Mercury 40hp Tach Install

logain

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May 29, 2010
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I recently purchased a 1983 40hp Mercury for my boat. The motor was setup for a pontoon boat originally. I had to buy a separate control box for it. I picked one up from a guy on craigslist. Control box is working just fine with the motor. I recently bought a tach wiring harness to plug into my control box that matched the pinout of my controller. I bought a Faria tach off of Ebay and it just came in. Looking at the instructions from Faria, I need 3 wires. One for ignition terminal, one for ground, and one for signal. My harness is a 4 plug, 4 wire, but only two wires are actually equipped with connectors, solid white and black. The other two, a brown and white with a stripe, are tucked away and taped up. What do I need to do to make this work. :confused: Any help would be greatly appreciated as this is my first time having such a setup. I have only had tiller 6-9.9's up to this point. :)

S 4 PN Tach Harness 84 69108A5 Mercury Mariner Outboard Boat Motor Part 3 | eBay


Faria TC9817A Inboard Outboard Boat Tachometer Gauge | eBay

http://faria-instruments.com/site_manuals/IS0012u.pdf
 

gm280

Supreme Mariner
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Jun 26, 2011
Messages
14,585
Re: Mercury 40hp Tach Install

Not knowing you setup, you may have bought the wrong type tachometer for your engine. It looks like you bought a tach for an Inboard/Outboard engine instead of an Out Board engine. So that tach may not work properly... An OB tach from Faria will have a switchable selector switch on the back of the tach to select your OB's pulse output. Not sure about the I/O tachs though. I'd say to contact Faria and find out if it will work and they can tell you how to set it up if it will work with your OB... They will know one way or the other...
 

alanfox55

Chief Petty Officer
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Nov 17, 2011
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476
Re: Mercury 40hp Tach Install

I'll give you what little bit of information I have related to your problem. Using a service manual for the 1994-2002 30/40 hp model if your Tach/Accessories connector has a female end there must be another cable that runs to the tach. The harness connector I'm looking at has five wires but only three go to the Tach...purple to the Batt post,black to the ground and gray to the sender post. It seems to me if your cable coming out of your control box has a female end you could just cut off that end then have the lose wires you need for your tach. Keep in mind that harness is for more than just the tach so you don't need all of the wires if a tach is all your wiring. Hope this is some help to you and good luck.
 

logain

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May 29, 2010
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Re: Mercury 40hp Tach Install

The tach I bought does have the switch for 12 pole outboard option which Faria's list shows I should be set to. The wiring harness I have comes that way new. It is a 4 pin on the control box as is the wiring harness I purchased. The guy I bought the control box from said he had been using it on a mid 80's 9.9. I don't recall him saying anything about using a tach with it and don't recall seeing one on his boat. He was in the middle of putting a 40hp Johnson onto his boat when I showed up. His 9.9 went out and he got a good deal on the 40 and no longer needed the merc controller. I have been trying to find the install instructions for the harness, but have had 0 luck so far.
 

MH Hawker

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Jul 13, 2011
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5,516
Re: Mercury 40hp Tach Install

This is a standard instrument print for the commander 2000 / 3000 series.
What controller do you have post a picture of it.

comander 2000-3000 insterment wiring.jpg
 
Last edited:

logain

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May 29, 2010
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Re: Mercury 40hp Tach Install

I got lucky and found the instructions. I had to remove the tape from the brown wire. It had the loop connector on it. The brown wire was for the signal. Guess it was taped up so wire could be used for lights only for speedo. Thanks everyone for the help. :)
 

logain

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May 29, 2010
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Re: Mercury 40hp Tach Install

Looks like I did possibly buy wrong tach. I was doing 28mph full throttle. Tach read 3k
 

MH Hawker

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Re: Mercury 40hp Tach Install

sounds more like its not on the right pole / pulse setting
 

logain

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May 29, 2010
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Re: Mercury 40hp Tach Install

I was certain I had it set on the proper setting per faria instructions.
 

logain

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May 29, 2010
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Re: Mercury 40hp Tach Install

I checked the sticker on the tach and it was on the right setting. Idle is about 600 rpm or so. Is that right?
 
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