erratic tach

[Jcris]

I've noticed some erratic readings from the instrument panel lately. The tach seems normal at idle but when I'm cruising along at 3000 rpm's it will jump to 4800 or so. Then it will begin to slowly work it's way back to 3000. The voltage meter also doesn't register more than about 11 volts. The fuel gauge registers normally most of the time but will occasionally twitch. I'm not sure if these issues are related or not. These instruments and all senders are original to the boat (21 years old).
I'm going to check the grounds and then the alternator. I'll check the alternator with a DVM at the battery. Looking for battery voltage first and then test with motor running. I'm thinking charge voltage back to battery should be about a 13.5 vdc? The alternator bearings sound good and are not rough. Maybe the voltage regulator is on it's way out? How long do these instruments last? What about the senders? Could they be contributing to these issues. Any troubleshooting tips greatly appreciated

 

[mBurns]

one thing you could try before replacing -- pull the cluster back from the dash and wipe the switch on the back of the tach --- take it all the way clockwise, and all the way counterclockwise a few times. then set it back at your engine (how many cylinders). fixed my erratic tach issues.
while you've got it out, i would also check tightness and general cleanliness of the connections.

if you're seeing 13V on the alternator, its good. finding a good ground may take you some time if you haven't done it before. unlike cars, the "frame" of the boat is not grounded. best of luck

 

[JoLin]

I'm going to check the grounds and then the alternator. I'll check the alternator with a DVM at the battery. Looking for battery voltage first and then test with motor running. I'm thinking charge voltage back to battery should be about a 13.5 vdc? The alternator bearings sound good and are not rough. Maybe the voltage regulator is on it's way out? How long do these instruments last? What about the senders? Could they be contributing to these issues. Any troubleshooting tips greatly appreciated

Make sure all your connections are clean and that you run the engine up to 1200+ rpm to 'excite' (energize) the alternator. Reading at the battery with alternator running should be 13.8-14.8 volts. Period. If lower, you have high resistance in a connection, or the alternator is bad. If higher, your voltage regulator (usually an internal part of the alternator) is shot. Both conditions point to bad connections or bad alternator.

My .02

 

[Jcris]

Thanks guy's
I'll be checking today and will report findings.

 

[Jcris]

Checked the alternator this morning and got a consistent 14.68 vdc with any number of accessories turned on with motor running at approximately 1200 rpm's.
Battery was at 12.79 before starting motor. All the connections to the fuse block, gauges and battery are clean and tight. There are at 6 wires on the ground post of the tach which seems odd but I'm unsure. They are stacked on top of each other which makes it difficult to really tell how good the connections are. Would it be prudent to remove those ground wires and run them to the ground buss on the fuss block? What would be best?
Also, the volt meter/gauge on the dash shows 12 vdc when the alternator is showing 14.68 vdc to the battery. How accurate can I expect that gauge to be?