Alternator and Tach mystery

[Dean Abramson]

We have had a few occurrences when at low revs, the engine runs unevenly, and the tach needle jumps all around. Jumps around really fast and erratically.

Sometimes when this has happened, this scenario occurs: I am charging Batt1, but then when I switch to Batt 2, the problem disappears. Or vice versa.

Then there was this. We were motoring along normally, at around 2400 rpm. A slight breeze came up, so I stopped the boat so I could better feel where the wind was coming from, in hopes of sailing. To stop the boat, I used some reverse. Decided there was not enough true wind after all. Put the boat in forward, and then, even though the tach said we were at 2400 again, the boat was slow and seemed to be laboring, and really, the revs did not seem as high as it showed. (I am pretty in tune with what 2400 rpm feels/sounds like on this boat.) We lost more than a knot of speed compared to what we'd had. At low revs, the tach jumped around again. (I cannot remember if I tried having the batt switch in the other position. Btw, I always charge 1 or 2, never Both.) I decided that when I'd stopped the boat, we must have wrapped some kelp around the prop. (There was lots floating around.) Proceeded to an anchorage. Dove on the prop, and there was signifigant kelp, but less than I imagined; but maybe when I stopped the boat there, a bunch fell off, before I had a chance to see it. Anyway, I cleared the prop. Started up again. Tach jumped around at low revs until I switched to charging other Batt. Then boat, tach, motor, all felt right and we went full speed again.

The tach condition is clearly affected by the charging load. Does the tach measure actual engine RPM, or somehow deduces it based on what the alternator is doing? I realize that the kelp-and-prop thing MAY be a red herring and unrelated, but... hell, I am stumped by this. By the way, the ammeter on the engine panel has NEVER worked; however, I have always good charging of my batts (2 12V AGMs), and that continues. We just came back from 37 days out, and we never had charging issues per se. We stayed stationery for three days at a time without any charging, without having solar panels or a wind-turbine, without being particularly power-frugal. We have no refrigeration, but we do have pressure water, a stereo, and read a lot at night with old-fashioned bulbs.

Any thoughts on this? Why would our tach act this way sometimes? This is new this year. We just changed the alternator belt; that made no difference.

???

Dean

[JWSutcliffe]

Dean:

The tach is connected to the alternator AC output before it is converted to DC by the diodes. Assuming you still have the Universal M25 and standard wiring harness, an erratic tach reading could be due to a bad connection somewhere between the alternator TACH post and the alternator (i.e. the crummy wiring harness connectors), a bad ground connection, or bad brushes in the alternator. If your alternator has not been overhauled in some years it may be time. I had my standard alternator done last year at a local auto electric repair shop for $100.

[JWSutcliffe]

Look here for the wiring diagram:

http://www.marinedieseldirect.com/unive ... agram.html

[Dean Abramson]

Sailed today. Checked both batteries on volt gauge before starting engine; both showing over 12 volts. Started engine on Batt1. Tach was happy. Switched to Batt2. Tach jumped all around again. Did that only when switch set to Batt2. (So I just went back to Batt1.)

How can charging a different battery cause the tach to behave differently? It would seem to me if this is a tach or alternator issue, it should not matter which batt we are charging.

Totally confused.

Dean

[JWSutcliffe]

Dean:

As a mechanical engineer, my electrical knowledge is less than precise. However, I had read that the amount of current being passed by worn alternator brushes will have an effect on the quality of the AC waveform. It may be that one of your batteries is more fully charged than the other, so after starting on battery 1 it took more charging current than battery 2. I normally have both batteries on charge at the slip, so I never noticed a difference. I do know that before having my alternator rebuilt last year I occasionally noticed some tach fluctuations, and now have none of that. One of my projects for this winter will be to completely rewire the whole mess and finally eliminate the connectors and frequent starting problems associated with that. For more useful information, read Mainesail's website on the pitfalls (and dangers) of the now aging Universal M25 wiring harness.

Good luck with it.

[Dean Abramson]

Thanks, Skip. I know way less about electricity than you, but that sounds like an explanation.

On your advice, I have taken a look at the Maine Sail article. Very interesting. I have an unconnected orange wire hanging near the alternator. I think it is the one that used to connect to the ammeter. My ammeter has never worked. I am thinking maybe someone saw the danger in the original wiring, and "fixed" it by simply cutting out the ammeter.

I appreciate your input.

Dean

[Phil Shedd]

Dean

First I would check to see what the voltage is at each battery while under power . This should be very near 14-15 volts if the batteries are charged. There may be a voltage difference with the # 2 battery if so it may be starting to get weak but still hold a charge. This means it will show 12 volts but will not give a great deal of current (amp hours) . A load test would show this .

http://www.harborfreight.com/100-amp-6- ... -9191.html

The ammeter should show something when charging unless you think the charge wire off the alternator goes directly to the positive wire on the starter. You should check this out. I would check this out just so you will know. I not sure if we talked about this but I installed a ground wire from the alternator to the batteries this year . This made a big difference in charge rates. I think connection between the engine block and alternator was not that good because of rust on the bracket.

Hope this helps . Coming out next Thursday hope to get out on Saturday and Sunday

Phil

[John M]

When you switch batteries and the tach reads correctly does the engine smooth out as well?

[Dean Abramson]

John, that's a very good question, and the answer is yes.

Since I made the original post, I have spent a lot of time with a friend of mine who is both a marine surveyor and a university professor in a scientific field. He really knows electricity.

My friend says that the most likely scenario was as follows:
When this behavior occurred, the first battery I tried to charge was already fully charged. The voltage regulator senses this, and cuts off power to the charging circuit. After a tiny fraction of a second, the voltage regulator (if I understand my friend correctly) tries again; cuts off again; tries again; cuts off again. And so forth. This sends erratic pulses to the charging circuit and alternator, doing two things: 1) the tach is going on and off, super rapidly; hence the jumping needle; 2) Likewise, the alternator is being turned off and on, and this makes the engine run rough, as the load on the belt keeps fluctuating.

I tend to believe this theory now, because the oddness happened only a few times, and looking back, I think that it is very likely that since I normally use Batt 1 only for starting, and then let it charge for 20 minutes or so before switching to Batt 2; Batt 1 was fully charged when I started, and caused the issue. (Maybe the last time I had charged Batt 1, I had inadvertently let it charge longer than usual.) At other times I might have fully charged Batt 2, then noticed the issue on that side.

I am now pretty sure that the kelp issue was unrelated in reality, but that particular day I had the voltage-regulator thing happening AND a bunch of kelp on the prop, so the two conspired to make the engine run particularly rough and slow us down. We had motored all day, and I think the offending battery must have been all topped up.

The boat has been put to bed for the winter. If I get more data in the spring, I will report back. Most of the weirdness occurred on the last day of our season, so I have had no chance to test this new theory. But it seems right to me.

Dean