Tachometer Troubles

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Jon Bastien

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Jun 3, 2004
71
Hunter 290 Tampa, FL
Hello all,

I've been having some issues with my tachometer since I bought the boat. It's a Yanmar 2GM20F with about 350 hours; there is a tachometer sender in the bell housing- Not off the alternator.

For the first three days we owned the boat, the tach worked just fine; Then, it quit working altogether (although the hour-meter still works just fine). Wiggle wires, poke around a little, re-seat connections... and now the tach SORTA works. WHEN it feels like working, it reads about half what I estimate the actual RPM to be. (At idle, it displays about 400. At cruising speed, it shows about 1100 for a few seconds... and then quits working.)

SO, I decided today was a good day to try a few tests.

First, I used my ohm meter to verify that the sensor wire had good connectivity- No obvious issues there, even when wiggling connectors.

Next, bypass the entire sensor wire with a jumper wire straight from the sensor to the back of the tach. No change in readings; still showing idle at 400 and approximate 'cruise' at 1100; thus, I've eliminated the 'sensor' wire as a possibility. Only other wires going to tach are Power and Ground, and they both have good connectivity.

Unfortunately, my multimeter won't do KHz (Cycles), so I couldn't try to determine if the sensor is working...

Does anyone have any ideas what I should try next? Does this seem like a bad sensor, or a bad display?

Thanks in advance,
--Jon
 
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Jon Bastien

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Jun 3, 2004
71
Hunter 290 Tampa, FL
OK, so I grabbed the multimeter and started poking around the tach sensor...

Resistance across the two terminals on the sensor is 1.540 ohms; AC Voltage from the sensor varies between 3.9v and 5.5v when the engine is running (dependent on RPM). According to everything I can find, these are within the normal range-- Eliminating the sensor as the culprit.

This leaves the gauge and the wiring...

After looking at the diagram, I think I may be more confused than I was when I started. :doh: :D

Apparently, on the Hunter 290 the designers elected to use a custom, Hunter-specific instrument panel with a custom tachometer that doesn't really correspond directly to the Yanmar wiring diagrams. The closest approximation is the Yanmar 'B' panel, but the wiring diagram for that looks like it may have errors:

- From the Tach Sensor, there are two wires- Orange and Blue/Red. (This matches what is on my engine as well.)

- The Orange wire appears to be the sensor wire, and connects more-or-less directly to the tachometer 'Sensor' stud. This matches the wiring diagram.

- The Blue/Red wire is where things get confusing. This wire, according to the diagram, ALSO connects to a stud on the back of the tachometer... but there isn't a stud on the back of the gauge to connect this wire to. It also, according to the diagram, appears to connect to the Orange wire on the path to the tach- in effect, shorting them out (I think this part of the diagram is incorrect).

On the boat, the Blue/Red wire seems to be a ground- a continuity test between the wire at the tach sensor and an engine mounting stud indicate a clear path... When trying to trace the physical wire, it gets lost in a loom, so I just assumed it was supposed to be that way (although I don't know why anything screwed directly into the engine block would need a ground reference wire...).

Starting to think the Blue/Red wire isn't supposed to be a ground... but I'm not sure where to find that information, or where that wire may actually need to go.

Anyone got any ideas?

--Jon
 
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Don Lucas

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Jan 12, 2011
930
Hunter 410 full time cruiser
On my last boat the tach stopped working right and I tranced it down to a corroded connection in the wire harness (over time 50% of the connection in that connected got jumped out).

On my current boat the tach works good until the engine runs a while and I'm convinced that it is the tach itself.
 
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