Tachometer question

[gobster]

I recently bought a Cajun Bass Boat that the tach wasn't working. I got under the dash today to see if the wires are connected and to the right posts. Everything looked good so I started checking for continuity from the tach to the engine. Everything appeared to be ok until I checked the ground. When I checked to make sure that there was a ground from the engine to the tach, the digital ohmmeter went blank. Checked it a couple times to make sure and it always went blank. Out of curiosity, I checked to see if there was power to the ground and it showed that there was some voltage there. So I connected a wire to the ground post and grounded it to the engine and the tach still didn't work. Don't ask me why (because I don't know), I disconnected the ground at the tach and cranked the engine and the tach seems to be working fine without the ground hooked up. Tried it a couple times to make sure. I haven't had it on the water to check at different throttle settings but will check that later this week if the weather permits. So my question is, is there a danger to damaging the tach or to get inaccurate readings without the ground wire hooked up?
I had read that there should be a dial on the back of the tack to adjust the tach to the engine, but this tach doesn't have that feature. It is a Cajun branded tach and from appearances is the original one to the boat based on the branding on it.

 

[makomark]

...Out of curiosity, I checked to see if there was power to the ground and it showed that there was some voltage there. ...

what does "some voltage" mean?

without a proper power supply connected, it is possible (but not highly likely) to damage a tachometer...using a tachometer without a proper ground is very likely to provide an erroneous output value...

Many times, a malfunctioning tachometer can be remedied by cycling the selector switch a few times...some even use their favorite spray can of water dispersant...

 

[gobster]

Thanks for the response. When I checked for voltage at the ground post (with the ground wire connected), there was about 9.5 volts. Every time that I checked for continuity for the ground post to a good ground, the screen on my meter would go blank. If I disconnected the ground wire from the post and checked just the wire, the continuity showed 0.00 indicating that the ground wire itself was good. So I left the ground off the back of the tach, cranked the engine and it seemed to be reading fine. The back light was even working. There isn't any type of selector switch on the back of the tach to cycle so that isn't an option. I haven't had a chance to take it on the water to see how it does when running down the lake.

 

[makomark]

Be careful making assumptions based on measurements with modern instruments (DVMs), especially continuity ...

a 'low' resistance measurement may be indicated with the DVM but with a modest increase in current (from a light bulb), the same path could have significant resistance....and make the continuity measurements with no voltage or current applied to the circuit...

its very possible the tach is still being grounded thru the back light's wiring....

The meter going blank suggested its input limits have been exceeded....