'00 VMAX 150 OX analog tach problem

I recently purchased a boat with a 2000 VMAX 150 OX on the back. The tach worked fine when I purchased the rig. I cleaned the nasty boat carpeting up (with a pressure washer) but didn't spray down any of the gauges or anything. I got the dinged up prop rebuilt and some other minor stuff, but nothing to do with the dash/gauges. On the last trip out the tach is reading way high. At 1/2 throttle it's pegged out at 7,000 RPM. Any ideas?

[boats.net]

Probably a bad tach.

Verifying bad tach

Is there a way to verify the tach is bad or good? I have a volt/ohm meter and have access to a digital one as well. Thanks.

[boats.net]

You check voltage(AC voltage) on green wire going to tach to see if voltage is steady.

all tachs work the same way, they count the number of pulses then divide that number by 1 for a 4 cylinder, 1.5 for a 6 and 2 for an 8. even a few drops of water can carry voltage and it sounds like you're jumping between two of the contact points. Try drying everything with a hair dryer

Thanks for all the tips!

Just wanted to say how much I appreciate all the helpful folks on the board. It's great to find a board where people are knowledgeable and doing what they can to help others. I'm going to try drying the tack and I am assuming there are probably some little switches somewhere on the back so the same tach can be used on different engines. I may try switching them to another position and back as well.