Bouncing Tach Needle

[gettinthere]

I get intermittent bouncing of the tach needle.
Nothing had been changed prior to this starting. System had been putting out full rating of the alternator.
When the needle starts bouncing, victron battery monitor shows no charging. When the needle stops bouncing, charging starts. This pattern repeats roughly every few minutes. Seems to be much more prevalent at idle and fast idle.
I have a backup voltage regulator, same thing on either regulator. I took the alternator to Battery Shack, Marathon. All diodes tested fine.
I put on a new belt and it's VERY tight, no change. Replaced or double checked all wires/connections. Solid.
Battery bank is around 60 to 80% state of charge.
what am I missing?

[Sailmonkey]

Ours does that when the alternator is in float and the solar or wind kicks in some more juice.

[belizesailor]

Originally Posted by gettinthere

I get intermittent bouncing of the tach needle.
Nothing had been changed prior to this starting. System had been putting out full rating of the alternator.
When the needle starts bouncing, victron battery monitor shows no charging. When the needle stops bouncing, charging starts. This pattern repeats roughly every few minutes. Seems to be much more prevalent at idle and fast idle.
I have a backup voltage regulator, same thing on either regulator. I took the alternator to Battery Shack, Marathon. All diodes tested fine.
I put on a new belt and it's VERY tight, no change. Replaced or double checked all wires/connections. Solid.
Battery bank is around 60 to 80% state of charge.
what am I missing?

My first guess would be the regulator, but swapping regulators should eliminamte this possibility. Same make/model of regulators? My Next Step regulator sometimes causes the tach to misbehave when approaching full charge.

Could be a simple loose/corroded connection somewhere between alt and tach. Or the tach itself is having issues.

Also, don't get the belt too tight -- you can cause excessive bearing wear -- you want some play in the belt. How much depends upon installation specifics, but typically about 10mm total movement (both directions in total) when you apply pressure in the middle of the belt.

[Lowcountry]

My tach bounces when my battery banks are topped off.

[slacker]

I think its pretty normal. When the batteries are really full the regulator goes into float. During float the regulator turns on and off the current to the alternator field. This happens very fast. Since your tach is driven by an impulse from the alternator this manifest itself as a bouncing (on-off-on-off) of the tach needle.

Next time you have an episode of "bouncy tach syndrome" turn on something that draws a big load. This will temporaly force the regulator out of float and should end the bouncy-bounce.

[gettinthere]

Actually this happens more when the state of charge is down. I stated in the OP that I'm at 60 to 75% SOC when this happens. If we motor for a long time, SOC comes up eventually and the bouncing stops.

[GordMay]

Check the ground at the Tachometer/Gauge cluster, or (better yet) run a temporary gnd wire from Tach to Battery Negative.
If there is a pulse dial on the back of the Tach, give it a turn or two, then put it back to your correct setting.

[Nemo55]

Are you running a diesel or gas engine,,,,also,,,make sure you have total 1/2 inch play in the center of your drive belt unless its on its own self tensioner.
Too tight of a belt can lead to failure of the Alternator shaft bearing.
If you are running a gas engine,,i have seen bouncing tack from a bad spark plug lead grounding itself.

[Sailmonkey]

Originally Posted by gettinthere

Actually this happens more when the state of charge is down. I stated in the OP that I'm at 60 to 75% SOC when this happens. If we motor for a long time, SOC comes up eventually and the bouncing stops.

Now that I'm thinking more about it, is there a temp sensor on the alternator? Ours goes on the blink when adjusting current down for temperature.

[gettinthere]

Diesel engine
no temp sensor
Gord has the most compelling solution...will check that soon.

Thanks all

[gettinthere]

Gord, sorry that didn't work either. I ran a jumper directly from tach neg to batt neg.
BUT, that got me looking at the back of my old Det-Mar tach. There is a 4 step adjustment on the back. Just for grins, I switched it from far over one side, 4 steps to the other end. Tach reads about 100rpm's higher now...AND the bouncing needle SEEMS to have been cured! I'll be running the engine several hours tomorrow, we'll see.

[ovrmyhead]

Originally Posted by gettinthere

Gord, sorry that didn't work either. I ran a jumper directly from tach neg to batt neg.
BUT, that got me looking at the back of my old Det-Mar tach. There is a 4 step adjustment on the back. Just for grins, I switched it from far over one side, 4 steps to the other end. Tach reads about 100rpm's higher now...AND the bouncing needle SEEMS to have been cured! I'll be running the engine several hours tomorrow, we'll see.

What make is your tach? Mine is reading low, gonna pull it out tomorrow. Fingers are crossed for something simple like this

[gettinthere]

Det-Mar

[belizesailor]

Originally Posted by gettinthere

Det-Mar

Did the fix prove to be permanent?

[gettinthere]

Define permanent?

The fix worked for the 10 minutes I ran yesterday. I would normally bounce in that time frame.
getting underway this pm for an hour or two...we'll see.