Rpm gauge

planesailing

I have got a strange issue with my VDO RPM gauge... when engine is in neutral the RPM  gauge works perfectly and rises as I rev the engine, however when i put the boat in gear it works if I'm going below 5 mph but as soon as i increase the throttle the gauge either stops working or bounces all over the place. Any ideas?!?

Stodge

I'd check the back of the gauge for a switch with 4-6-8 on it.  That's for setting the number of cylinders.  Switch it back and forth several times and see if that helps.  Otherwise you'll need to find where the tach is connected to the engine and make sure that connection is good.  

planesailing

Thanks @Stodge! I'll try pulling the gauge and checking the cylinder setting tomorrow. Am I right in thinking that the RPM gauge gets its reading from the alternator??? Volvo Penta 5.7 GS 2001 model. Gauges are VDO

Stodge

It should come from the distributor.  And what I meant was not just check for the correct setting of the switch,but move it back and forth.  They can get a little corrosion inside.  I'd double check the wires or plug going to the tach as well.  

planesailing

Yes I knew what you meant by recycling the cylinder setting on the tach. Will double check all connections there and will check distributor to see if that's where it's getting its reading from. The thing that baffles me is it works fine in neutral but just plays up when in gear. Strange!

Stodge

more vibration at higher RPM might make a poor connection fail.