ABS light on gauges erratic
This Chevrolet Cavalier exhibited a speedometer needle that moved up and down, a flashing warning lamp, an ABS light on, and a trouble code for low system voltage. Any time you see erratic behavior in an electrical circuit when you turn on a high power device like a heater blower motor, you should immediately suspect a poor ground somewhere in the circuit. Corroded ground connections can function just fine when there’s very little power flowing through them. But that same corrosion can drop voltage flow by 2-3 volts when you start pump lots of electrons through the rust.
In this case, a voltage drop test conducted on the ground connection with the blower on high showed a 2.2 volt drop. That’s enough of a voltage drop to cause a “low voltage” condition to the instrument cluster and the ABS system.
If you were to rely just on the trouble code (Low system voltage ABS), you’d think you had a bum battery or alternator. After spending hundreds, you’d still have the problem. So clean all ground connections first. Then reset the trouble code and see how things work.