Merc tach question

[craigman]

Fellas,
trying to hook up the factory tach on my boat.
1974 Reinell
1974 Merc 850
Tach is Merc also (Thunderbolt stamped on back side of tach)

The controll box, it has a three wire hookup. Black, brown,white
I'm assuming:
black=ground
white=positive
brown=tach pickup

Controll box has the loom with the three wires that go to the tach.
Problem is, it only has 2 hookups. (well three,,, one for the night light)
One terminal says "positive", and the other says "negative".

On the back of the tach where it says "positive", would that mean "tach pickup" terminal?

Thanks for any help!
Craig

 

[craigman]

Re: Merc tach question

Nobody?

 

[craigman]

Re: Merc tach question

Bump..

 

[Moody Blue]

Re: Merc tach question

Got a picture?

A little unclear about this statement, "Controll box has the loom with the three wires that go to the tach.
Problem is, it only has 2 hookups. (well three,,, one for the night light)"

So the tach is already connected?

 

[douglas1978]

Re: Merc tach question

i have a 1971 650 with the tach that plugs into the control box has 3 wires. Looking from the backside of the tach green is hooked up to the left post white is for the light and black on the right post hopes this helps since are wires are different colors

 

[craigman]

Re: Merc tach question

Yes, i have a wire loom that comes out of the controll box with three wires that go to the tach.
White, black, brown.

On the back of the tach, it just has two post. One says "positive", the other post says "negative".
And then one wire goes to the night light.

On most tachs i've seen their is a "lead" terminal. Mine only has "positive" and "negative".

I'm "assuming",
white=night light
black=ground
brown= tach lead?

Please correct me if i'm wrong, but i'm "assuming" the brown wire would go the the "positive" on my tach?

I've never seen a tach like this before. Like i said, all the tachs i've seen have a "lead" terminal.

Is there any way to test a tach? Maybe with a ohm meter?

Craig

 

[cr2k]

Re: Merc tach question

That could be right. Brown is the tach signal wire. White is the light wire and black is a ground wire.

My name is Craig, I guess you know yours.

 

[craigman]

Re: Merc tach question

Ya, i thought the tach wire was the brown one.
And i'm also assuming the "positive" terminal on the back of the tach is the tach lead.

Well,, that's how i hooked it up and i'm not getting anything.
So i'm guessing the tach is bad. Time for a new one. Darn!

Is their any way to test the tach?

Thanks for the replys!
Craig

 

[Chris1956]

Re: Merc tach question

Craig, Merc switched the type of Tach that used on their motors o/a 1974 or so. They went from one powered by the switchbox, to one powered by the stator.

You should look under the wrap cowl and see where your brown wire is attached. I would connect it to the switchbox and retest. If it is already connected there, probably the tach is bad.

 

[craigman]

Re: Merc tach question

Chris,
i'm pretty sure this tach is original to the boat. The back of the tach is stamped "thunderbolt".
And it has the factory loom coming out the front of the controll box going up to the tach.
I'm pretty sure i have it hooked up correctly. Kinda of a bummer, i was hoping to used the factory tach. It's pretty cool looking!

Oh well,, guess it's time for a new tach.

Craig

 

[craigman]

Re: Merc tach question

Ok,
now i'm thinking it could be a wiring issue.
Last time i had it at the local shop getting the water pump changed, and the guy was telling me my engine had a new wiring harness on it. So it's not original.
Just to eliminate any kind of wiring issues, i'd like to just run some jumper wires strait to the motor.
Question is, where would the tach "lead" hook up to on the motor?

Thanks as always!
Craig

 

[Chris1956]

Re: Merc tach question

Craig, According to my Merc Service manual, the brown tach wire should connect to the brown terminal between the green and red termianls, on the switchbox, for motors earlier than 1976. That will work for the old Merc Tach. If you buy a newer tach, move the brown wire to the rectifier and connect it to one of the stator (yellow) wires.

 

[craigman]

Re: Merc tach question

Thanks Chris!
i'll have to give that a try today.

Craig

 

[craigman]

Re: Merc tach question

Got it fixed!
but boy,,, this one throws me for a loop...
just a quick run down of what i "thought"
white= positive for night light
black=ground to "ground" terminal on back of tach
brown=tach lead to "positive" terminal on back of tach

This is what made it work:
white=night light
black=tach lead to "positive" terminal on tach
brown=to "negative" terminal on tach

Figure that one out!

Craig