Ok so it seems that the problem is with the Z-drivers. MEasuring the amps is possible, but usually it's just easier to check the Vref, as you already have done. 0.84v itself doesn't tell much, it depends of the stepper drivers chips. For example with A4988 driver, 0.84v can mean either 1.05A or 2.1A, depending of the resistor on the pololu pcb. If the Z-drivers are indeed A4988, try to turn to pot so that the voltage lowers to something like 0.4V. Do it with the printer powered off just to be safe. Try turning a little at the time to see which way to go. On my pololu's counterclockwise lowers the vref.