Bypassing dump valve in modulator allowing fluid into the low-pressure accumulator causes low pedal on vehicles equipped with Kelsey Hayes EBC310/EBC325/EBC410 ABS systems. The EBC310/EBC325 system is on GM, Ford, and some Dodge light trucks and SUVs. The EBC410 system is on the Ford Windstar. There are slight differences in the external look of the units, as shown in Figures 45.1 and 45.2. The main difference we are concerned with for the dump valve diagnosis is the number of rubber caps. The modulators in question can be equipped with 2, 3, or 4 rubber caps, as shown in Figures 45.1, 45.2, and 45.3. Understanding where the low-pressure accumulators are is key to accurate testing. The rubber caps marked with arrows are the low-pressure accumulators. To perform the dump valve diagnosis, follow the steps below:
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1. Locate the low-pressure accumulator caps. You find them on the brake line end of the modulator. Remove either of the rubber caps covering the cap vent.
2. Insert a straightened paper clip into the cap screw vent hole until it bottoms out.
3. Have someone start the vehicle and apply the brakes while slowly increasing brake pedal pressure. NOTE: Be sure to duplicate the fade for the test to be accurate.
4. If the paper clip pushes out, the dump valve bypasses, you’ll need to replace the modulator. If the paper clip does not move, perform steps 2 and 3 on the other low-pressure accumulator.
When it comes to intermittent low brake pedals on vehicles equipped with Delco VI ABS (See Figure 46.1), the low brake comes from EMB (Electromagnetic brake) or ESB (Expansion spring brake) not holding the motor in homed position.
Solution: This condition is usually not straightforward to diagnose. The primary reason for this is its intermittent nature. The most effective method of diagnosing it will be to look for supporting symptoms of the condition. The symptoms that will support the EMB or ESB as the cause of the low pedal will be:
1. ABS light on with either a code 38 (left front), code 41 (right front), or a code 42 (rear brake circuit).
2. Problem occurs only ONCE per drive cycle.
3. The occurrence of the problem should be timed with the ABS light coming on.
4. There should be an underhood noise associated with the occurrence as the piston causes the gears to the wind.
You can also diagnose it by looking for the symptoms that don’t support the EMB or ESB brakes as being the cause but do support the master cylinder as the cause:
1. If you can duplicate the problem with each pump of the brake pedal or more than once without resetting the ABS, the problem is the master cylinder.
2. If the problem occurs without a code, noise from the modulator/motor pack, the problem is the master cylinder.
ABS failures rarely cause conventional brake problems, but it is wise to be aware of those that can. The lack of this knowledge can lead to many hours of wasted effort and frustration. While most technicians are familiar with the RWAL/RABS dump valve causing excessive pedal travel, few are aware of a similar problem with the Delco VI. The Delco VI is the most common ABS in use today based on the number of units in the field. It is also unique in how it modulates the brake pressure during an ABS stop.
Instead of solenoid sets and a pump motor assembly, the Delco VI uses motor-driven pistons (Figure 46.2). The pistons are driven up and down by high-speed bidirectional motors to modulate the brake pressure during an ABS stop. During standard braking, the pistons are held in their uppermost or “homed” position by two types of motor brake assemblies. The motor brakes prevent the pressure in the system from pushing the pistons down during standard braking.
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