* Improper Bleed Procedure: ABS systems require a very specific bleeding procedure. Air trapped in the ABS modulator or lines can cause erratic behavior, including wheel lockup. If the system wasn't properly bled using a pressure bleeder (often necessary for ABS) or following the manufacturer's instructions, this is a prime suspect.
* Incorrect ABS Sensor Installation/Gap: The ABS wheel speed sensors need to be installed correctly with the proper air gap to the tone ring on the wheel hub. An incorrect gap, damaged sensor, or a dirty tone ring can prevent the sensor from reading the wheel speed accurately, leading to ABS malfunction and lockup.
* Faulty ABS Modulator/Pump: While you've installed new brakes, the ABS modulator/pump is a complex unit prone to failure. Though less likely if *all* the braking components were new, a faulty modulator, even a new one (though rare), could be sending incorrect signals to the ABS control unit. It's possible a previously damaged component could have been overlooked during the initial work.
* Wiring Issues: Incorrect wiring during installation can disrupt the signals within the ABS system. A damaged wire, a short circuit, or a loose connection can cause misinterpretations of wheel speed, leading to unwanted lockups.
* Master Cylinder Issues: While less likely to cause *only* rear lockup, a faulty master cylinder, especially if the car has a combined master cylinder for front and rear brakes (proportioning valve issues could also cause this), can lead to uneven braking pressure and lockups.
* Brake Proportioning Valve Problems: This valve controls the brake pressure distribution between front and rear brakes. A malfunctioning proportioning valve, particularly if it's separate from the master cylinder, can cause disproportionate braking force to the rear wheels, potentially leading to lockup at low speeds.
* Low Fluid: Ensure the brake fluid level is correct. Low fluid can disrupt the ABS system's operation.
Troubleshooting Steps:
1. Re-bleed the ABS system: This is the first and most crucial step. Consult a repair manual specific to your 1992 Oldsmobile Achieva S for the proper bleeding procedure. A pressure bleeder is highly recommended.
2. Inspect ABS Sensors and Tone Rings: Check for proper sensor installation and gap, clean the tone rings, and inspect the sensors for damage.
3. Check Wiring: Carefully examine all wiring connections related to the ABS system for damage, shorts, or loose connections.
4. Have the system scanned: Take the car to a mechanic with a scan tool capable of reading ABS codes. This will identify any fault codes stored in the ABS control unit, which can pinpoint the problem.
5. Inspect the Master Cylinder and Proportioning Valve: Have a mechanic check these components if the above steps don't solve the issue.
Warning: Driving with a malfunctioning ABS system is unsafe. If you cannot resolve the issue, have a qualified mechanic diagnose and repair the problem before driving the vehicle further. Ignoring the problem could lead to an accident.
