ABS Non-Trouble Code Problems

By Bill Williams
Technical Contributor

The majority of ABS systems are designed to store a trouble code, disable themselves and turn the ABS warning light on in the event of a problem with the ABS. Another important aspect of ABS operation is that if a failure occurs with the ABS system, it should not affect the base brake system. There are exceptions to every rule and this article will cover what is perhaps the biggest exception - where a failure in the ABS system does not store a code and does affect base brake operation.

The vehicle population affected by these systems includes pickup trucks, SUVs and some minivans equipped with the Kelsey-Hayes two-wheel and four-wheel ABS systems. These systems include the EBC2, EBC4, EBC310, EBC325, EBC410, EBC430 versions. The Kelsey-Hayes EBC2 rear wheel ABS systems have been used on Ford, Chevy/GMC, Dodge, Isuzu, Mazda, Nissan and Suzuki light trucks and a few SUVs and minivans since 1987.

This vehicle pool represents a large population, so the chances of running into one of these vehicles is pretty high.

All ABS systems prevent wheel lockup by modulating the pressure to the wheel experiencing excessive slip. By monitoring the frequency output of each wheel-speed sensor, the ECU can decide if an individual wheel slip exceeds a desired threshold. When such a threshold is exceeded at a particular wheel, the action the ECU will take involves controlling the HCU (hydraulic control unit) or modulator. The HCU is the part of the ABS system that is responsible for cycling the brakes during an ABS stop. The method in which the modulator accomplishes this varies from one ABS manufacturer to another.

The EBC2 system is a one-channel rear-wheel system. It is the simplest of all modern ABS systems. It is designed to prevent rear wheel lockup to allow straight line stopping. The EBC4, EBC310 and EBC325 systems can cycle each front wheel individually while the rear wheels are cycled together. They are called three-channel systems because of this.

A pair of valves, known as the isolation and dump valves (see Figure 1), control each channel. So, in the three-channel EBC4, EBC310 and EBC325 systems, there are three pairs of valves. The EBC410 & EBC430 systems are also three-channel systems, but have four pairs of valves. These systems are used on Windstar minivans. These vans are equipped with diagonal split hydraulic systems which require one pair of valves for each wheel. If either rear wheel experiences excessive wheel slip, both rear wheels are cycled together.

In all of the Kelsey-Hayes ABS systems listed, the isolation valve is placed between the driver's foot and the wheel(s) controlled by that valve. It is a normally open valve meaning it has no effect on braking unless the ABS system engages during an excessive wheel slip scenario. The dump valve controls brake fluid flow in and out of a part of the HCU known as the low-pressure accumulator. It is a normally closed valve, that prevents brake fluid from the base brake system from entering the low-pressure accumulator during non-ABS braking (acceptable wheel slip braking). The low-pressure accumulator is comprised of a spring-loaded piston. It is a storage vessel used to store brake fluid during the release phase of the ABS event. The dump valve is located between the isolation valve and the wheel.

The process of preventing wheel lockup involves four stages. The four stages used to cycle the wheel are apply, hold, release and reapply. The apply stage of the process happens every time the brake pedal is applied (See Figure 2).

If a wheel is experiencing excessive wheel slip, as seen through the wheel-speed sensor frequency output, the ECU will put the wheel into the hold stage by isolating it from the driver's brake-apply pressure (See Figure 3). This is done by closing the normally open isolation valve to the wheel experiencing the excessive wheel slip. Once closed the driver's brake-apply pressure no longer controls the braking to the wheel. When the isolation valve was closed, the brake was in an applied position, meaning there was pressure to the wheel. The closing of the isolation valve trapped that pressure at the wheel. In many cases, this trapped pressure is enough to cause the wheel to continue to experience the excessive wheel slip. If the ECU sees this through the signal produced by the wheel-speed sensor, it will go into the third stage of the process.

The ECU will pulse the normally closed dump valve. Each pulse of the dump valve releases the pressure to the locking wheel into the low-pressure accumulator (See Figure 4). The lowering of the pressure to the wheel experiencing the excessive slip reduces the braking to the wheel which allows it to speed up. The EBC2 and early-model EBC4 systems use reset switches to signal the end of one cycle while the others use the wheel-speed sensor signal. When the end of a cycle is seen, the ECU starts the process over again by closing the dump valve and opening the isolation valve.

The above explanation is somewhat simplistic, but it gives an idea of what is supposed to happen. With this foundation we can now discuss the failure that takes place. All of these systems can experience what is commonly referred to as a by-passing dump valve. Remember, the dump valve is a normally closed valve that controls fluid flow into the low pressure accumulator. If the dump valve leaks during normal braking, fluid will be allowed into the low pressure accumulator.

The fluid pressure will overcome the accumulator spring and result in filling the low-pressure accumulator resulting in a low- or no-pedal condition. This failure could be described as an "internal leak." The affect this type of failure will have on the brake pedal height depends on the volume of the low pressure accumulator and the displacement of the master cylinder. The larger the volume of the accumulator, the greater the impact. I have measured the accumulator volumes for most of these systems and have listed the results in Figure 5.

Since it is a single channel system, the EBC2 system has only one accumulator. The remaining three channel systems use three dump valves and two low pressure accumulators. The EBC4, EBC310 and EBC325 systems have one accumulator for the rear channel, while the front brakes share a common accumulator. The EBC410 and EBC430 system have one front and the opposite rear wheel sharing one accumulator, while the other two wheels share the remaining accumulator. The testing I have performed has shown the impact on brake pedal height to vary between just under two inches to almost five inches of additional pedal travel. The EBC310 system used on the F-Series pickup has the largest volume accumulator due to the dual piston calipers. If either front dump valve fails, it results in almost five inches of additional pedal travel.

In many cases, those who experience a by-passing dump valve will commonly confuse the condition with a defective master cylinder. Many a master cylinder has been installed to try and correct this problem. Accurate diagnosis comes from first understanding the problem and then knowing how to diagnose it. The problem has to do with the dump valve seat. Over time, corrosion can occur on the dump valve seat that could prevent it from sealing off the low pressure accumulator. Figure 6 shows a dump valve from an EBC310 system. As you can see, it doesn't take much to cause a problem due to the size of the dump valve seat. The dump valve can by-pass if corrosion occurs on this seat or a foreign object lodges between the dump valve plunger ball and the dump valve seat. In either case, the resulting symptom will be excessive pedal travel which is often described as "the brake pedal goes to the floor."

The first step in accurate diagnosis involves understanding that the pedal travel caused by a dump valve failure will typically be different enough from a by-passing cup seal in a master cylinder to point you in the right direction. When a primary cup seal by-passes in a master cylinder, it is almost always while the vehicle is stationary and the pedal pressure is low. The primary cup seal's ability to seal is at its weakest under these conditions.

The typical description of a by-passing cup seal is that the pedal sank to floor while the vehicle was sitting at a stop light. A by-passing dump valve can occur at any time, regardless of pedal pressure, and often shows up more often under higher pressures than lower pressures. It is common for a by-passing dump valve to cause a loss of pedal as the vehicle is slowing down. This should be one of the first clues it is not a master cylinder. Always make sure to conduct a thorough customer interview to determine when the pedal loss is occurring.

The next step in the diagnostic process will depend on whether you are able to duplicate the problem in the shop. We will discuss this procedure first. The diagnostic steps taken will depend on the type of modulator.

Problem that can be duplicated in shop:

EBC2 - Rear wheel

These vehicles use two different design modulators. The steps used in the diagnosis of the dump valve will depend of the modulator design. The first step is to locate the modulator and determine its type. GMs and Chevys place the modulator under the master cylinder, while Fords and most Dodges place it on the left-hand frame rail near the driver's feet. Most imports locate the valve on the right-hand frame rail, back near the axle. Once the valve is located, use the following information to diagnose it. (Note: The "Type" designations are assigned by me for reference in this article and are not Kelsey-Hayes designations.)

Type 1: Rear-Wheel ABS "Torpedo Style"

This valve is one of two designs used on vehicles equipped with rear-wheel ABS. It is identified by its cylindrical shape, bleeder screw and large cap screw, as shown in Figure 7. To perform the dump valve diagnosis, follow the steps below:

  1. Remove the 1-1/4" cap screw at the end of the valve body. Remove the low-pressure accumulator spring.

  2. Insert a screwdriver or similar tool into the back side of the low-pressure accumulator piston.

    Have someone start the vehicle and apply the brakes, while slowly increasing brake pedal pressure. The brake pedal must sink to the floor for the test to be accurate.

  3. If the low-pressure accumulator piston pushes against the screwdriver as the brake pedal is being applied, the dump valve is by-passing and the modulator will have to be replaced.

EBC2 - Type 1: Rear-Wheel ABS "Torpedo Style"

Follow the steps below to perform a dynamic test on the EBC2 "Torpedo" style valve.

  1. Remove the cap screw and low-pressure accumulator spring. Re-install the cap screw.

  2. Test drive vehicle until pedal fade is duplicated.

  3. Remove cap screw and check low-pressure accumulator position. If piston is against cap screw, dump valve failure has occurred and the modulator will have to be replaced. It is very important not to engage the ABS during this test. Low pressure accumulator movement occurs during the ABS cycle.

Type 2: Rear-Wheel ABS "Block Style"

This is the other valve used on light trucks equipped with rear-wheel ABS. It is identified by its block-shaped body. It is important to make sure the valve you have located is the ABS modulator and not the combination valve (See Figure 8). To perform the dump valve diagnosis, follow the steps below:

  1. Locate the low-pressure accumulator cap screw. Remove the rubber cap covering the cap vent.

  2. Insert a straightened paperclip into the cap screw vent hole until it bottoms out, as shown in Figure 9.

  3. Have someone start the vehicle and apply the brakes while slowly increasing brake pedal pressure. The brake pedal must sink to the floor for the test to be accurate.

  4. If the paper clip pushes out as the brake pedal is applied, the dump valve is by-passing and the modulator will have to be replaced.

Confirmation of diagnosis on EBC2 modulators

If the previous steps indicate a by-passing dump valve, the modulator will have to be replaced. Before replacing the modulator, the system should be checked for additional causes of excessive pedal travel. The most effective method to accomplish this is to temporarily "fix" the modulator. This can be accomplished by following the steps below.

  1. Remove the cap screw and low-pressure accumulator spring.

  2. Insert a solid spacer in place of the spring to prevent the low-pressure accumulator piston from moving. Install the cap screw (see Figure 10).

  3. Start the vehicle and check the brake pedal. If pedal height and feel is restored, all that is necessary is modulator replacement. If the pedal is better, but still low, perform a line lock test to determine cause of additional pedal travel. NOTE: The spacers are to be used as a diagnostic technique only. They are never to be left in place.

EBC4 (4WAL)

The EBC4 system was only used on Chevy and GMC vehicles from 1988 to 1995. It is equipped with three dump valves and two low-pressure accumulators. The method used to diagnose a by-passing dump valve will depend on the vehicle application. The mounting location of the EBCM determines whether access to the low-pressure accumulators is possible.

C,K,R,V Series with EBC4

These vehicles mount the EBCM, as shown in Figure 11. This mounting position makes it possible to check the low pressure accumulators by following the steps below:

  1. Remove the six (6) mounting bolts that attach the mounting bracket to the EBCM.

  2. Remove the four (4) mounting bracket to fender well bolts.

  3. Remove the bracket.

  4. Remove the 1-1/4" cap screw of the low-pressure accumulator facing the front of the vehicle (See Figure 11). Remove the low-pressure accumulator spring.

  5. Insert a screwdriver, or similar tool, into the backside of the low-pressure accumulator piston.

  6. Have someone start the vehicle and apply the brake pedal while slowly increasing brake pedal pressure. The brake pedal must sink to the floor for the test to be accurate.

  7. If the low-pressure accumulator piston pushes against the screwdriver as the brake pedal is being applied, the dump valve is by-passing and the EBCM will have to be replaced. If no movement is felt, go to step 8.

  8. Repeat steps 4-7 on the low-pressure accumulator facing the rear of the vehicle.

G Series with EBC4

These vehicles mount the EBCM as shown in Figure 12. This mounting position makes it possible to check the low pressure accumulators by following the steps listed below:

  1. Remove the 1-1/4" cap screw of the low-pressure accumulator facing the left side of the vehicle (See Figure 12). Remove the low-pressure accumulator spring.

  2. Insert a screwdriver or similar tool into the back side of the low-pressure accumulator piston.

  3. Have someone start the vehicle and apply the brake pedal while slowly increasing brake pedal pressure. The brake pedal fade must be duplicated for the test to be accurate.

  4. If the low-pressure accumulator piston pushes against the screwdriver as the brake pedal is being applied, the dump valve is by-passing and the EBCM will have to be replaced. If no movement is felt, go to step 6.

  5. Repeat steps 1-5 on the low-pressure accumulator facing the right side of the vehicle (See Figure 13).

S & T Series with EBC4

These vehicles mount the EBCM as shown in Figure 14. This mounting position makes it impractical to check the low pressure accumulators directly. They have to be checked by a process of elimination. Perform the diagnosis by following the steps below:

  1. Perform a line lock test. If the pedal sinks between 1" to 1-1/2" and then gets firm, go to step 2. If the pedal is high and hard, the problem is at one of more of the wheels. Finish line lock test to determine source.

  2. Remove brake lines at master cylinder. Install plugs or dummy lines and bleed at each outlet. Start the truck and apply brake pedal. If brake pedal is high and hard, the problem is between master cylinder and line locks. This would point to the EBCM as the cause.

EBC310 (GM)

The GM version of the EBC310 mounts the combination valve to the front of the HCU as shown in Figure 15. On C,K,R and V series, it is mounted as shown in Figure 16. On S and T series trucks, it is mounted as shown in Figure 15.

  1. Locate the low-pressure accumulator caps. They will be located on the brake line end of the modulator. On S and T series trucks, remove either of the rubber caps covering the cap vent. On C,K,R and V series, do NOT remove the rubber caps because you will not be able to put one of them back on.

  2. Insert a straightened paperclip into the cap screw vent hole until it bottoms out, as shown in Figure 17. On C,K,R and V series, sharpen the tip of the paperclip and poke the paperclip through the rubber cap. The hole created will be "self sealing".

  3. Have someone start the vehicle and apply the brakes while slowly increasing brake pedal pressure. The brake pedal must sink to the floor for the test to be accurate.

  4. If the paper clip pushes out, the dump valve is by-passing and the EHCU will have to be replaced. If the paper clip does not move, then perform steps 2 and 3 on the other low-pressure accumulator.

EBC310 (Ford)

The Ford EBC310 appears to have three low-pressure accumulators, as shown in Figure 18. These units only have two low-pressure accumulators. They are the center two end caps. To diagnose a by-passing dump valve, follow the same steps as for the GMC EBC310.

EBC410 (Ford)

The Ford EBC410 appears to have four low-pressure accumulators, as shown in Figure 19. This is NOT correct. These units only have two low-pressure accumulators. They are the center two end caps. To diagnose a by-passing dump valve, follow the same steps as for the GMC EBC310.

EBC325 & 430

These modulators are equipped with two low-pressure accumulators, as shown in Figure 20. Use the same tests as outlined above for both the EBC325 and EBC430.

Problems that CANNOT be duplicated in shop:

Dump valve failure can occur on an intermittent basis making diagnosis in the shop difficult, if not impossible. If the failure is intermittent in nature and can only be duplicated on a road test, use the steps below. Two designations will be made to simplify the diagnosis.

Vented Cap Screw Design (EBC2 - Block Style, EBC310, EBC325, EBC410 & EBC430)

  1. Insert the straightened paper clip through the rubber vent cap, as shown in Figure 21, until it bottoms out. On dual accumulator modulators, insert paper clips in both rubber caps.

  2. Use a magic marker or piece of tape to mark the position of the paper clip in reference to the rubber cap as shown in Figure 22.

  3. Test drive the vehicle until the pedal fade is duplicated. If the ABS cycles during the road test, the test will have to started over again. Fluid is supposed to flow into the low pressure accumulator during and ABS event. Push the paperclip(s) back in and road test the vehicle.

  4. Check the position of the paper clip(s). If either paper clip has been pushed out, dump valve failure has occurred and the modulator requires replacement. If no movement has taken place, the source of the pedal fade is not ABS related.

It is important to become familiar with these systems and the procedures used to diagnose them. The reasons for this are due to large population of vehicles equipped with this system, the frequency of failure and the cost of replacement.