Tool Briefing: Maintaining and repairing electric vehicle braking systems

March 13, 2015
Information on how regenerative braking affects the braking system, what some of the common maintenance and repair issues are and what tools are required to diagnose and repair the system.

Editor's Note: This article was orginally published March 13, 2015. Some of the information may no longer be relevant, so please use it at your discretion.

Many of the brake issues and symptoms you will see on hybrid electric vehicle (HEV) brake systems are the same as with other vehicles, such as vibrations, noises and sticking calipers, to name a few. The difference can sometimes be what actually caused the issues.

Heat and brakes do not get along, so you might think symptoms such as vibrations and brake fluid contamination are caused by heat as it is in most vehicles.

Since brakes are not used as much to slow an HEV, heat is not generated as quickly. Therefore you must look for other causes of these issues. Here, we look at a 2011 Toyota Highlander Hybrid that came into the shop with a customer complaint of a vibration and noise seemingly coming from the rear of the vehicle.

Step 1: Obtain vehicle-specific repair info and disable ABS self-test function

As with any brake service, you should obtain the specific repair information for the vehicle. We will use the Mitchell 1 ProDemand system to not only look up the details and specifications, but also any TSBs that may pertain to the vehicle. Since much of the technology used in HEV braking systems is relatively new, it is a good idea to check TSBs as well as assure you understand the safety procedures for each vehicle you repair.

Hybrids are just like every other vehicle because you need to remove the wheels and calipers to perform a thorough brake inspection. But like many other late-model vehicles, including most HEVs, an additional step is necessary.

In many cases, the Antilock Brake System (ABS) is utilized to supply brake fluid under higher pressure than normal. Designed within the ABS system is a self-test function that will pressurize the system to assure there are no leaks and the system is functioning normally. Many of the manufacturers utilize the driver’s door opening to send a signal through the CAN bus to start the self-test function and energize the ABS pump.

This can turn into a problem if the technician does not disable the self-test feature. If servicing a caliper and the door were to be opened, the high pressure will flow to the caliper and could cause it to activate. The best case scenario is the caliper blows the seal; the worst case is causing a severe injury to the technician by pinching hands or fingers.

Another caution in the repair information relates to using extra care when using magnets around ABS speed sensors, as the magnet can affect the performance of a speed sensor. Because of this, it may be a good idea to use a clamp mount on your dial indicator instead of a magnetic mount when checking rotor run-out.

Step 2: Relieve system pressure and inspect brake system

According to service information for our Highlander, there are no TSBs relating to our customer’s issues, but there are some specific procedures necessary to safely work on the brake system. These include disengaging the Electronic Brake Control (EBC) system by removing the relays and bleeding down the pressure in the brake fluid accumulator with your scan tool.

Prior to collapsing the calipers to install new pads, you will also need to remove some of the fluid from the reservoir, keeping the level at the “min” line. You also should be aware that there are capacitors in the ABS used to supply back-up battery power in case of power loss, so make sure you wait five to 10 minutes for the capacitors to bleed off to ensure there is no power in the system. Once the system is disabled, you can safely remove the calipers and inspect the system in the same manner as any other vehicle.

Part of any brake inspection should include checking the run-out of the rotors that can cause vibrations, noise or excessive wear. The front brakes checked out well within specifications for pad and rotor wear, and run-out was under 0.001" on both sides.

Checking the rear brakes was next on the agenda. Using a dial indicator on the inside and outside of the rotor surface showed only 0.001” overall run-out on the left rear. The right rear rotor showed 0.003” on the inside and 0.004” on the outside surfaces.

It is necessary to measure both the inside and outside surfaces to assure that the rotor can be resurfaced as well as where the high and low points are in relation to each other. The amount of run-out and the relationship of the high and low points will determine if the rotor has enough material to be safely resurfaced. We have found one of the easiest tools to measure brake pad wear is the OTC 6596 Brake Pad Gauge. Use it to measure the pad to backing plate thickness (or pad to rivet) on both the leading and trailing edge of each pad.

The results of our inspection showed that the left rear brake pads were worn evenly, but the right inner pad was worn to the backing plate and the outer pad was worn unevenly, 0.003” more on the leading edge than the trailing edge. This is a common issue with hybrid vehicles, as the brakes do not get “exercised” as much and the calipers will eventually bind, causing the caliper to not release and wear the pads.

Additionally, the caliper slides can rust and corrode, causing the caliper to slide unevenly creating irregular wear on the brake pads. We have both conditions on this vehicle indicating a caliper that is not releasing completely and is also binding on the slides, causing uneven pad wear.

Step 3: Replace the mechanical components

Replacing the calipers will be necessary. First clean the slides, pins and mounts with a mild abrasive pad, such as a green Scotch Brite pad or a wire brush, to remove corrosion and rust without damaging the parts.

Take care when you remove the brake line from the caliper so as not to damage the fitting or hose. Many technicians also use vice grips or hose pinch pliers on brake hoses so they don’t leak fluid, but DO NOT do this as the hose could be damaged.

Don’t forget it is also necessary to clean the wheel hub face and rotor mounting surface. We use the 3M Scotch-Brite Roloc Brake Hub Cleaning Disc Kit that attaches to either an air drill or die grinder to perform this task, as it allows thorough cleaning around the wheel studs, permitting the rotor to sit evenly on the hub.

Mount the rotors and torque the lug nuts (we usually install the “acorn” upside down to make sure we do not damage the lug nuts) following the manufacturers torque spec and rotation sequence. You will need to measure the run-out of the rotor with your dial indicator both on the inside and outside surfaces. It is also necessary to use either a micrometer or a digital Vernier caliper to measure the thickness of the rotor. Once you determine the overall run-out (out of parallel) and thickness of the rotor you will be able to determine if there is enough rotor material available to use your brake lathe to surface the rotor. Install a dial indicator onto the caliper mount and check the rotor run-out to make sure it is within specs. If not, mark the rotor in the high spot then remove the rotor and rotate it on the hub.

Using the dial indicator again measure the rotor. If the high spot is in the same location the rotor is warped. If it has moved, it is caused by the hub or axle being bent or possibly having contamination on the mounting face. Warping can cause a vibration leading to uneven and possibly premature pad wear. Surface the brake rotors using either your on-car brake lathe or an off-car stationary lathe, making sure that the rotors are within specifications after surfacing.

Once the rotors are installed and checked for proper alignment, install the pads and calipers using new hardware and the recommended lubricant on the slides and pins. Use a torque wrench to tighten the mounting bolts to proper specs to assure they are tight, but not tightened too much to cause the bolts to stretch. At this point, you will need to bleed the system so leave the wheels off and keep the nuts on the rotors to hold them in place.

Step 4: Bleed the system with a scan tool

Most hybrid manufacturers do not recommend the use of a vacuum bleeder, but using either a pressure bleeder with the correct adapters or bleeding manually are approved procedures. Either bleeding method will require the use of a scan tool in order to bleed the system properly.

Even though there are some manual bleeding procedures that may be used without a scan tool, using a bi-directional scan tool to make sure all of the air is out of the system is recommended, ensuring all of the ABS solenoids have been activated and allowing the brake fluid to flow through the entire system. Follow the manufacturers recommended procedures by obtaining the information from your repair information source specific to the vehicle, as well as bleeding and reactivating the system procedures performed in the correct order.

For the Highlander, we will use the Toyota TechStream Scan Tool to go through the entire bleeding and reactivation process. As with most “active” tests and procedures, using any aftermarket scan tool with bi-directional capabilities is just as efficient as the factory scan tool. Most active, or bi-directional functions, performed with any scan tool use the programming within the vehicle’s software to activate tasks within the module you are working with.

After performing your brake repair, bleed the system using the following procedure:

  1. Use only DOT 3 brake fluid, unless otherwise specified.
  2. Fill the reservoir to assure during the bleeding process the fluid remains between the “min” and “max” lines.
  3. Make sure the EBC is disabled while using the scan tool.
  4. Move the shift lever to the “P” (Park) position and apply the parking brake.
  5. Connect the scan tool to the DLC with the ignition “off.”
  6. Turn the ignition “on” and start the scan tool (DO NOT start the engine).

Go to the following menus:

  • Diagnosis
  • ECB Utility: ECB Invalid. This turns off the Electronic Brake Control so air cannot enter the actuator when the pump runs which will make it more difficult to bleed the system

You are now ready to initialize the bleeding sequence. Using the scan tool enter the following menus:

  • Diagnosis
  • Air Bleeding. Select “Usual” if just the front or rear brakes have been installed or repaired.

Bleed the brakes in the order listed in your repair information, in this case: left rear, right rear, left front and right front. Repeat the procedure until all of the air is completely out of the system.

Steps for manually bleeding the brake system

  1. Depress and hold the brake pedal. Allow the solenoid to operate for approximately 30 seconds, with the brake pedal depressed, bleed the rear brake system using the bleeder screw on the left rear disc brake caliper while the pump motor and solenoid are operating. Repeat the procedures until air is completely bled from the left rear caliper. Repeat the entire process on the right rear. The ECB warning light will come on and the buzzer will sound while bleeding, but that does not indicate a malfunction.
  2. Follow the procedure in the same manner for the front brakes, starting with the left front, then the right front.
  3. When finished install the two relays back into the brake controller and cancel the "EBC Disable" function using the scan tool.
  4. Clear DTCs.
  5. Perform Accumulator Zero Down with the scan tool. This allows the correct amount of fluid to remain in the reservoir and accumulator. Perform accumulator zero down by following the steps displayed on the scan tool. The procedure to perform the zero-down is the same as disabling the EBC with your scan tool, with the added step of selecting “Zero Down” within the ECB Utility menu.

Accumulator pressure will be released and accumulated repeatedly, which allows the fluid inside the accumulator to circulate. The pump motor pressurizes the accumulator every time the ignition switch is turned from off to on, doing this four or five times will assure all of the air is removed from the accumulator and will allow proper filling of the reservoir. After performing accumulator zero down (accumulator depressurizing), return the fluid in the accumulator back to the reservoir by turning on the key and adjust the fluid level in the master cylinder reservoir to the “max” line.

If the Zero Down procedure is not performed, the accumulator will return fluid to the reservoir and could overfill the system.

In this case, we used the TechStream scan tool, but many of the aftermarket tools will work just as well, if not better. Aftermarket tools, such as the AutoEnginuity, in many cases work faster than a factory tool. This is primarily due to the technician being more familiar with the tool because he or she is more adept at using it. In addition, most aftermarket tools boot up faster than a factory tool and are ready to function much quicker than a factory scan tool. 

Finalize the job by clearing DTCs and road testing the vehicle to make sure the customer’s concerns were resolved.

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