Author Topic: Outboard tach (Read 13755 times)

Jason · « **on:** April 30, 2011, 11:19:25 PM »

I don't think my old tach in the V172 is very accurate. Is there a way I can check to see if the tach is accurate? I hear outboard tachs are different then I/O's. I have a timing light with a built in tach, will that work?

Rich_V174SS · « **Reply #1 on:** April 30, 2011, 11:46:48 PM »

Outboards and I/O's provide the tach signal from different places. On an I/O the tach reads the pulses created by the ignition coil for each firing of a spark plug so the tach must be set for the number of cylinders on the engine to read correctly. On all modern outboards the tach signal comes from the stator's charging output in which case a tach would have to be set for 12-pole alternator. However, on older Mercs that use a distributor for ignition I'm not exactly sure where the tach signal originates from so I'm not sure which setting on the tach works for them. Your timing light might be a good tool for comparison since it reads the pules going through the spark plug leads.

Jason · « **Reply #2 on:** May 01, 2011, 12:10:00 AM »

Quote from: Rich_V174SS on April 30, 2011, 11:46:48 PM

........Your timing light might be a good tool for comparison since it reads the pules going through the spark plug leads.

hmmmm. your right. I might just have to divide the number by two as a 2 stroke is firing every revolution where as a 4 stroke is every other.

Maybe I'll just have to follow the tach signal wire and see where it leads me.

Hyperacme · « **Reply #3 on:** May 01, 2011, 06:29:28 AM »

Check your email Jason ....

Jerry · « **Reply #4 on:** May 01, 2011, 11:36:10 AM »

On my FireFlite with a 75 Johnson the Mag wire grounds to kill and my standard auto tach trigger wire goes to the ignition switch. It's set on 8 cyl and works great.

wiliermdb · « **Reply #5 on:** May 01, 2011, 12:32:42 PM »

Go to this site as they may have a wiring diagram for your motor.

Hope this helps.

http://www.maxrules.com/MERCwireindex.html

wiliermdb · « **Reply #6 on:** May 01, 2011, 12:35:26 PM »

Found this solution several times. Also may work for you.

Tachometer

Tachometer - if you no longer have the original tachometer and have purchased an aftermarket tachometer, chances are that it will not register. To make the tachometer work on the Mercury 650Electric start, you need to move a

brown wire from the switch box to the rectifier terminal. The rectifier has two Yellow / Red terminals. The brown wire should go to any of those two.

For this engine (Merc 650 1975), the tachometer should have the selector set to 5 in order to read the 12 pole engine. I used a Faria #TC9120A model number and it works great. Any new tachometer you buy for this particular engine should be able to be adjusted to read 12 pole alternators. If you have the incorrect pole settings, it will probably not register or register incorrectly according to Faria. I have heard of some people frying their tachometers, just be sure you have yours set correctly before powering it up.

Try it without making the wire switch and it probably will not register. It is a simple change.

Jason · « **Reply #7 on:** May 01, 2011, 05:46:56 PM »

Thanks for all your help guys. Jerry, I don't think an auto one would work for mine unless I found one that would work for a V12!

Willermdb: Is your's an SS? Mine is. I think I'll use my test light for now to at least test my current tach to see if it's accurate as I can go off the spark plug wire and divide the number by two. If i have to replace the tach i will try what you say with moving the brown wire.

wiliermdb · « **Reply #8 on:** May 01, 2011, 06:26:05 PM »

I may have missed something here. Do you have an I/O or an outboard? The info I found and posted was related to older Mercury outboards. I'll see if I can find anything for I/O's for you if that's the case.

Marc

wiliermdb · « **Reply #9 on:** May 01, 2011, 06:31:13 PM »

Found this info for anyone that wants/needs MerCruiser wiring diagrams.

http://www.boatfix.com/merc/Servmanl/18/18E4R2.PDF

wiliermdb · « **Reply #10 on:** May 01, 2011, 06:33:09 PM »

Quote from: 74GlasCarlSS on May 01, 2011, 05:46:56 PM

Thanks for all your help guys. Jerry, I don't think an auto one would work for mine unless I found one that would work for a V12!

Willermdb: Is your's an SS? Mine is. I think I'll use my test light for now to at least test my current tach to see if it's accurate as I can go off the spark plug wire and divide the number by two. If i have to replace the tach i will try what you say with moving the brown wire.

I have a '77 CVX 16 with a '96 Evinrude 88 SPL on it now. I have the original Evinrude 115 on a stand in the workshop. Need to fix the cav. plate and repaint the motor to make it look really nice.

Jason · « **Reply #11 on:** May 01, 2011, 07:13:34 PM »

Oh, I have an an outboard. But mercury had a 950 and a 950SS. I have the 950SS which has a different style ignition system. Same story for the 650 you have or had. I think the non-SS style engines lack the ignition switch box. Also looks like the rectifiers are a little different too.

wiliermdb · « **Reply #12 on:** May 01, 2011, 09:46:05 PM »

Jason,

Look at this link here and it shows the entire wiring diagram for your 950SS. Sorry I can't help with the Merc stuff much as I have two Evinrudes, which I have become very familiar with over the past year.

http://www.maxrules.com/oldmercs/Wiring/0_1965a/10.pdf

This will take you directly to the 950SS wiring diagram in PDF form. Print it out and bring it out to the boat to trace your wires.

Hopefully this will give you an idea of what's going on with the wiring.

Marc

Tonka Jim · « **Reply #13 on:** May 02, 2011, 09:46:16 AM »

The shift control on my 1977 merc has a place in the front to plug a tach in.

Jason · « **Reply #14 on:** May 02, 2011, 12:16:33 PM »

Mine does not even have a control box. Must be behind the panel. No idea if it has the tach port though. Not sure how they did it in 1967. I'll have to peak.

MarkS · « **Reply #15 on:** May 02, 2011, 12:25:13 PM »

I've got a new (out of box but not installed) Stewart Warner 6 grand white face white bezel, marine tach if anybody needs one. Settings for 6 pole, 12 pole, plus standard ignition systems if I recall. With instructions, PM me for negotiation $ (very reasonable!)

Jason · « **Reply #16 on:** May 02, 2011, 02:04:04 PM »

Mark, Do you know the hole diameter of the one you have? If mine turns out to be fautly I would be very interested in the one you have. I am gonna run mine tonight to test it out.

Rich_V174SS · « **Reply #17 on:** May 02, 2011, 02:20:30 PM »

Quote from: 74GlasCarlSS on May 02, 2011, 12:16:33 PM

Mine does not even have a control box. Must be behind the panel. No idea if it has the tach port though. Not sure how they did it in 1967. I'll have to peak.

That control box does not contain any wiring except for a neutral safety switch, so no it won't have a tach harness connection. It's the same box I have in my V174 except with a different handle because mine has the T&T buttons in it. They're usually used for I/O engines but will work fine for outboards if all the electrical wiring is mounted elsewhere like on the dash.

MarkS · « **Reply #18 on:** May 02, 2011, 03:41:29 PM »

I'll measure it tonight Jason, I'm thinking it's 4". (Was a dead fit into the O.E. hole in the Starflite, I just decided to go with black on black to match the other gauges I installed.)

Eric_Michael · « **Reply #19 on:** May 02, 2011, 04:43:02 PM »

Quote from: 74GlasCarlSS on May 01, 2011, 12:10:00 AM

hmmmm. your right. I might just have to divide the number by two as a 2 stroke is firing every revolution where as a 4 stroke is every other.

Naw. Have fun with it. Put a top-fuel racing tach in it. Let everyone think your motor red-lines at 10,000 RPM. Tell people it is a special racing motor. Enjoy the envy.