Erratic tachomether

[M0bi0us0ne]

My tachometer started being erratic, especially over 3k. It wildly swings, but the actual engine rpm stays constant. I cleaned all the ground contacts, but it did not help. The alternator is an Arco Marine with an internal regulator, and it is about 5 years old. The battery charges fine, and I have no issues starting the boat up. I have attached short video
View attachment IMG_1500.MOV
Any suggestions on how to fix this?

Thanks

 

[alldodge]

Your trim gauge is doing the same thing
Are any of the other gauges doing the same?

 

[flashback]

Time to start cleaning electrical connections.

 

[Redrig]

Time to start cleaning electrical connections.

Agreed. I had this happen a few years ago on my boat as well. I just spent an hour cleaning up every ground that I could find. I don't know exactly which one fixed it, but after cleaning them all up, that problem went away.

 

[airshot]

Years back, my tach started acting up, giving eratic readings. It was suggested I look for the pole switch on the back, mark the current setting, then using a screw driver move the pole setting screw back and forth to clean the inside contacts. It appears some light corrosion can build up and cause eratic readings. After some serious twisting back and forth, my teach read more steady. Be sure to turn it back to the original setting where you started. Took care of my issue, might be worth a try for you.

 

[M0bi0us0ne]

Your trim gauge is doing the same thing
Are any of the other gauges doing the same?

That’s the fuel gauge, the are no baffles in the tank so it jumps around a bit if there are waves. Everything else works as expected

 

[M0bi0us0ne]

Agreed. I had this happen a few years ago on my boat as well. I just spent an hour cleaning up every ground that I could find. I don't know exactly which one fixed it, but after cleaning them all up, that problem went away.

I have already cleaned all the ground contacts I could find. No change

 

[kd4pbs]

There's probably a switch on the back which is changed for the number of cylinders the engine has. Exercise that switch.

 

[Redrig]

How is your actual ground cable ? The big one from engine block to battery post ? Those can get corroded big time. Especially when auto style cable is used.

When this happens does the rpm and engine noise change at all, even in the slightest ? When mine was happening , not only were the gauges going crazy , but my volts would peg high and surging , my blower motor was noticably louder as it was happening.

I have already cleaned all the ground contacts I could find. No change

 

[airshot]

There's probably a switch on the back which is changed for the number of cylinders the engine has. Exercise that switch.

There are contacts inside that you can't get to that can corrode. By moving that back and forth vigorisly will help clean those contacts. Be sure you return to the original contact when you started. That worked on my tach when it started reading wonky.....

 

[dingbat]

I have already cleaned all the ground contacts I could find. No change

It take two (wires) to tango

 

[M0bi0us0ne]

There's probably a switch on the back which is changed for the number of cylinders the engine has. Exercise that switch.

Done that, no luck

 

[M0bi0us0ne]

How is your actual ground cable ? The big one from engine block to battery post ? Those can get corroded big time. Especially when auto style cable is used.

When this happens does the rpm and engine noise change at all, even in the slightest ? When mine was happening , not only were the gauges going crazy , but my volts would peg high and surging , my blower motor was noticably louder as it was happening.

The rpm do not change and it is the only instrument acting up. The cable are silicon sleeves and tinned. Changed them a couple of years ago they visually look good. The boat is on a fresh water lake so not that much corrosion happening in general.

 

[dingbat]

Surprised everyone just assumes it's a cable problem.
Could very well be something wrong with where/how the signal is generated.

If this was an outboard, I'd say change the voltage regulator.

Where is the tach signal generated on this particular motor?
Ignition module?
Coil?

 

[M0bi0us0ne]

Surprised everyone just assumes it's a cable problem.
Could very well be something wrong with where/how the signal is generated.

If this was an outboard, I'd say change the voltage regulator.

Where is the tach signal generated on this particular motor?
Ignition module?
Coil?

No clue, I replaced the ignition coil, and that did not help. The VReg is built in the alternator, which is relatively new.
Could it be a problem with the distributor?

 

[alldodge]

Your trim gauge is doing the same thing
Are any of the other gauges doing the same?

Fuel or trim same question, the fuel gauge jumps around similar to the tach, not like fuel is sloshing back n forth

Could it be a problem with the distributor?

Doubt it's the Dizzy
Take a hand held tach and connect it to the gray wire feeding the boats tach

 

[Redrig]

Complete longshot here. Do you have points ?

I had defective point once and it made my boat barely run and my tach was all over the place.

Basically one of the rivets in the points came came lose and was wobbling around on the arm. This is a video I made once I finally found out the problem . A 2 dollar part that ruined a vacation.

 

[M0bi0us0ne]

Complete longshot here. Do you have points ?

I had defective point once and it made my boat barely run and my tach was all over the place.

Basically one of the rivets in the points came came lose and was wobbling around on the arm. This is a video I made once I finally found out the problem . A 2 dollar part that ruined a vacation.

I do have a point, I will check it

 

[M0bi0us0ne]

Crazy idea, is it possible that after 50+ years, my tach simply broke?
How do I open it? There are two screws in the back, but they simply release the mechanism inside. I can't figure out how to open the casing. Is it sealed or glued?

 

[M0bi0us0ne]

Fuel or trim same question, the fuel gauge jumps around similar to the tach, not like fuel is sloshing back n forth


Doubt it's the Dizzy
Take a hand held tach and connect it to the gray wire feeding the boats tach

Trust me that is normal for the fuel gauge especially past half tank. If there are no waves and the boat does not move it stays perfectly stable