posted 02-17-2013 09:26 AM ET (US)        

It is sometimes reported that certain data being monitored on a NMEA-2000 network becomes unavailable on an intermittent basis. One likely cause for this data loss is the device providing the data becomes unavailable on the network. A reasonable cause for a device becoming unavailable on a NMEA-2000 is from loss of electrical connection to the network. The cause of the break in electrical connection of a NMEA-2000 device to its network may be due to a number of circumstances. Without sophisticated and typically difficult to obtain electronic monitoring equipment, the average boater cannot perform much diagnostic testing. There is a simple explanation for loss of electrical connection to the network: a physical discontinuity in the wiring. Because in typical NMEA-2000 network wiring all the electrical connections are made as part of multi-pin prefabricated wiring appliances, a reasonable place to look more closely for a problem is in those wiring appliances.

METHOD FOR LOCATING INTERMITTENT NETWORK WIRING APPLIANCES

To check the cables, connectors, and Network-T wiring appliances associated with a device whose data is becoming lost intermittently, operate the network and the device in their normal modes. Observe the data from the device on a display. Firmly hold in one hand the various wiring appliances associated with the device whose data is intermittently lost, and using the other hand, grasp one of wiring appliances and slowly wiggle it. Be certain to hold the other components motionless when you wiggle the one component. For cables, slowly manipulate the cable in your fingers, flexing and bending, while moving slowly along the length of the cable. While doing this, keep all other wiring devices as motionless as possible. For cable connectors, hold the wiring appliance they connect to as motionless as possible, while slowing rocking or wiggling the connector from the device or the connector on either side of network T-connector. While doing this, observe the display of the data from the device and watch for any data loss. If there is an intermittent connection in one of the wiring appliances, you may be able to discover the wiring appliances with the bad connection using this method.

As an adjunct to the wiggle-test, all wiring in the network must be given a close visual inspection. Check all cables for signs of damage from abrasion, particularly cables that connect to mechanical devices that have motion or vibration, such as a network cable that connects to an outboard engine data source. Check all cables that pass through openings in bulkheads or cowlings for signs of wear and abrasion. In outboard engines, check the path of the cable for proper clearance and avoidance of other electrical cables that carry high-voltage, such as ignition spark wiring to spark plugs. Check that network cables are tightly bonded to other cables that carry strong electrical signals, such as coaxial transmission lines for radios, cables to SONAR transducers, or cables carrying high currents such as primary power distribution cables, particularly to high current devices like electrical motors.

If the wiggle test and close visual inspection do not reveal any particular wiring appliance defect or abrasion, proceed to test further by substitution. Move all the wiring appliances associated with the device whose data is being lost intermittently to another data source which shows no loss. For example, if on a boat with twin outboard engines with one engine showing intermittent loss of data, rearrange the wiring devices so that the only change is in the engines. Resume normal operation. Continue to observe for loss of data. If data loss reoccurs, evaluate the situation as follows:

--if the intermittent data continues to come from the original device showing the problem, the source of the problem is likely in that device;

--if the intermittent data now comes from the new device to which the suspect wiring has been moved, the source of the problem is likely in the wiring appliances that were moved.

If the substitution test indicates a problem in the wiring, repeat the diagnostic wiggle test, concentrating on the suspect wiring appliances. Or, replace wiring devices one at a time in that segment of the network wiring, continue operation, and monitor for data loss. As each wiring appliance is replaced, and if the loss of data stops occurring, then the wiring appliance you last replaced is likely to be the cause.

To verify that you have found the actual cause, reinstall the suspect wiring appliance. Repeat the wiggle test, concentrating on that appliance. If this does not reveal a defect, then leave the wiring alone. Return to normal operation, Monitor for data loss. If data loss returns, you very likely have correctly identified the source as the suspect wiring appliance you just reinstalled. Replace the suspect wiring appliance and continue operation. If no further data loss occurs, you have remedied the problem.

Locating intermittent wiring can be difficult, time consuming, and frustrating. I had a situation of intermittent connection in my boat's NMEA-2000 network. It took me half a day of very carefully wiggling and manipulating wiring appliances until I located the actual cause of the problem. After replacing that defective wiring appliance, the network returned to normal and reliable operation.

If you cannot locate a source of the problem in the wiring appliances, and if the substitution test shows the problem remains with the original device, these results indicate a problem in network device itself. Further testing requires use of sophisticated electronics that can monitor and record the data stream coming from the suspected device. Most boaters will not own or will not have access test equipment of this sophistication. It is unlikely that more detailed diagnostic testing of the suspected device can be done with rudimentary methods. At this point, the suspect device may have to be taken to a qualified service technician for further analysis and repair.