I have 2001 that intermittently has issues with the tachometer not working and the depth/speed constantly resetting and going into demo mode. Sometimes I start the engine and it works great, sometimes it works for a little while then the tach craps out and the depth/speed goes into demo mode, sometimes neither of them work from the get go. Definitely think this is probably just a grounding issue, but this is my first season owning the boat and I want to understand what I’m looking at. Since the issue seems to include both the tach and depth/speed, I think it’s part of the larger electrical bridle that both these units connect to. Does that terminate at the battery to check for ground? What should I look for regarding corrosion/junk ground lines? Thanks in advance!
It’s a 2005 Honda BF50A engine, the depth finder is the old Standard Horizon MD-100, and it’s a physical dial tach like what you see in a car. While they are old, and possibly original to the boat, they both do function… sometimes!
The tach on the BF50A is just reading a signal from the #2 pulser coil. You probably have the Honda OEM tach. That eliminates all NMEA2000 cabling and networking issues that you might have had with a BF50D.
IIRC the depth and speed signal on the MD-100 are coming from a transducer and a wheel-driven speed log mounted on the transom. The cable is separate from the wiring that connects the remote control to the engine, although they probably follow the same path from the pedestal to the engine well. The problem is probably not in that wire. Again, IIRC it will be a large, round (possibly coax) cable with an additional wire in or bundled with it (for the speed log).
Ideally a new pair of 12v wires would have been run from the fuse block to power the MD-100 but the installer probably tapped into the 12V and ground wires in the Honda harness that were already in the pedestal. Take the smoked plastic cover off the port side of the pedestal and trace the wires coming off the MD-100. Three will go to the transducer and two will be going somewhere else. The two wires will be the + (should be red with an inline fuse) and - (black) that power the display. I suspect you will find that they are tied into the grey(?) and black wires going to the back of the tachometer. The black wire should be ground and IIRC the grey wire is pulling 12V off the rectifier. Look for a loose or corroded connection in one or both of those connections. My money is on the ground wire.
Good luck.
Bill
2001 26X Simple Interest
Honda BF40D
"If I were in a hurry I would not have bought a sailboat." Me
Just remembered something else to check. I think there are a couple of other (small) wires coming off the MD-100 that are probably not attached to anything. They are for the NMEA0183 network that is probably not connected in your boat. Just make sure that none of them are shorting out.
Bill
2001 26X Simple Interest
Honda BF40D
"If I were in a hurry I would not have bought a sailboat." Me
that intermittently has issues with the tachometer not working and the depth/speed constantly resetting and going into demo mode. Sometimes I start the engine and it works great, sometimes it works for a little while then the tach craps out and the depth/speed goes into demo mode, sometimes neither of them work from the get go. Definitely think this is probably just a grounding issue, but this is my first season owning the boat and I want to understand what I’m looking at. Since the issue seems to include both the tach and depth/speed, I think it’s part of the larger 
electrical bridle 
