I designed fuel indication systems for automotive in the past but not boats but I believe they are similar. There are two basic designs, bi-metal and air-core. The bi-metal is the cheapest and may be what you have. The sender in the tank changes resistance with the fuel level and changes the current going through the gauge bi-metal. The varying current causes varying heat that bends the bimetal. The needle is attached to the bi-metal and moves accordingly. You can usually tell if it's a bi-metal by the angle of display. If it's 45 degrees or less then it's probably bi-metal. A bi-metal system also requires a regulator of some kind to control the nominal heat, typically another bi-metal that alternates opening/closing to produce a square wave whose effective voltage can be adjusted by tightening or loosening the preload on the bi-metal element. The regulator is usually what fails in these systems. Shorting or opening the sender will move the needle but it will take a little while and will only work if the regulator hasn't already failed. The other system, air-core, is similar to the meters used in an analog VOM. It has a permanent magnet attached to the pointer shaft and then two coils wound 90 degrees to each other around the magnet. The varying resistance of the sender changes the current through the coils causing a change in the magnetic field and thus the position of the pointer (the pointer is the resultant vector). These type of gauges require no regulation and react rather quickly when the sender is grounded or open. These systems are usually have 90 degrees of movement or display. In any of these, the ground is always suspect. The tank is metal but the sender is usually mounted on a rubber seal. This insulates the sender base. If the ground is not connected to the sender base, the ground path is broken. Grounds also have the nasty habit of corroding. Hopefully my long winded description will lead you to the proper solution. --- John Nall wrote: > Yeah, I know -- this is really off-topic. And I apologise, But > there are > a lot of very knowledgeable people on this list, and perhaps one of > them > will be kind enough to give me a general answer. > > A couple of years ago, I had a new outboard motor installed on my old > power > boat. The installation included an instrument panel, and said panel > included a fuel gauge. It was never hooked up, however -- merely a > standard thing that was included. > > The other day, I had to have a leak repaired in the fuel tank, and I > asked > the guy to put in a new sending unit and hook it up to the fuel gauge > for > me. I picked the boat up today, and the fuel gauge does not budge, > although the tank is about 3/4 full. (The guy who put in the sending > unit > said he would be glad to put it in, but that he didn't know anything > about > fuel gauges, and if it didn't work it would be my problem). > > How would I go about trouble shooting the fuel gauge? The sending > unit > itself is merely a float inside the tank, with a wire going to the > gauge, > isn't it? I would assume that as the float moves up or down, there > is a > varying resistance. And I would assume that the fuel gauge itself is > merely an amp meter with the current varying as the resistance of the > sending unit changes. I further assume that one side of the meter > would go > to ground, while the other side would be hot (going through the > resistance). So the whole thing should be able to be checked, using > merely > a multimeter. > > Are these assumptions correct? Or am I missing something vital? > > Thanks for any tips, > John > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. ===== Chuck Hellebuyck Electronic Products chuck@elproducts.com **** Home of the BasicBoard Development Platform********* Great for learning, teaching and developing electronic, programmable robotics and other embedded applications. Starter packages from $149.95. http://www.elproducts.com __________________________________ Do you Yahoo!? SBC Yahoo! DSL - Now only $29.95 per month! http://sbc.yahoo.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.