[heze]

Hello
Does anyone have know how the electronics inside a tachometer of a Mercury engine works? I'm busy with designing and creating a display with the motor-properties (rpm, speed, temp). Now I would like to know how I can measure the rpm of the engine. The engine is a 1989 50 hp 3 cyl Merc.
What I basically want to know is: how are the pulses from the rectifier converted to a voltage or a frequency which drives the gauge? When I know this, I can measure the rpm of the engine.

Thanks in advance,
Regards Hendrik (The Netherlands)

[Silvertip]

Certainly you can measure the engine RPM. A tach is doing that is it not? A tach consists of a pulse counter that has an input, pulses from the rectifier. You will be measuring six pulses per engine revolution. I suggest you do a search on-line for "tachometer schematic". There will be any number of them to choose from. Or, you could use a pickup that consists of a coil of wire around one of the spark plug wires and count those pulses (one per engine rev).

[bhile]

Tachs work from AC pulses from the stator not the rectifier. The rectifier is just a convenient tie in point. Most Mercs give 6 pulses per revolution.

[heze]

I know how a tachometer works (counting pulses), but how will the pulses be counted? I would like to solve the problem at the same way as Mercury does, and not a pickup wire at the coil.

My question is: Does someone can tell me how a Mercury tachometer works inside, and with which parts? Do they really count pulses, or do they place a big capacitor between the tach-wire and ground, and use the voltage on the capacitor to drive the gauge? How do they prevent the circuit from overloading? Which electronic parts are used?

Thanks for thinking!
regards Hendrik

[Silvertip]

I wasn't going to do your research for you but I happened to have this link handy. It's for a car but if you can do design work like you imply you can, you should be able to figure out the input to accommodate 6 pulses/rev instead of the 3 pulses per rev on this six cylinder/four stroke engine. Virtually any tach that is less than 30 years old will have at least one integrated circuit in it which you will have no idea what it consists of unless you can obtain the data sheet.

http://rad.davis.home.comcast.net/earlytach.html