1986 Merc 60hp 3cyl Tach question

J

jhodge

Seaman
Joined
Mar 6, 2009
Messages
63
Tach does not work and for a good reason. Wires cut off about 3 inches from back of tach. Maybe its junk but need to test it. It is an OMC with the numbers 173604 printed on a sticker on the back. There is a 5 position switch with the settings of H, 6, 4, 3, 2. Set it on 3 for 3 cyl? The wire colors are black, grey, blue and violet. Assumption is black is ground? Grey is pulse from the rectifier? Blue is night light? Violet is 12v positve from the key switch? Does anyone know if it is compatable with my application, are my guesses at wiring correct, and is the guess at the switch position correct? Thanks for any help. JH
 
J

jhodge

Seaman
Joined
Mar 6, 2009
Messages
63
Re: 1986 Merc 60hp 3cyl Tach question

Been doing some research and found that my guesses at wiring connections is pretty much right on, but the 5 position switch is not as guessed. Apparently the 2, 3, 4, 6, H positions is not equivalent to number of cylinders. They have something to do with the number of pulses emitted by the stator. Which I undestand to be controlled by the number of windings that supply current to the rectifier. I did a temporary hookup today and did a little test ride. The tach is working but recording too high. At idle the rpm was 1200 and the switch was set on 3. Never got around to see what it was doing at WOT but suspect it was pegged. So, will try position 6 and see if that seems more reasonable. At the same time I was trying to check the WOT max spark setting, but it was too light for the timing light to be effective. Meant to check voltage at the battery at the same time to see if the rectifier was working, but forgot to do the check. So sorta had success with one out of three. Will have to make another test run and be more careful about the tasks selected to do. Foggy in the mornig so may be bettter for uning the timing light. JH
 
J

jhodge

Seaman
Joined
Mar 6, 2009
Messages
63
Re: 1986 Merc 60hp 3cyl Tach question

An update to this problem. I kept fiddling with this tach and had almost given up when I suspected the rectifier was bad. It would kinda read off one AC lead but not the other one and there never seemed to be a charging voltage on the battery. Took the rectifier off and used the menu in the manual for checking it out. Each and every way turned out 'rectifier bad, replace'. I've read on here that others have gone to Radio Shack to get a rectifier. I looked them up online and they have a full wave bridge rectifier that handles up to 50 volts and 25 amps. Well within the range of my power output. Only problem is it has 4 connections, a single mounting place and spades rather than the bolt and nut connectors. The extra connector is a ground connector, where the original rectifier was grounded thru its base. Snip, snip, crimp, crimp, a little shrink tubing, drill a hole and tap it and back in business for 3.29 plus tax. Tach now works as designed and the voltgage at idle is 13.3volts. Time for a celebratory beer. JH
 
H

henrye718

Petty Officer 1st Class
Joined
May 24, 2009
Messages
207
Re: 1986 Merc 60hp 3cyl Tach question

I have seen the radio shack rectifier mod but remember there being some disadvantages, maybe even major draw backs over time? I dont remember 100%. The next step is a universal voltage regulator, they say is the way to go.
 
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