Tach gives erratic reading

[Harlan]

99% of the time my tach reads fine but on rare occasion it will suddenly jump up and down. Motor is at 2000 rpm +/-, tach jumps up to maybe 3000, down to 1500, back up - after a minute or so reads fine again and stays there. Engine is Universal M25 with original Motorola alt. (Tach is alternator take off.) All connections including grounds have been checked and are clean and solid. Belt is tight (slipping couldn't happen this fast nor cause higher reading anyway.) Voltmeter reads steady even while this is happening.

Any thoughts?

 

[LeslieTroyer]

I’d check all the wires for corrosion -

If it only dropped occasionally it might be belt slip, but because it jumps up also I suspect an electrical gremlin, if wires are ok I’d suspect the tach.

Les

 

[capta]

I agree with Les. If it was more consistent, I'd suspect a diode in the alternator, but yours sounds much more like a connection problem.

 

[agprice22]

Wires can draw corrosion up into them while the connections still look clean. 1/2” of corrosion between the strands can cause significant problems.
Maybe try making some test wires, and see if the problem repeats. I have many test cables ready for times when I cannot figure out a connection or electrical behavior problem. They make troubleshooting easier.

 

[Ken Cross]

What kind of tach do you have?
If you have a flywheel pickup, the pickup or the tach could be an issue. Put a meter on the tach input at the tach and look for AC voltage. If it's low it could cause this. Possibly someone else can chime in about the correct levels, but as I remember it should be a couple volts. If it's an alternator pickup, I suspect the instrument.

Ken

 

[capta]

What kind of tach do you have?
If you have a flywheel pickup, the pickup or the tach could be an issue. Put a meter on the tach input at the tach and look for AC voltage. If it's low it could cause this. Possibly someone else can chime in about the correct levels, but as I remember it should be a couple volts. If it's an alternator pickup, I suspect the instrument.

Ken

"(Tach is alternator take off.)" from original post.

 

[jssailem]

@capta certainly by now you would know, no one really reads the messages here. They skim them then draw conclusions. Then they type their ideas on to the screen. Sometimes the do not even bother to read them to see if the autocorret has repaired their typing foibles correctly.
But it is all ok... We are sailing folk. We can take it or leave it. Sail a little off course, then tack back hoping for the wind in the right spot to take us to our desires.

 

[justsomeguy]

...no one really reads the messages here.

Ayup.

 

[Michael Davis]

no one really reads the messages

Reading this one carefully John. Sun Gypsy has the same problem. I have a new Tach. on board but am now thinking I'll replace the 34 year old wiring as well. Something else to fill my time this winter!

 

[jssailem]

After your adventure I would think rest is in order along with a keg of fine beer.

And your computer to follow SBO through the winter doldrums.

 

[Ken Cross]

After your adventure I would think rest is in order along with a keg of fine beer.

And your computer to follow SBO through the winter doldrums.

Winder doldrums? It's gorgeous out there today. I just came back from a 5-mile kayak and got too much sun.
I guess it depends on where you live.

Ken

 

[Michael Davis]

I guess it depends on where you live.

I'm with you Ken. Just waiting for out injector pump to be rebuilt then we'll bring the boat home from Ucluelet. The winter job list is getting longer and there is still a couple of things to finish up from last year but with this weather we'll continue sailing 'til it changes.
If I get a whole keg I may need some help John. Wouldn't want it to go flat!
SBO is a given!

 

[LeslieTroyer]

Beer I like beer

Les

 

[Displacesailor]

I had that problems years ago with the same type engine on a thirty footer. Problem went away when we replace the battery cable because it was too short to easily remove for winter storage?