System Test

Check that all in-line fuses or circuit breakers are operational and supply power. With control switch turned "ON", measure voltage of Purple wire at rear window grids. There should be approximately 12-13 volts, if not check wiring harness, control switch or relay timer.
Connect negative lead of voltmeter to ground. At midpoint of grid lines voltage should be approximately six volts. If voltage is 12-13 volts, there is a break between midpoint and ground. If voltage is zero, there is a break between midpoint and input side of grid lines.
Disconnect switch leads and check that it is switching properly. If switch is operating normally, install a new relay timer and check operation of system. If system now operates normally, replace relay timer.