How to test a tachometer

[Mickeyrouse]

I have one and I use it, by Starrett. Knowing what it reads is not the problem. I’m trying to get my tachometer to do that for me.

[Dsanduril]

Catalina Direct has some excerpts from an older Teleflex technical manual on testing tachs and hour meters. Nothing on the combined instruments, but it may be useful with slight adjustments. Check out the 8th page (page 338 in the original document).

[dw1979]

Originally Posted by GordMay

I cannot imagine how that would work. Could you explain?

When I start my engine (FL120) and the engine starts running, there is no response from the tac until oil pressure builds up enough pressure to make the circuit. In an over temp situation the sensor breaks the circuit and drops out the tac. Just another way to grab your attention that something isn’t right.

jp

[Mickeyrouse]

Dsanduril, that is the most helpful thing I’ve seen yet. It puts everyone’s comments- all worthwhile and correct, apparently- in to context.

[tradrockrat]

This is a timely thread! Thanks to all of you who know what you're doing...lol

My tach always read far faster than actual RPM's, but the alternator went bad and I just had it completely rebuilt - It STILL reads fast. The guy who rebuilt it replaced all the guts, converted it to self exciting, and said he tested the tach sender and it read the proper voltage. And yup - tach is still reading way fast. So I'll get cracking on adjustments, checking voltage at the instrument, etc.

Thanks again!

[StoneCrab]

Originally Posted by Mickeyrouse

BTW, the alternator had been recently rebuilt by an alt. shop with Balmar parts.

There you go... problem after a recent rebuild......

Sounds like something wasn't installed properly, part failed or a wire came loose.

I'd still suggest taking the tach and alternator in to a rebuild shop or back to the shop that did the rebuild. They can spin the alternator to test it. If they are willing to test the tach with it, so much the better.

Find a shop that has been in business since prohibition. The rebuilding business isn't thriving in the USA due to cheap parts from overseas, most people just buy an offmarket new unit and turn in the core. This isn't good for the old one man shops that used to rebuild the alternators.

They are still around to service farm equipment and big truck alternnators where the price hasn't come down as much.

[Mickeyrouse]

Sorry, but that’s not the problem. The tach had gone to sleep before alt. re-build. And it is still asleep when I swap my back-up tach in, same make and model. It just didn’t wake it up. Incidentally, Teleflex has mfg’rd to my knowledge at least three methods of driving tachs: the count-the-flywheel-teeth method (also called magnetic), the A/C pulse method ( which mine is) and, really ancient but still seen, direct cable drive, which I had on a Westerbeke 4-107.
Don’t let this bit of tach trivia lull you into thinking I know anything about tachometers. If I did, I’d be sailing instead of doing this.
And I make regular visits to the alt shop, dragging both alternators. Who has been there since God was little.

[Mickeyrouse]

Oops- late nite posting error. Meant to say “ back up alternator”

[Dalestr]

The main thing is to verify what is presented at the tach for the Stator signal. We believe you have an alternator driven Tach signal on your boat and it runs through the Balmar regulator so, using AC volts setting on a DVM, see what is on the signal wire at the tach. You said at one post the signal terminal was connected to IGN+. That would be incorrect. The alternator driven tach connects to Ign+, Gnd, Stator signal, and +Lamp. Four separate connections. In some cases the Ign+ to the voltage regulator may go through an oil pressure switch (this may be true also for the fuel pump). Any chance you can get an o-scope or maybe a scope display type meter connected to the signal terminal (VS. Gnd)? If your DVM has a frequency function you could see that it too increases with RPM.
If you don't see a proper AC pulsed signal at the Tach connection, like 4-10Vac on the meter, then bypass the Vreg entirely and see what you get . Maybe a broken stator wire from vibration or bad connection? You don't need to run this signal through the regulator to operate the tach. Not sure if Balmar reg. effects the signal at all -- might just use it as an input. Some hourmeters in tachs will not increment unless a proper stator signal exists to ensure engine is running. If you want to bench test the tach, you need a DC power supply and a signal generator as a starting point.

[Mickeyrouse]

I appreciate your thoughtful and detailed response. You are right, I mis-spoke about the IGN terminal connection.
I have since checked the stator output from the alternator proper and get only .33 volts (fluctuating) reading on my DVM on the AC scale. Similar reading from the signal output from the charge controller. Since I’m getting output from the alternator, it’s pointing to a connection problem inside the case.