Summary

The four LEDs on the front of the JK-1MD/MED or JK-1HD blink right to left and then back, from left to right, in a rotating pattern.

Cause

This indicates a short between the A1-A2 (door station input/output), or the B1-B2 (sub-station input/output) wiring.

Solution

Check for loose, disconnected, or incorrectly polarized wires. A bench test can also prove this by providing a short run of fresh wire, in a controlled environment, between each piece of equipment.

The LEDs in question are located in the highlighted area of the master station, found below.