Occasionally, there will be a need to replace an ABS modulator or other hydraulic component, such as a pump/motor or even a master cylinder, on some systems. When this need arises, it will be essential you are prepared for the task. This article provides an overview of some of the techniques, tools and procedures required for these tasks.
Typically, when performing wheel-based hydraulic repairs, no special steps are needed to bleed the ABS modulator. This is true as long as no air is allowed to enter the modulator. If air is allowed to enter, it may require special steps and/or tools to remove it. There is a simple step when performing upstream hydraulic repairs that will prevent the system from draining, thus avoiding some of these headaches.
Figure 1 shows a brake pedal depressor installed on a vehicle receiving hydraulic service. This simple step moves the cup seals in the master cylinder past the vent ports which blocks the fluid path to the reservoir. This will prevent the system from draining during service. The only thing to keep in mind is that the brake lights are on so if the job is going to be a long one, it is best to disconnect the stoplight switch.
While it is not possible to cover all the ABS systems in use, we can cover representative systems from the major categories when it comes to bleeding. We will do this on a case study basis.
Case Study 1 - Modulator replacement on a Kelsey Hayes BC310/EBC325/EBC410 - 4-wheel ABS system.
These three systems represent the majority of light trucks equipped with 4-wheel ABS from 1995 up to the early 2000's. Including the EBC410 adds Windstar vans from 1995 to 1999. These systems are three channel (except EBC410 - 4 channel) systems that use two position solenoid valves for pressure modulation, two low pressure accumulators and also incorporates a two-piston pump. Each channel uses an isolation and dump valve so the HCU has six total valves, while the four-channel EBC410 has four pairs (See Figure 2).
In terms of bleeding, the HCU (hydraulic Control Unit) can be divided into two parts, as shown in Figure 3. The green section represents the area that can be bled through wheel based bleeding. This area includes the isolation valves. The red section represents the areas that will not be effectively bled when performing wheel-based bleeding. Figure 4 shows the fluid path through the left front wheel channel. The fluid flow during wheel-based bleeding is from the master cylinder through the normally open isolation valve and out to the system. The fluid paths to the dump valve, pump and low pressure accumulator represent a "dead end" and will not bleed without additional steps (Figure 5).
The majority of ABS modulators not equipped with bleeders will require cycling of the valves and activation of the pump to bleed out these "hidden" areas. For example, the Ford version of the EBC310 requires the "bleed function" to be performed through the NGS scan tool (See Figure 6). This function cycles each of the dump valves and then energizes the pump to bleed these hidden areas. Other systems, such as the TEVES VI and some of the import systems, require the use of a dedicated bleed tool to perform a similar process.
Once the bleed function has been performed, the system should be bled at the wheels using the appropriate sequence.
Case Study 2 - Modulator Replacement on a Delco VI - 4-wheel ABS system.
The Delco VI ABS system (See Figure 7) is a four-wheel, three-channel system that was used from 1992 until 1999 on a large number of General Motors FWD vehicles. In addition, the system was used on the "F" series Camaro and Firebird and many Saturn vehicles.
The Delco VI represents the group of modulators equipped with bleeder screw(s). While this group isn't very large, there are enough of them to include in this article. As with any bleeder screw, these are placed at the highest point of the modulator, which on the Delco VI is at the solenoids.
Bleeding of the Delco VI will be required under the following conditions:
Master cylinder replacement;
Gear replacement; and
System runs dry.
While this article's purpose is not to cover the process of modulator replacement I want to take a minute and cover the most important step about putting this system back together. It is important before installing the motor pack to rotate the gears on the modulator in a counterclockwise direction until they stop (Figure 8). This places all of the pistons in what is referred to as the home position. This step is not that important for the front circuits, but is critical for the rears (center gear). Making sure the pistons are in the home position ensures that the fluid path to the rear brakes is open to allow bleeding the system without a scan tool (See Figure 9).
There are two other methods of homing the rear circuit pistons. The first is to use a bi-directional scan tool, such as the Tech 1 or 2, to command the rear circuit motor to home the piston. The other method should only be used when necessary. If the rear pistons are not in their homed position, the front wheel circuits can be bled until a firm pedal is obtained.
Once the pedal is firm, test drive the vehicle above 4 mph and the ECU will self test which includes homing the pistons. Caution should be used because once the rear pistons are homed, the rear wheel circuits are added back into the system and if air is present, expect a low pedal.
Bleeding should start with bench bleeding the master cylinder/modulator assembly. Use the following steps to bench bleed the assembly:
Mount assembly in vise and fill reservoir with DOT 3 fluid from a sealed container.
The outlet lines of the modulator are marked on the motor pack (See Figure 10). Plug off the LR & RF ports. Install bench bleeding kit into LF & RR outlets and place tubes in reservoir. Care should be taken not to get brake fluid on the motor pack.
Use a screwdriver to depress the master piston in about 1" and then release about 1/2". Repeat this process until a solid fluid flow is seen out of both LF & RR outlet ports.
Depress master piston 1/8" and then use short 1/8" strokes making sure not to release piston to stop position. This helps remove the air from the low pressure areas of the master cylinder.
Reverse the plugs and bleeding kit so that the LR & RF outlets are being bled. Repeat steps 3 & 4.
Once the assembly is bled, install it back on the vehicle and bleed the system out using the following steps:
The sequence on most of the FWD models using the Delco VI is RR,LR,RF,LF. Bleed the system using the appropriate sequence.
Once the wheels are bled bleed the modulator. Start with the rear bleeder valve. Once the air has been removed, repeat the process with the front bleeder on the modulator.
Confirm pedal height and feel. Repeat steps 1 or 2 as necessary.
This system should present much difficulty in getting a good pedal back when using the above procedures.
Case Study 3 - Modulator replacement on a Kelsey Hayes EBC4 - 4 wheel ABS system
The EBC4 ABS system is a GM system used on light trucks and vans from the late 80's to the mid 90's (See Figure 11). It represents a unique type of modulator in that it has bleeder screws that allow bleeding of the hidden areas of the modulator. The process is not as straight forward as it sounds. Getting fluid to these bleeders requires a couple of extra steps. The modulator will have to be bled if it has been replaced, a new master has been installed or air has entered it. To bleed the EBC4 system use the following steps:
Bleed the master cylinder before the BPMV (Brake Pressure Modulation Valve). The master cylinder can be bled through the brake lines. Loosen the line fitting at the secondary outlet and depress the brake pedal no more than 1/3 the distance. Repeat this process until no air is seen making sure to wait 15 seconds between each stroke when foot bleeding (These vehicles have QTU master cylinders). Repeat this process at the primary outlet.
High Pressure Accumulators
Bleed at the wheels. The sequence for these vehicles is RR,LR,RF,LF. Bleeding at the wheels is done before the BPMV to remove the air in the base brake system. This process will also bleed the non-isolated portion of the BPMV.
With the ignition off, install the high pressure accumulator depressors (See Figure 12). Open internal bleed screws (See Figure 13). This allows fluid from the brake system to flow into the hidden areas of the modulator and to the modulator bleeder screws.
Open the left modulator bleed screw and depress the brake pedal no more than 3/4". Close the bleeder screw and release the brake pedal. Wait 15 seconds and repeat the process until no air is seen. This can take a while so be patient. Close the left internal bleed screw.
Repeat step 4 on the right modulator bleed screw.
Once complete, evaluate the pedal height and feel. Repeat process is necessary.
This system can also be bled using a bi-directional scan tool. The process is usually called the "system function test." This process opens the valves and cycles the pump to purge air from the hidden areas of the modulator and should be run three times to make sure all air is removed. The system must be rebled after performing this process.
Before we cover the last two case studies, we should take a minute and discuss one of the things these two studies have in common - high pressure accumulators. Many of the first passenger car four-wheel ABS systems introduced were equipped with high pressure accumulators. A high pressure accumulator is a storage vessel designed to hold brake fluid under high pressures. The typical construction is a steel "bulb" with a threaded outlet. Inside this bulb is a thick rubber diaphragm that divides the bulb into two areas. The top area is filled with pressurized nitrogen gas. The bottom area is where the fluid is stored (See Figure 14).
The brake fluid in the accumulator is used for providing power assist, pressure modulation during an ABS stop and in some systems to apply the rear brakes. The pump/motor is responsible for filling the accumulator and is controlled by a pressure switch. The pressure switch will have a high and low cutoff so the volume of brake fluid in the accumulator will stay within a certain range I call the "flex zone" (See Figure 15).
The most widely known aspect of the high pressure accumulator is that it should be discharged before performing hydraulic service. The accumulator or master cylinder will generally have a warning label (See Figure 16) advising that the brake pedal be pumped up to 50 times with the ignition OFF to depressurize the accumulator. Generally, all you have to do is pump the brake
pedal until it is hard - once hard the accumulator is discharged (See Figure 17). You should be aware that on these older systems depressurizing the accumulator can cause failure of the diaphragm. Figure 15 shows the normal range of operation for the diaphragm while Figure 17 shows the discharged position. Discharging the accumulator causes it to stretch farther than it has in normal operation which can cause failure.
Case Study 4 - Delco Powermaster III ABS System
This system is representative of the early integral ABS systems where the ABS and base brake components were combined into one unit (See Figure 18). The ABS is not only responsible for pressure modulation, but is also responsible for power assist and rear brake apply. Looking at Figure 19 shows three brake line outlets from the unit. The two from the master cylinder (#1 & #2) supply each front brake. Outlet #3 supplies the rear brakes from the boost chamber.
Repeat step 3 at the left front bleeder.
The unit is also equipped with two bleeder screws. These bleeders allow the isolation valves to be bled after service. This is covered in the following steps.
Bleeding the front brakes:
Turn ignition switch to the Off position, then depressurize the system by pumping the brake pedal until hard.
Clean reservoir and diaphragm assembly, then remove reservoir cover assembly. Fill reservoir to proper level then reinstall cover.
Start at the right front bleeder. Bleed until no air is seen. Check, then add brake fluid if necessary.
Bleeding the rear brakes:
The rear brakes are bled using a charged accumulator. Follow the steps below to bleed the rear brakes:
Turn ignition switch to the On position, allowing pump to run. If pump runs longer than 60 seconds, inspect for leaks or pump for malfunctions. Slightly depress the brake pedal. Note: Don't push the brake pedal all the way down or too quickly as this depressurizes the accumulator.
Bleeding the isolation valves:
Open the right rear bleeder screw and bleed right rear until no air is seen.
Repeat step 2 with the left rear bleeder.
Start with the inner bleeder screw (towards engine) (See Figure 20). Turn ignition on and lightly apply the brake pedal. Open the inner bleeder screw and allow fluid to run until no air is seen. Close the bleeder screw.
Repeat this process on the outer bleeder screw (See Figure 19).
Bleeding the boost section:
Depressurize the accumulator again by pumping pedal with ignition off until pedal becomes hard.
Wait for two minutes to allow fluid in reservoir to de-aerate and then refill reservoir.
Apply moderate pressure to the brake pedal and then turn ignition on for three seconds.
Case Study 5 - Nippondenso, Nissan and Sumitomo - Honda/Acura Version
Repeat step 1 ten times.
Confirm pedal height and feel.
Honda and Acura vehicles have used the system shown in Figure 21 from 1987 to 1997. This is a non-integral system that incorporates the use of a pump and high pressure accumulator, as well as a remote fluid reservoir. To bleed this system follow the steps below:
Pump brake pedal with ignition off to deplete the vacuum in the booster. Fill both the master cylinder and modulator reservoir with DOT 3 brake fluid to the "Max" level. Check fluid levels in both reservoirs frequently during the bleeding process to prevent air from entering the system.
Generic ABS Bleed Techniques
If the master cylinder has been replaced or air is thought to be in the master, begin by bleeding the master at the brake lines. Start with the secondary section. Loosen the line fitting and depress the brake pedal three quarters. Repeat until no air is seen and then bleed primary section with same technique.
The HCU (hydraulic control unit) is bled next. The HCU is equipped with a bleeder screw that allows removal of trapped air (See Figure 22). It is best to attach a bleeder hose to the HCU bleeder to control the fluid discharge. Start engine then slowly open bleeder 1/8 to 1/4 turn to allow fluid and air to escape.
Repeat step 3 in small increments until no air is seen. Turn ignition off and proceed to step 5.
Caution: The brake fluid in the HCU is under high pressure. Don't open the bleeder more than one quarter turn or too quickly or brake fluid may be discharged under high pressure.
Bleed the rest of the system using the published bleed sequence.
Confirm pedal height and feel. Rebleed if necessary.
Regardless of the type of ABS system you are working on, it is often useful to have some generic techniques available to you when trying to restore pedal height after hydraulic service. It is not practical for a shop that provides service to all makes and models to have all of the scan tools and bleeding tools required in the service manuals. So what is a shop to do when faced with this scenario?
The first step in the process is to determine what is necessary - before the service is started. Looking up the bleeding procedures before you start will prepare you for what is necessary to bleed the modulator according to the manufacturer. If you are not equipped with these tools, you have to make a decision on whether to proceed. I would say on the majority of ABS systems in use, there is a work around. I covered this back in April, but since the topic applies again I will restate it.
The procedure involves using the ABS to bleed itself. Perform the hydraulic service and bleed at the wheels using the published sequence. If, and only if a safe pedal can be obtained, proceed to the next step.
The next step involves cycling the ABS on a loose surface repeatedly. This will engage the ABS and the cycling of the valves and pump will move any trapped air out of the isolated areas and into the hydraulic system. It must be understood that this will affect pedal height and feel and caution should be used driving the vehicle. Rebleed the system and pedal height should be restored. This method will not work if the ABS system is disabled due to a fault code, so make sure to check the ABS light before counting on this method to bail you out.
There is another method that, while not widely used, can be useful from time to time. It involves manually opening the appropriate valves while energizing the pump. This is the same process most scan tools use to remove the trapped air.
This method can only be used on ABS systems where the ECU and HCU are stand-alone components. These systems will allow access to the valve and pump wiring where systems, such as the Kelsey Hayes systems mentioned earlier will not. The Kelsey Hayes EBC310, 325, 410 and 430 combine the solenoid pack and ECU into one unit. There is no way to access the wiring to the solenoids.
If you are going to use this method, follow these steps:
Have an assistant apply and hold the brake pedal;
In closing, ABS bleeding should not present a great degree of difficulty if you follow some simple rules:
Energize the first dump valve - the brake pedal should fall during this step;
While the dump valve is energized jumper the pump so it runs - the brake pedal should rise during this step;
Repeat this process 3 times for each dump valve; and
Bleed the system at the wheels.
Determine the amount and type of hydraulic service needed;
Look the system bleeding requirements up before performing the work;
Determine if you have the ability to perform the necessary bleeding, either using the factory method or an alternate method; and
If not replacing the modulator, take the steps necessary to keep air out of the modulator.