Is my bounc­ing tacho ter­mi­nal?

Canal Boat - - Back Cabin -

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.

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