Tach question

[RoughFishAssassins]

I would like to add a tach to my 1977 115hp Evinrude. My question is I suppose that I cant use a typical automotive tach can I? I have a 5" racing tach that is not being used, It is adjustable from 4 cylinders to 12 cylinders. Otherwise does anyone know where I can find a fairly cheap on that would work?

 

[Silvertip]

Re: Tach question

The only way that tach will work is if you connect the tach signal wire to one of the coils. If and only IF the tach recognizes that signal will it register. Since the lowest setting for the tach is for a 4 cylinder (two pulses per engine revolution) the best you can do with this tach is to multiply the reading by 2. Why? Because your engine is a two stroke so the coil you have the tach connected to fires once each engine revolution so it would read 1/2 the actual rpm. Outboard tachometers do not rely on the number of cylinders and engine has since they read pulses from the charging system.

 

[metriccrescentwrench]

Re: Tach question

Whoa - don't connect your tach to the coils, the primary side of the coils gets a voltage peak of well over 100 volts.

Here's a typical V4 wiring diagram (they're from the links at the top of the Johnson/Evinrude forum) http://maxrules.com/graphics/omc/wiring/80_81_V4.jpg

Yours may not be identical to that but it should at least give you an idea of what's going on. The tach signal is the grey wire on the 3-pin connector on your remote control. The tach signal comes from the 12 V charging coils on the stator, I believe your flywheel has 6 magnets so I believe your tach might work if it's on the 12 cylinder setting, try it and see.


If you end up buying an outboard tach take a look in iboat's online store, they're not all that costly.

 

[emdsapmgr]

Re: Tach question

You need a marine type tach. Most marine dealerships have access to Teleflex gauges. Teleflex makes a nice assortment of inexpensive tachs. Check them out on the web. On your 1977, the tach gets it's signal from the grey wire (tach signal wire) which the factory left loose inside the control box. You will need to find a pos and ground from the dash. The marine tachs count the pulses from the battery charging system. The instructions will tell you how to set the tach for your engine. (usually set the tach to #6 position.)

 

[RoughFishAssassins]

Re: Tach question

Sounds good, thanks for the info.
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