Crazy Tachometer Display

A couple years back I installed a digital OMC tachometer. Yesterday at the lake I noticed some weird numbers on the display while cruising. At low to mid range rpm the tach display is normal, but when I accelerate past 30 mph (about 4,500 rpm) the tachometer starts displaying random numbers. The display refreshes about every second and when it does I will see something like this: 1284, 5296, 3851, 5417, 2149 etc. Of course those are not the actual numbers I am seeing on the display, but you can tell the numbers are completely random and don't make any sense. If I pull back on the throttle to get under 30 mph (somewhere under 4,500 rpm) I notice the display starts to show me reasonable RPM's that seem accurate again. If I try to accelerate the random RPM display returns. It feels like 30 mph and 4,500 RPM is the point where the problem starts on acceleration and where the problem ends on deceleration. The whole time my tach is displaying these random RPM numbers, my motor is running flawlessly. I could not sense a change in the performance of the motor. Any ideas on what is going on here? Do I have a bad tach, bad stator, bad battery, or bad rectifier? Please share your thoughts.

The motor is a 1978 Johnson 140 hp.

Re: Crazy Tachometer Display

Probably a loose or corroded wiring problem. Start by cleaning the engine grounds, then disconnect and clean the connectors in the large red harness plug at the engine. Also be sure that the connections on the tachometer are clean and in good shape.

When the engine is running it does it on its own, without a direct relationship to the forward wiring, battery, tachometer or charging system. The only wires in the ignition harness that directly effects the engine's ignition system when running are the kill wire and ground.

Be sure to disconnect the battery while working on the wiring.

Re: Crazy Tachometer Display

Possible that your rectifier is going bad. The rectifier works off the charging side of the stator, not the ignition side-that's why the engine still runs fine. Erratic tachs often happen when the rectifier quits. It provides pulses to drive the tach and also provides the charge to the battery. You may find your battery is not being recharged properly. If you have a voltmeter, the volts should read 13,14, 15 volts after running it for a while. If the meter shows 12.3, after running, then the rectifier is not recharging it properly.

Re: Crazy Tachometer Display

The first thing I thought of was your rectifier. I am not sure which type you have but check out these two FAQ's to see if you can troubleshoot your rectifier. Or follow your manual.

http://forums.iboats.com/showthread.php?t=190638
http://forums.iboats.com/showthread.php?t=190636

But I agree with ezeke that you should check, clean, and tighten all wiring in the charge and tach circuits. When in doubt, go with the easiest (and sometime obvious) solutions first

Re: Crazy Tachometer Display

No question that you could have a bad rectifier and what usually kills them is bad wiring, bad grounds, bad cables and/or poor contacts.

You can test your unregulated rectifier by disconnecting it and following Joe Reeve's instructions here: http://forums.iboats.com/showthread.php?t=190638