Tach slow/alternator not producing

[starfish62]

Odd problem. A mechanic put a meter on the engine, and at 1,000 RPMs tach reading the engine was turning 1,000. At 1,200 Tach the engine was turning about 1,500. At 1,900 tach the engine was turning 2,870. The (50 amp, single-wire) alternator was putting out about 12-14 amps when the engine was rev'd up. Ground fault? Any ideas?

[NoTies]

The alternator will only be putting out as many amps as is necessary so this may not be a problem. Alternator driven tahcos should be calibrated at cruising revs not at idle so this may also not be a problem. All that being said if I ever need to take an alternator off for any reason I always get it bench tested & cleaned/checked.

[never monday]

The alternator counts the AC sign wave peaks and applies a multiple to it to give you a reading. On the back of the tach there should be a calibration pot. Set it for max rated RPM, if you can achieve it. and live with how it falls out.

[senormechanico]

Obvious question. Is the belt slipping?

[Alan Wheeler]

Obvious question. Is the belt slipping?

Good comment. Obviouse but so often overlooked.
Also is the belt in good condition.

[starfish62]

Alternator is brand new, as is the belt. Belt is properly tensioned and not slipping. I can yank the alt & get it bench tested, but it is literally brand new (less than 10 hours on it).

[Lancerbye]

The alternator driven tachs are an analog device that that have to be calibrated to the alternator. The output of a single wire alternator is rectified and regulated before leaving the alternator. The tach works on a small voltage output change between 12 to 13.8 volts. If you just changed the alternator. Is the new one a higher output type with a smaller pulley diameter? The problem with the Tach is a span calibration error. If you had a true digital type that counted pulses from a magnetic sensor the accuracy would be greater and voltage fluctuations would not affect it.

[Alan Wheeler]

OK, so if this is new, anychance you have fitted a smart charging device to the Alternator???

[starfish62]

Hey Wheels, just signed the acceptance papers Thursday. Don't really know what the mechanic did before we started looking at it. The regulator is internal, and I haven't changed anything there (going to, but haven't, yet). What puzzles me is that the tach is accurate at low rpms & then is off by 30% at max, and the alternator isn't putting out, even though it's new.

[senormechanico]

How about a proper ground from the alternator case?

Also, does the Little Mermaid wear an algebra?

[starfish62]

Hey SM, I'll check that. Would that affect the tach?

[Lancerbye]

If you have a high resistance connection anywhere, be it the grounding or the signal, the voltage drop across that resistive connection will increase as the signal increases taking away from the signal input causing an increasing error.

[senormechanico]

Just got back from taking company out for a sail. Whooeeee! Had to reef with people not sailors in the cockpit, but we did ok. Instantly got 23 knots apparent as soon as we got out of the jetty doing ten knots.

Short steep 5' chop on the way home, but did almost 8 knots closehauled through it. Waves were breaking across all 3 hulls and we did some launches off the waves as well, but the company loved it! They must think that's normal sailing.


Where was I? Oh yeah, the ground connection has to be good or the alternator will think the batteries are charged. The field will not have any excitation voltage, hence no magnetic field for the tach to read either. At least not much. Just make sure the negative connection (which MAY be the case, but maybe not) is grounded.

Steve B.

[starfish62]

I can't get on the boat till tomorrow, but will check the ground & hope that's it. Hopefully the ground's just junked up & some sandpaper & CRC can remedy things.

S/M: 10 knots! Nice work. What are you sailing? At 10 I'd be surfing/out of control and very nervous, but I have a cruiser mindset.

[senormechanico]

starfish,

It's a Dragonfly 1000 folding trimaran. It's done low twenty's with the original owner, but I've never had the nerve to go over 15 or so.

Dragonfly - Quorning Boat

Steve B.