A. Yes, rotor inspection should never be a guesswork proposition. First of all, start with a rotor thickness check so you can determine whether the rotor is even in the serviceable thickness range. You can perform this using a rotor micrometer, which has one flat contact point and a pointed contact point on the other side. Take measurements at eight different locations throughout one complete rotation of the rotor. If the rotor has any grooves, make sure the pointed contact point of the micrometer touches in the groove, not on the shoulder. After you take your series of measurements, compare them to specifications. If the rotor measures at or below its specified discard thickness, replace the rotor. If your measurements show thickness variations outside the manufacturer’s specifications—called parallelism—this will also call for rotor replacement.
Q. Is it better to use a rotor micrometer or a dial indicator for determining rotor parallelism?
A. The two tools don’t really match up “eye-to-eye,” because they’re used differently. A micrometer is the tool to use to check for parallelism, but a dial indicator is the tool to use to determine whether excessive lateral runout exists. Lateral runout occurs when a rotor spins off its axis. Excessive parallelism and lateral runout can both cause a brake pedal pulsation, but for different reasons. That’s why you should have both tools in your brake diagnostic arsenal in order to properly diagnose a pedal pulsation problem.
Q. We need to service the rear brakes on VWs and Audis with electronic parking brake systems. Is there a special tool for these or do you use a standard caliper piston retracting tool?
A. Don’t force the caliper pistons back in using conventional methods on these cars or you will damage the calipers! To properly retract the caliper pistons, use either an appropriate scan tool or dedicated electronic retraction tool specifically made for this purpose.
Q. Brake fluid flushing seems to be more critical, the more I read. What should I consider if I plan to take on this service?
A. First of all, shop around for a quality pressure bleeding system that’s easy to use and fits all the vehicles you service. If you can, try before you buy because some aspects of using a tool just never appear until you’ve actually had some hands-on time with the tool. Next, consider the scan tools you may have and their capabilities. Proper flushing/bleeding of a system with antilock brakes (ABS) may require interacting with the brake hydraulic system through the system pump and solenoids. Always check service information to be sure of what the procedure requires for each make and model. Jumping into a system flush or bleed without knowing what you’re up against could cause you some major headaches. At the very least, you may not be able to get all the old fluid out of the system. Even worse, you could introduce a problem into the hydraulic system, causing a spongy pedal that’s difficult to get rid of. If you have any doubts whatsoever about performing the procedure due to a lack of equipment or information, steer clear. Proceed only when you know you have all the right stuff.
Q. What do I need in order to diagnose ABS systems?
A. A multi-featured scan tool is an excellent starting place for ABS diagnostics. To be effective, your scan tool needs to perform three essential functions: 1) retrieve ABS trouble codes, 2) display ABS diagnostic data, and 3) perform various system function tests. Some scan tools also have the capability of capturing and displaying ABS “freeze frame” data, which is the data that transpired at the time an ABS trouble code registered. This data can be used to help reveal more specific details when pinpointing the cause of an ABS code. From there, a digital multimeter (DMM) can be a helpful diagnostic ally, helping you to perform pinpoint tests that lead to the root of the ABS problem. For example, let’s say an ABS trouble code clues you towards a bad wheel speed sensor. Using appropriate diagnostic information, you would then follow the flowcharts and steps to check the sensor and its circuit.
Thanks for checking out this month’s Tool Q&A column. Remember, this is your column, because it’s based off your questions. So, let PTEN know what’s on your mind when it comes to tools and equipment.