1984 Hunter 31 Tachometer not working.

[plealman5]

I'll begin by saying that the P.O. did a lot of questionable wiring on this boat; wiring with extension cords & wiring most circuits/ equipment directly to the battery. The motor instrument panel has several issues. The PO seems to have by-passed the key switch. The fan switch doesn.t seem to work, and the tachometer doesn't work. What troubleshooting things should I start with to get my tach & other items working?
Paul L.

 

[dlochner]

What engine do you have? And do you have the shop manual for it?

 

[plealman5]

A Yanmar 2GM ? F. No Manual. I bought the boat "well used" in 2021, from someone who was given the boat for a debt owed.

 

[dlochner]

Take a look at the Owner's Resource section on the SBO landing page. There are downloads by brand and model and a general downloads list. I'm pretty certain there is a manual somewhere in there.

Downloads

Downloads

sbo.sailboatowners.com

 

[kloudie1]

As Dave says, Lots of great info in Owners Resources.. The Yanmar tach on the GM series is notoriously inaccurate and intermittent ..
The tach reads the number of teeth on the flywheel and calculates RPM .. The wiring connections between the tach and the pickup are usually corroded .. Attached is the tach pickup spec and stuff..

 

[dlochner]

As Dave says, Lots of great info in Owners Resources.. The Yanmar tach on the GM series is notoriously inaccurate and intermittent ..
The tach reads the number of teeth on the flywheel and calculates RPM .. The wiring connections between the tach and the pickup are usually corroded .. Attached is the tach pickup spec and stuff..

It is the same with my 3JH2E, the tach will magically start working and then sort of work and then not work. While Yanmar engines have a well deserved good reputation, the same can not be said for their wiring harnesses.

In addition to the Tach issue, the wiring for the starter solenoid is also a known problem. The issue is voltage drop in the circuit due to small wire size, age and corrosion. By passing the key and going directly to the start switch may have been the PO's attempt to deal with that problem. I took a different approach and directly tapped the main DC+ bus to run power to the key switch and start switch and then to the solenoid. The factory wiring gets power from the starter DC+ cable connection, then to the key switch, and back to the solenoid.

If the idiot lights and fan do not work the likely cause is no power to the key switch. The key switch also powers the idiot lights, night light on the tach, and the fan, if there is not power going to the key switch none of those will work.

 

[plealman5]

Thank you for the info.

 

[Indysailor]

I'd probably focus first on dealing with the stuff that can cause fires, and in the process it will probably lead you through the tach problem. But just to get started on that, start at the gauge and follow pos/ neg wiring, tightening and cleaning connections to eliminate continuity as the culprit.

I think generally on the back of the gauge you should find a power wire to ignition, a ground, maybe an illumination, then a signal (green?) to the alternator. This is a guess, figuring it's an older setup.

 

[marcham]

A couple things.... the tach is either off a magnetic sensor or off the flywheel (most likely) or off the alternator.

The magnetic flywheel sensors need periodic cleaning (I do mine every 2 years) and require very accurate adjustment.

The wiring behind the tach can be prone to corrosion, make sure all the pins are clean and test that you have a good ground (0 Ohm).

The tach needs 12v, do you have 12v at the tach?

Do you have a fuse at the panel? some do, some don't.

The engine wiring harness is copper strands. After about 30 years, it gets stiff and brittle from the heat and the vibrations, and you get micro breaks all along the wiring. When it gets to 30, it's time to replace. Marine tinned wire is more flexible and will last longer. Buy from Yanmar for about $500, or make your own for about $350 in wiring and connectors.

Start at the engine, then test the panel (grounds, 12V), then the wiring harness. You'll need one of these (20 ft) to test the grounds (and a good quality multimeter):

Amazon.ca

The service manual for the GM series is available online. It has an excellent wiring diagram, easy to follow. Everything is color coded. Try the tach on a buddy's boat with the same engine. This could be a 20 minute job or a 3 day job....

You don't buy the boat, you buy the previous owner's mistakes

 

[SFS]

Check all your connections. My tach stopped working one day, with no other failures. I assumed I'd have to buy a new one. A day later, I'm looking at the engine, and notice a terminal post up high somewhere (can't recall where exactly, but I'm thinking on the block, rather than the alternator). The ring terminal at the top of the stack of connectors on this post seemed to be just slightly off horizontal. I reached up, and the nut securing all the connectors was pretty loose. I tightened it, and bang, I had a tach back. Never gave me another problem.