ABS Accumulators
ABS accumulators come in two types, those implemented on integral ABS systems and those sometimes used with non-integral ABS systems. There's a big difference between the two in both design and the amount of pressure accumulated by the accumulator. The integral unit includes an electric pump that provides high pressure power assist as well as provide pressure for the hold-release-reapply cycle. The non-integral ABS systems sometimes use low pressure, 50 PSI spring loaded accumulators to provide pressure for the hold-release-reapply cycle.
An integral ABS unit contains the master cylinder, modulator, electric pump, and accumulator in a single modular assembly. The electric pump is operated by a relay that receives a signal from the ABS control module. These pumps create pressure for the hold-release-reapply cycle and usually provide pressure for brake assist as well. They store the accumulated pressure in the accumulator.
The integral high pressure accumulator has a low pressure and a high pressure switch. These switches 
allow the ABS control module to keep the pressure between a set parameter. The parameters vary, usually between 1000 PSI and 1600 PSI, when pressures get too low the control module will illuminate the amber ABS light. Some accumulators reach pressures as high as 2700 PSI. They utilize a nitrogen gas charge that is separated from the pressurized hydraulic fluid by a thick rubber diaphragm. When filled with pressurized fluid from the pump, these accumulators can reach dangerous pressures and must be handled with care. They must be depressurized by pulling the relay and pumping the brake pedal 25-40 times before any repairs can be performed. Always check manufacturer service manuals and TSB's before repairing any ABS system.
Most of today's vehicles use non-integral units. These units may contain a low pressure spring loaded accumulator. These accumulators hold pressure during the hold-release-reapply cycle and usually do not have to be depressurized before service. Always check manufacturer service manuals and TSB's before troubleshooting on any ABS system.
