Troubleshooting ABS Modulator Mechanical Problems - Page 2 of 2

Troubleshooting ABS Modulator Mechanical Problems

Mechanical issues with the ABS modulator, or hydraulic control units (HCU), are rare, but they can happen. With more vehicles on the roads with older ABS systems that have not seen proper brake fluid maintenance, mechanical issues are becoming more common.


Since pressure from the master cylinder has been bled off, the pump in the ABS modulator will spool up and apply pressure. The outlet valve is closed and the inlet valve is opened. The pump applies pressure to the wheel. If the wheel is still outside the wheel slip parameters, the cycle will start over. This happens very quickly. The operation of the solenoids and pump will cause a “kick back” or pulsation in the pedal.

The copper ions have attached to this valve
The copper ions have attached to this valve

Diagnostic Modes

If the inlet/isolation valve is stuck open, it will not affect normal braking in any way; it will only hurt the ABS system. This could lead to a pulling condition during ABS activation. If an outlet/dump valve is stuck open in one circuit, this could cause a pull condition during normal braking. This is due to the loss of brake pressure at a wheel.

If more than one valve leaks or is not able to seat, it could cause a longer than normal stop if the ABS is activated.

This isolation valve was unable to seal
This isolation valve was unable to seal


Sometimes a stuck or defective solenoid or pump will set a code. This code will typically deactivate the ABS system. Testing the unit with a scan tool with bi-directional control might be the best way to confirm the condition of the ABS modulator.

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