Tach not working....


Member III
My tachometer was previously working a few weeks ago. Engine runs fine, but tach needle dances all over the place. It's not a huge priority to fix because I know how the engine sounds at cruising RPM, but it would be nice to get it working again. Can anyone give me pointers on what to check first ? Is there a sensor somewhere on the shaft or engine ?

Frank Langer

1984 Ericson 30+, Nanaimo, BC
Whenever electronics start misbehaving, the first thing I check is the groundwire connection. I would start there, then check any other connections, including the possibility of a wire chafing somewhere.


Sustaining Member
The tach signal wire comes off the alternator. It’s a small (16-18g) wire. Mine’s grey and original, but check the wiring diagram in one of the ‘Remove trailer plug’ threads here. I have a second small wire for my external voltage regulator, the ‘Field’, which exits my non-stock alternator at the same point. I had the two switched on their alternator plugs after some wiring work - no tach AND no alternator output. The only other alternator wire is the BIG red output cable.

Kenneth K

Sustaining Member
Blogs Author
The alternator signal to the tach is a single gray wire (like Mark said). But the Tach gauge itself has its own (+) and (-) leads on the back side that are fed from the engine panel. The gauge (+) and (-) leads don't come directly off the trailer connectors but are usually daisy-chained from the posts of other instruments on the back of panel (this daisy-chaining makes the leads prone to more than one possible loose connection). The quickest way to troubleshoot the gauge ground may be to temporarily string a new ground wire between the tach GND and the engine block. If that fixes the problem, THEN start working your way through the chain of connection behind the panel.

20170116_181819.2.jpg Tach signal wire

20190319_195711.jpg Gauge "Pos" (= IGN), "Gnd" and Signal posts. You can also see the Range/calibration adjustment screws Christian mentioned.

20180628_181835.jpg Daisy-chained wires

If re-doing the engine panel (in whole or in part) it can be helpful to collect all the grounds from the panel onto a ground bus bar. Here, the gauges, panel lights, blower, and panel accessories (temp and pressure monitors, bilge warning light) each have their own path to the ground bus. This makes future troubleshooting much easier.

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