trouble down below?

higgs · Apr 16, 2010

Is it just the tach dropping or is the engine, too.

Stu Jackson · Apr 16, 2010

check the wiring at the back of the alternator and the alternator's ground to the engine

also sounds like a slipping belt

Stu Jackson · Apr 16, 2010

OK, but if it continues. The next step is to check the alternator itself, especially the bearings - easy check, just remove the belt and try to spin. Obvious conclusions from that simple test! Good luck and happy hunting.

rpwillia · Apr 16, 2010

Do an RTFM

Most of the tach use the alternator to determine the engine speed. Check your engine manual to be sure, then if it happens again you have a way to troubleshoot. It could also be the connector (as noted above) where it is touching, but not falling all the way off.

sailingdog · Apr 17, 2010

There is no coil on a diesel engine.. it doesn't use spark plugs...

thereisabelle said:
I know i'm not the sharpest tool in the shed.. in my younger days doing tune-ups on cars the tach and dwell meter hooked to the coil and ground. this allowed setting the angle for correct point gap and showed the engine rpm. now i'm dealing with deisel.does this change the route or origin of the circut? all manuals on the beta are onboard.. 2 hr. drive from the think tank. I would be there now but the plant sale is this weekend. thnks

druid · Apr 17, 2010

All good stuff here. One other thing: if the batteries are fully charged, the alternator basically "kicks out" - puts out little or no output. So the Tach is left with no input. Could be the regulator is pretty much shutting down the alternator and the tach isn't getting enough signal. Nothing much you can do about it - that's just the way alternator-based tachs work.

druid

Stu Jackson · Apr 17, 2010

thereisabelle said:
all manuals on the beta are onboard.. 2 hr. drive from the think tank.

Good luck on the plant sale. These days of the internet and online manuals, there's pretty much no more excuse for "the manual's on the boat."

I think it's pretty important to build up an electronic and hard copy library. I don't have a computer on my boat, nor a laptop (eegads, how does this fellow live???) but have found almost everything I need in terms of manuals on the internet.

Sorry about the soapbox, but this has come up for me in responding to questions quite a few times just recently.

I simply recommend that whenever you get something new for your boats, by all means keep the manuals onboard for use, but either copy or download that material for use at home. Better yet, keep the originals at home and copies on the boat.

Dan Johnson · Apr 17, 2010

Stu...

since it's not that practical to make copies of all the manuals for all the systems on the boat, I keep mine aboard inside quart size Zip-Lock bags to prevent any water absorption and mildew formation. I use a bigger bag when needed for something bigger than a booklet size.

PS: you up and around and back to sailing yet?

Bill Roosa · Apr 18, 2010

tachometer 101

There is only one 'type' of tach. It measures frequency fluctuations represented by voltage changes in the sensing circuit. You have to provide a source of fluctuating frequency of proper voltage level to get it to operate.
The fluctuating voltage can be supplied by;
a) a dedicated tack sender which sends 1-2 volt AC current to the tack
b) the alternator unrectified output terminal commonly called "tack" or "T" or something similar usually next to the regulator input.
c) the low side of the ignition coil (na for diesel engines)

Typical output is 1-2 volts alternating current.

Problems and how to fix
for a) it either works or don't. I've never seen one that is intermittent. Intermittent is a wiring problem. If it does not work at all check the magnet on the flywheel (it needs to be there) check the output of the sender with a multimeter set to lowest vlotage setting on AC there should be some output. Check for the same output at the tack unit. Check for corroded terminals. 1-2 volts is easly sucked up by one corroded terminal
for b) you have to make sure the alternator is operating properly first. If some of the diodes are bad it will still operate at lowere amp output but make your tack read lower that actual RPMs. A sliping belt will show as a veriation in speed when there is none as the regulator turns the alternator on and off. There should always be enough voltage residual to drive the tach circuit though. If the alternator checks out, then test for an AC current between the tach terminal on the alternator and ground (not the regulator terminal right next to it!!!!) It will read 12 volts DC but you are not interested in the DC component, switch to AC and make sure there is a 1-2 volt AC ripple voltage on top of the 12 volt DC charging voltage The alternator would have to be in operation for this to happen so you will need to fabricate a plug that allows the regulator to control the alternator and you to probe the tack terminal. If you get a signal test for the same signal at the tack unit. Check the terminals for corrosion. The pulley size has to be accounted for on new instlations also. There is a frequency shift feature on most tacks to account for the alternaotr RPM being a multiple of the engine PRM due to different sized pulleys.
c) is right out.

I've heard several folks imply that the alternator shuts off and the tack will not work when the batteries are charged. It is my experience that the residual magnetism in the alternator rotor is sufficient to produce a 1-2 volt ripple in the 12 volt output even when the requlator is not suppling current to the rotor. The tack should not drop out just due to the regulator turning off the alternator unless you ran the engine for a very long time with the regulator off. Like a year or two.
Inverter wiring near or in the same circuit can also cause tachs to act up. They produce AC which can be picked up inductively (wires to close together) or through backfeeding into the DC system. Try turning the inverter off and see if that fixes the problem. OR try turning it on and seeing if it causes a problem.