Strange readings for tachometer

so I bought a boat a while back as a fix it up project and it didn't come with a motor so i bought one from a guy it came with a throttle control box. when i re did the boat i realized the current tachometer in the boat would not pug into the control box. So i opened up the control box cut the wires from the plug and manually rewired the tach. when i turn the key the tach needle moves to the zero but when i start the engine it show any readings. So i checked the sender wire with the motor running with my multimeter and it was reading pulses that ranged from .22 volts all the way to 14. I have not checked the rectifier or stator yet I was wondering what your thoughts where on the problem before i tore to deep into the problem. I also have no clue if the tach worked on the boat that it was on or not so i might have a bad tach i read error when i check for any ohms between wires on the tach also even when reversing the leads.
My motor is a 76 850 mercury thunderbolt and my control box is a mercruiser and my tach is an old quick silver. Thank you for any help.

When you say it's a "Mercruiser" control,I presume you mean the rectangular one,like this one,with a cobra head if it fits into a '77 850.Is the tach this one and wired like this?

You will note that the signal wire goes to the NEG - Pole AND the NEG - black wire goes to the POS + pole.This is the way these old tachs are wired.The white 12v+ is on a loop though the lamp,which is missing from this tach of mine

haventaclue said:

You will note that the signal wire goes to the NEG - Pole AND the NEG - black wire goes to the POS + pole.This is the way these old tachs are wired.The white 12v+ is on a loop though the lamp,which is missing from this tach of mine

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not exactly here is my tach and this is the control box that I have
On the inside of my control box there is a thick black wire which i assume is the ground since it is bolted to the box then there is a thin white, and a thin black wire. on my tach I have 3 wires one grey which is the sender, one purple which is the battery and one black which is the ground

There should be a brown wire in the harness from the engine to that rubber plug on the control box.If not,you could run a wire from the rectifier to the tach. Set the pointer on the button on the back of the tach to 6P

haventaclue said:

There should be a brown wire in the harness from the engine to that rubber plug on the control box.If not,you could run a wire from the rectifier to the tach. Set the pointer on the button on the back of the tach to 6P

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my tack only has a 1-4 settings 1 being 4cyl-4 cycle 2 is 6 cyl-4cycle 3 is 8cyl-4cycle and 4 is 12 pole im guessing i need to be on the 1 is that correct? and where exactly does that brown wire your talking about connect on the tach? my rectifier has three wires on it one red on the ++ and the other two are yellow. Ill check when i get home if theres an extra wire in the box but i dont belive there is. the plug in on the box only had a three prong plug installed.

Tach signal wire connects to either of the yellow wires on the rectifier.Set pointer to 4. 12 pole as that is the number of poles on your stator,and if your tach had 6P/pulses post that is where you would point the arrow.Your engine is 2 stroke/cycle and all the other settings are for 4 stroke/cycle.
There should have been three wires in the plug,Brown= tach signal,Black= ground and white =12v+. Of the top of my head I can't remember the orientation but I'll look at mine tomorrow morning.

haventaclue said:

Tach signal wire connects to either of the yellow wires on the rectifier.Set pointer to 4. 12 pole as that is the number of poles on your stator,and if your tach had 6P/pulses post that is where you would point the arrow.Your engine is 2 stroke/cycle and all the other settings are for 4 stroke/cycle.
There should have been three wires in the plug,Brown= tach signal,Black= ground and white =12v+. Of the top of my head I can't remember the orientation but I'll look at mine tomorrow morning.

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Alright ill check it out i just got my hands on the wiring diagram and see what you mean. ill go do some testing tonight and get back to you with what i find.

haventaclue said:

Tach signal wire connects to either of the yellow wires on the rectifier.Set pointer to 4. 12 pole as that is the number of poles on your stator,and if your tach had 6P/pulses post that is where you would point the arrow.Your engine is 2 stroke/cycle and all the other settings are for 4 stroke/cycle.
There should have been three wires in the plug,Brown= tach signal,Black= ground and white =12v+. Of the top of my head I can't remember the orientation but I'll look at mine tomorrow morning.

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So I just looked at it and there was a brown wire in the control box so i hooked that up to the grey wire on my tach then the white wire from the control box to the purple on my tach and then black to black. when i turn the key the needle moves to zero but when i crank the engine over nothing. i read voltage pulses coming through the brown wire with my multimeter so im still thinking its a bad tach.

Hot goes to 12v pos your pic shows it connected to the light. The signal is A/C volts and takes a dva to properly read the voltage produced by the stator.

kimcrwbr1 said:

Hot goes to 12v pos your pic shows it connected to the light. The signal is A/C volts and takes a dva to properly read the voltage produced by the stator.

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hot is going to the 12 volt it just dosent look like it in the picture cause some electrical tape is covering the wire. should i test my stator?
and i went to put my control box back together after working on it tonight and it wont go back together correctly. when it does go back together all my throttle adjustments are way messed up. do you know the correct way to resemble one? there two different gears on mine and the throttle gears slide in a slot on the forward and reverse gear. im guessing i didn't put the gears back in the right spots.

Set the shift lever in neutral,set the roller on the throttle side to the 10 o'clock.Put bottom to bottom and close like a book,you'll probably have to jiggle the warm up lever a wee bit to get it to line up with the shift cam.Make sure the spring and indent roller are in place before you put it together,it's prone to taking a flying jump and are hard to find when it flies
As you look at the plug,the parallel pins are white =left,black = right horizontal =brown signal,or at least they should but sometimes past owners will have done strange and wonderful things.

haventaclue said:

Set the shift lever in neutral,set the roller on the throttle side to the 10 o'clock.Put bottom to bottom and close like a book,you'll probably have to jiggle the warm up lever a wee bit to get it to line up with the shift cam.Make sure the spring and indent roller are in place before you put it together,it's prone to taking a flying jump and are hard to find when it flies
As you look at the plug,the parallel pins are white =left,black = right horizontal =brown signal,or at least they should but sometimes past owners will have done strange and wonderful things.

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Mhmm so that was the thing that came shooting out when I opend it! Where exactly does that go? I was hoping it wouldn’t be needed when I put it back together. And what do you mean by the roller on the throttle side?

There is a flat bent spring that goes in the slot and the roller goes in the detent in the wheel that locates neutral.

kimcrwbr1 said:

There is a flat bent spring that goes in the slot and the roller goes in the detent in the wheel that locates neutral.

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alright shes back together and motor seems to run good. Thank you guys so much! I might go test my stator and then order up and new tach. do you guys know if i can instal a water temperature gague on my motor. i thought i should be able to remove one of the bolts on top of the motor and install the sender in one of thoese and run wires to the gague.

If the battery is charging just get a new tach. A temp guage is not practical if you have a steady stream out the telltale you should be fine?