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My digital speed gauge has quit working, thus preventing the miles per gallon from working on the fuel gauge. What does the speed gauge read to determine the mph and distance traveled. I have twin 150 HPDI's. Is there a lot of possible problems, or would a replacement of the gauge itself likely cure the problem. I know this is not much to go from, but it is all I have. Thanks.
What are the symptoms, ie does it read 0 or is the guage blank? If it's blank, look for a power problem (wiring or fuse). If it's reading 0, the sensor on the engine is probably plugged. I hear this is a common problem, although it hasn't happened to me yet. You can also rewire the system to use your GPS as the speed source.
The gauge intermittantly reads a speed, say 18 mph, but it does not vary, it just stays there. But also, it may just have two dashes where the speed would be read.
I've had this happen a few times on my 250HPDI and my old 200 Yamaha. Usually the problem is the tiny hole on the front side of the lower unit is clogged. Run some monofilament in there and see if this does not clear up your problem. For a permanent solution, if you have a GPS with an NMEA output, this can be wired to the Yamaha Fuel Management Gage. Do a search on this board or the Electronics Board for wiring instructions.
I have similar issues with my new 4 stroke Yamahas. The Yamaha speedo reads about 10% below my GPS also. The Yamaha is a pitot tube type inference of speed, like most boat speedos, and is subject to plugging as mentioned already. I plan to hook my GPS to the speedo/mpg guage, when I figure out how. If someone could provide a link to the proper thread with instructions, I would appreciate it. I searched and could not find it. I got some wiring instructions from Yamaha, but they are lacking.
If it is reading anything you first might want to check and see if the GPS is set for Knots or Miles per hour. Then check your Yam gauge for the same. Most boats have the boat speedo set for Miles per hour from the factory and most GPSs default to Knots per hour. So that is about a 10% difference with the GPS reading knots and the boat speedo reading MPH. I don't worry about the Yam speedo on my boat, sometimes it works great, and even seems really acurate, but then it might not work for weeks. I use the GPS for my speed. It depends on what concerns you. Do you want to know how fast your covering the waters surface or do you want to know how fast your covering ground and how long it is going to take to get to your destination. There usually is a difference depending on current conditions.
I would think it makes more sense to have the Fuel Managment Gage hooked up to the GPS anyway. If you're looking at mpg for range calcutations it's going to be way more accurate (Assuming of course you boat where there is some tide). GPH or Gallons Used obviously wouldn't make any differece.
That said mine's still plugged into the speedo because I hate working under theo console where the wiring is.
Yamaha Fuel Management System Gauge 
