Need Tachometer Troubleshooting Help

[lonnie]

I have a 1989 Volvo Penta AQ171. The Tachometer starts at Zero RPM with the motor Off, When I start the motor the Tach goes to 6000+ (even when Idling. When running the boat The tach bounces around between 4000 and 6000+. I've looked at the wiring diagram in the owners manual and there is a Tachometer Sending unit bolted on the engine block with 2 wires that go to an electronic ignition module. Out of the electronic ignition module are wires of which some go up to the Tach.

My questions is, Can someone help me trouble shoot this so I can determine if the Sender, the electronic ignition, Tach or wiring are at fault. Or is there any of these components that are known to usually go bad? I have a Volt meter. But I'm not sure if the output from any of these is a voltage.

Thanks for your help.

[namu mac]

Look at the back of your tach . If it is a multi cyl tach there will be a switch to choose between 4-6-8 cyl & 2 cycle applications. This switch can be changed by using a small screwdriver. Somtimes corrosion can bridge between the different contacts. If so use a screwdriver & change the setting back & forth-this will free up the contacts- set switch to proper setting & see if it works. If so you can take the tach out & solder between the correct contacts thus elinating the corrosion problem. Good Luck

[lonnie]

Thanks for the advice, I tried cleaning contacts at Tach end. Does anyone know what kind Of readings I can take from the 2 wires going into the tach? Should there be a votlate? There doesn't seem to be. Which is confusing to me as Something is making the tach move.

Thanks

[Orca]

Generally there will be a voltage tick everytime a spark plug is fired. It will be a square wave of pulses, and you will need an 0-scope (or a tach :grin:) to see it. Many auto multi-meters have built in tach functionality, so one of these may help you. Or you may be able to find someone with a scope to help you out.

The tach is responding to an average voltage per unit of time. If you have an old analog voltmeter with a sweeping needle, you may be able to see something, where your digital meter never will respond.