Is my bouncing tacho terminal?
QLast year I bought a well maintained narrowboat with a BMC 1.8 engine, Adverc battery management system and a Victron inverter. I think the alternator is a Lucas 12v 120amp with twin pulleys. I recently broke a terminal off the wire leading to the alternator, this caused the rev counter to bounce all over the place, but when I located the problem and reconnected the wire to the alternator the bouncing needle behaved properly for the week we were out. I then reconnected to mains electric hook up.
A fortnight later the tacho again started to “bounce” around, this time in a more pulsing fashion. The aforementioned wire was still connected and seemed to make no difference to the pulsing of the tacho needle. In this instance not long after we had left the marina, subsequently it happened after 3-plus hours of cruising. The problem only seems to occur when the leisure battery indicator suggests that the batteries are or are approaching 100 percent full. Sometimes the pulsing resolves itself other times it seems to persist. However I put a load on the system by running the microwave or water pump to see what would happen and the needle again settled down, sometimes for the duration that the load was applied and sometimes after the microwave/ water pump had stopped.
If I stop the engine for a while during a “pulsing” episode and run a tap etc. the problem does not restart immediately the engine is restarted, but will do after a while.
I went over to the boat yesterday and replaced the lead that I had previously broken and repaired. This time it was ratchet crimped and had a heat-resisting sleeve on the crimp. The terminal it went on was labelled D+, it was the only wire from that terminal. A heftier cable and a smaller cable come off the B+ terminal and there is a thin wire coming from a terminal labelled W. It ran for 45 mins or so after I changed the lead, it finished topping up the batteries from the 98.8 percent where I had left them on Monday (which didn’t take long) and just when I thought , problem solved the tacho started to bounce in time to a pulsing/surging type of sound quite distinct from the usual rhythmic diesel noise. During this episode which lasted perhaps 10 minutes or so I pulled the multiplug off the Adverc and it made no difference to the tacho’s bouncing. I was watching the control panel when suddenly there was a flash from the green light (not the Adverc one) that comes on with the start up and the bouncing and pulsing noise ceased. It restarted again after a further 12 minutes or so (can’t be sure) and lasted again for 5 mins (almost exactly). I didn’t see the light flash this time.
I hope I have provided all the details necessary. 1 starter battery & 4 x 120ah leisure batteries. Please let me know if I have missed off anything obvious/essential for a diagnosis. JOHN BARSBY
ATONY REPLIES: There are two thin wires connected to the alternator. A rev counter pulse wire connected to a terminal often marked W and the warning lamp wire connected to D+. B+ and if fitted B- are the main charging terminals and have very thick wires on them. If you knocked the D+ (warning lamp) wire off then the warning lamp would not come on unless it shorted to some metal and the alternator would not energise. If the alternator does not energise then the rev counter would read zero, not bounce. If the W (rev counter) terminal fell off then the rev counter would simply not work. If you disconnected the B+ or Bcable while the engine was running then you may have damaged the alternator. If the engine was stationary nothing could happen apart from possibly lots of sparks from the B+ wire. I suspect the detached wire was a simple coincidence.
I have seen and experienced this sort of thing when solar charging is involved when the engine is running. If the solar controller is delivering a higher charging voltage than the alternator then the alternator can keep shutting down and starting up again. Sometimes the warning buzzer also quietly chirps away to itself. However as you do not mention solar we must look elsewhere.
The D+ takes no further part in alternator operation once it starts charging.
The Adverc will not start working until the charging voltage reaches a certain threshold and a certain time has elapsed. This is what you found when you unplugged it - it was not working and the alternator was controlling itself. The flash from the light suggests a voltage spike from the alternator and the noise suggests the alternator – hence the engine going on and off load.
The noise could be from the alternator belt. If this is the case it may well cause the rev counter to pulse. However the green warning lamp is often for oil pressure. Charge warning lamps are usually red.
This is looking more and more like an alternator fault, possibly worn bushes or slip rings. So if you have a local starter and alternator repair shop start to think about taking it to get tested – but first you need to clean the mating surfaces of all the battery terminals, ensure all the charging system terminals are properly fitted to the cables and clean and tight. Also make sure the alternator belt is tight enough and the belt is in good condition.
If you have a large multi-plug on the main engine wiring harness pull it apart and make sure all the connections are clean and tight.
I do not see how knocking the warning lamp or rev counter wires off an alternator can damage the alternator unless in doing so you shorted one to metal while it was still connected to the alternator. Even then the alternator would have to be running and charging to do any damage.