Why is my AIS not working, and how can I troubleshoot it?
My AIS powers up, but I sometimes lose all the targets on my chartplotter, and another boat recently said they couldn't see me. How do I check whether the fault is in the AIS unit, the aerial or the connection to the plotter?
Answer
Most AIS problems stem from a poor power supply, no GPS fix, silent mode, an aerial fault or a broken data connection to the chartplotter. First establish whether the unit has stopped receiving, stopped transmitting or is failing to display targets, as each symptom calls for different checks.
Identify the AIS fault
Start at the AIS unit. Record its status lights and exact alarm messages before restarting it. Their meanings vary between models, so consult the unit's manual.
- No lights: check the supply, fuse and negative return.
- Targets visible on the AIS unit's own display or in diagnostic software, but absent from the plotter: check the data connection and plotter settings.
- Receiving targets but other boats cannot see you: establish whether you have a receiver or a transponder, then check silent mode, GPS status and transmission alarms.
- Neither receiving nor transmitting: check power and the shared VHF aerial path before looking for separate faults.
A receive-only AIS cannot broadcast your position. To test transmission, ask a nearby boat to look for your MMSI on its onboard AIS while you are underway. Allow several minutes for reports. Internet tracking websites have coverage gaps and delays, so your absence from a website does not prove a fault.
Check power and transmission settings
Measure voltage at the AIS power connector with the equipment switched on. A reading at the battery alone is not enough. On a nominal 12 V installation, a substantially lower reading at the unit indicates resistance in the wiring, fuse holder, switch or negative connection. Compare the reading with the manufacturer's permitted supply range.
Look for loose terminals, green corrosion and an inline fuse hidden behind the instrument panel. Use the specified fuse rating. If the unit restarts when other equipment operates, investigate voltage drop before replacing it.
A Class B transponder normally needs its own valid GNSS position before transmitting. A position on the chartplotter does not establish that the AIS has a fix. Check the AIS satellite or position-status page, GPS aerial connector and aerial location, particularly beneath metal or carbon-fibre structures.
Confirm that silent mode or a wired transmit-disable switch is off and that the correct MMSI has been programmed. MMSI changes often require a dealer or manufacturer reset. Class B reporting intervals depend on equipment type and vessel speed, so transmission is not continuous.
Inspect the aerial and splitter
Follow the VHF coaxial cable from the AIS to the aerial. Look for loose plugs, wet connectors, crushed cable and corrosion where the cable enters deck fittings. Water inside coax can seriously reduce range even when the outer jacket looks sound.