Excellent explanation. I can see now why corrosion free terminals are so important. It's very good to know the tach to coil connection may be at the ignition switch rather than at the coil. Clearly this would be another place to address the tach problem.
Thanx On Sep 15, 1:57 am, waterguy <[email protected]> wrote: > Cleaning all the contacts on the backs of your gauges and all the > contacts on sending units (fuel tank, water temp, oil press) can make > your instruments so much more reliable. Since most indicator gauges > are basically ohmmeters, measuring resistance, you don't want any > resistance at the terminals due to corrosion, since that will cause > the gauges to read false. > > An electric tach on a gasoline engine is a pulse voltmeter, reading > pulses from the coil. As each cylinder fires, the pulse drives the > tach needle upward against spring pressure, which pushes the needle > back -- this is a minute movement. As the engine speed increases, the > pulses come closer together, and the spring has less time between > upward bounces of the indicator needle to push it back down, thus > causing the needle to rise. If there is corrosion on the terminals, > the signal weakens, giving an inaccurately low reading. > > When I bought my boat, I found that if I set the throttles so the > tachs read exactly the same, the Judson synchronizer lamp indicated > that the engines were out-of-synch -- which was confirmed by the > pulsing woo-woo-woo-woo sound that out-of-synch engines make. > Cleaning the tach input connections, ground connections and coil > connections cured the problem. > > Diesel tachs are either pulse voltmeters (like the gas engine tach > described above) driven from the alternator or pure voltmeters driven > by a sending unit which is just a small generator driven by the engine > like on the Detroit Diesels. > > One little tip for gas engine boats with electric tachs (like most of > ours): since the tach is wired into the ignition system, a major > short to ground in the tach will fail the ignition by grounding the > coil. This little devil will drive you out of your mind: You'll have > an engine stall or no-start situation, you'll check for spark, > nothing. Aha, you'll say, I have an ignition system problem. You'll > pull the leads off the coil, check it for continuity and resistance. > It'll have continuity and resistance, so you'll figure your coil is > fine and you can look elsewhere for the problem. You'll reattach the > coil leads - one of which is the shorted-to-ground tach lead. > > Now you're really off to the races. You'll spend hours trying to > figure why you have no spark, replacing ballast resistors, points, > condensers (or electronic ignition modules), and on and on and on. > You'll spend days and dollars trying to diagnose it. You'll pull the > distributor out, take it in to an auto electric shop that has a > distributor testing machine, and hand them $50 or so to diagnose it, > only to have them tell you it's fine (which it is). Meanwhile the > problem is that short inside the tach grounding out the coil. > > Also, the connection from the coil to the tachometer may not be at the > coil, it may be made at the coil terminal on the ignition switch at > the helm. Best way to eliminate a shorted coil as a possible cause of > ignition failure is to check for resistance between the positive > terminal and the ground terminal on the tach. If there's no > resistance, the gauge is shorted and so is the coil. > > Limp-home mode if this happens at sea: disconnect the tach positive > lead from wherever it takes the signal (at the coil or at the > switch). ONLY if you can't find where the wire terminates on the > signal side, disconnect the positive lead from the tach, tape up the > terminal fitting real well, and secure the wire it to something non- > metallic so it can't flop around behind your instrument panel. Don't > disconnect the negative lead and leave the positive connected; > sometimes instruments will acquire an unintended ground through their > mounting bracket being in touch with something conductive, and if the > acquired ground is high resistance, you'll end up heating the circuit, > possibly causing a fire. > > Reason I mention this is that checking for resistance in the tach is > not one of the usual things suggested in troubleshooting ignition > failures, nor is it where most of us would likely look. Now y'all > know. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "UnifliteWorld" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/unifliteworld?hl=en -~----------~----~----~----~------~----~------~--~---
