Logo
FULL SITE MENU

Tachometer

B

bryanfrater

New member
Recently acquired a 1972 skiff craft with 302 I/O. Has a kiekhaefer Mercury instrument cluster. Tach does not work. My question concern wiring for it. The center posts connects to negative side of coil and it is grounded. No place for a positive connection, do not understand. Can hook up another tack with 3 wire attachment and works fine. Would like to use original tack. Thanks
Bryan
 
the tach gets its signal from the negative side of coil and should be piggybacked to ground to the other gauges,3rd wire would be for a light i thought
 
All tachometers regardless of manufacturer should have a minimum of 3 connectios

Power......................Ignition on power when key is in the on or run position.

Ground.....................Ground


Sender.....................from in your case the negative side of the coil


possible fourth connection would be the light..................
Based on the year of your boat the wire colors may be different then the picture I attached.
 
Some early tachs, were just voltmeters with an averaging/smoothing capacitor in them. +12 gets supplied to to the input of the tach every time the points open. New tachs (those with 270 degree needle sweeps and selector switches) have circuitry in them that needs a continuous "clean" +12 to operate... hence 3 wire.
 
Lear something new... Now how far back was that type of tachometer used?

Was this the 1920's??
 
Just looked at Manual #1 late 60's to early 70's.

It shows brown wire from coil -

Ground to tachometer case is black wire

one wire but no color on schematic from Key switch, run position to tach.

so three wire as far as the manual shows

If that is not how it is supposed to be not sure what to say..........

I was a yungin back then so before my time..........
 
Thanks for the info from manual 1. Does it show where the positive wire is connected. My problem I do not see where to do that on my tack.
Bryan
 
I'd like to see a pix of the front as well. Back in the day, some analog multimeters had an RPM scale. Also many "ignition analyzers" for points style engines are just meter movements with a smoothing capacitor.
 
bryanfrater said:
..................... My question concern wiring for it. The center posts connects to negative side of coil and it is grounded.
Click to expand...
Bryan..... FYI and FWIW:
The - side of the ignition coil is not actually taken directly to Negative (aka ground)
The - side of the ignition coil is the triggering side, and is taken to Negative (w/ a capacitor in the loop) during coil saturation.
As this circuit opens, the primary field collapses causing the secondary winding to discharge high energy that creates one spark event.
The electronic tachometer senses each cycle and converts the data into an RPM reading.

kghost said:
Boy

I could use a "smoothing capacitor" from time to time myself!!
Click to expand...

Excellent observation! :D


.
 
Last edited:
Here are 2 pictures. The center connects with a brown wire to the coil. From other posts it maybe a capacitor activated tack and just might be broken. If that is the case where would I find the capicator?
 

Attachments

  • image.jpg
    image.jpg
    67 KB · Views: 50
bryanfrater said:
Here are 2 pictures. The center connects with a brown wire to the coil. From other posts it maybe a capacitor activated tack and just might be broken. If that is the case where would I find the capicator?
Click to expand...

The capacitor I mentioned along with a likely diode and one or more resistors and capacitors are internal to the tach. That particular tach with its 90 degree needle sweep is typical of early non mechanical tachs. Its just the same meter movement that is in an analog multimeter. These first generation "electronic" tachs started to appear in boats in the early 1970 as a cost saving measure. They avoided expensive long runs of "speedometer" style cables to mechanical tachs. Eventually someone invented the two coil meter movement that allowed 270 degree meter needle movements. These movements required more complex circuitry behind the meter and and to await the development of inexpensive integrated circuits.

To test your tach to see if it works, connect one end of a 12 volt bulb to +12 with a wire. Connect the tachometer ground to a known good ground. Very briefly touch a wire connected to the other end of the bulb to the other meter post and tap it a few times rapidly.... The needle on the tach should "jump".
 
Back
Top