rpm gauge need to be replaced or zero'd?

[TThornhill]

hey all, i think i have an issue with my rpm gauge. i have a mercury 150 o/b on a 22' deck boat. according to my rpm gauge when im going about 15 mph steady, im sitting around 3000 rpm, i would think at that speed i should be below that. also when i trim up it can make an almost 1000 rpm addition . also at idle it sits a tad below 1000, and when i cut the motor off it still sits at 1000. anyone know why it might do this??

 

[Fireman431]

Re: rpm gauge need to be replaced or zero'd?

When you turn the key to the ON position, but don't start the motor, does the tach 'zero out'? Many tachs, especially teleflex tachs, stay at the last RPM input when you cut the key.

Is the tach the original for the boat? Has the previous owner replaced it? Is it the proper tach for an O/B setup? Looking at the model number will give you the answer you need. If it is the proper tach, make sure the dial switch on the back is at the correct setting. It is based on 1/2 the number of pulses from your engine's ignition system.

Check the make & model and go to their web site for a manual. Many also have a calibration port, but you need a tiny standard screwdriver to calibrate it. Watch for the terminal while doing it, they will zap the crap out of you.

http://www.ehow.com/how_7289350_troubleshoot-boat-tachometer.html

 

[Silvertip]

Re: rpm gauge need to be replaced or zero'd?

When you trim up boat speed increases. It does that because the engine speeds up -- hence the increased rpm. At idle it should be below 1000 rpm. Idle speed is in the 650 - 800 RPM range for most engines. Tachometers and many other gauges "float" when turned off -- meaning they sit wherever they were when the key was turned off. Even then they drift a bit. In the case of the tach, it should drop to zero when the key is turn to RUN but the engine not running. The calibration dial on the back of the tach should be set to 6P (6 pulse). You might turn that back and forth a bit to burnish the internal contacts as sometimes contact corrosion can result in inaccurate tach reading.

 

[NYBo]

Re: rpm gauge need to be replaced or zero'd?

Are you fully on plane at 15 MPH? If not, the tach reading may be correct. You might also have a spun prop hub.