My tachs have never been right since I've owned my boat but last year the port tach pretty much stopped working altogether. I've read several threads here about pulling the tachs and cycling a cylinder switch in back. Is this the same process for tachs with diesel engines? If not, can this re-calibration be accomplished some other way for tachs with diesels?
I'm tempted to just replace them since they are the original tachs. Are the Aetna digital tachs still the best ones to use? For those that have made this upgrade, which model is better 8905R or 8402?:
http://www.aetnaengineering.com/model-8905r-lcd-tachometer/
http://www.aetnaengineering.com/model-8402-led-tachometer/
Lastly, if I go this route and replace them, is it possible to advance the hour meter to the current number of hours on the engines?
If you have some electronics savvy, It's possible with a pulse generator. This is a small circuit board that sends out pulses, thus simulating an engine running. I think i got mine on Amazon. You hook it up on the bench and just let it run for the requisite number of hours. But don't over-shoot, because there is no way that I'm aware of to "wind" them backwards.