or consistently on the mark. As long as all the nuts are tightened evenly, they won't distort the rotor and hub. But if only one nut is overtightened, it can create runout problems.
TSB 00-05-23-002 (Feb 2000) covers front disc brake pulsation on 1997-'00 Chevy Malibu, '97-'99 Olds Cutlass, '99-'00 Olds Alero and '99-'00 Pontiac Grand Am. The pulsation is due to unevenly worn rotors, which is caused by aggressive pads that accelerate rotor wear. The fix is to replace the rotors, pads and hubs.
BRAKE FLUID -- GM's position of brake fluid is that they do not recommend changing it on most vehicles. The only exceptions are Metro and Tracker models. GM brake systems are sealed and should not need flushing. GM's goal is to reduce maintenance requirements, not add them.
Related Articles:
Click Here to See More Carley Automotive Technical Articles