Murcury 850 thunderbolt ignition tachometer

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Hotrod758

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I bought a us marine tachometer same 3 wires on back ground ignition and signal but it says for a 8cyl would this still work on thr 4cyl merc ?
 
merc850

merc850

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The signal wire runs from ONE of the YELLOW wires on the rectifier, if that's a new tach it probably means 8 poles which will work. What is the serial nr. of your motor? Did you find the tach with the nr?
 
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Hotrod758

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So the mercontroller has the plug built into it for a tach so I should be able to connect through that right? I just was curious if this tach would work and why it says it's for a 8cyl I posted a wore diagram so signal to signal ignition to ignition and ground to ground should work ? I get confused with wiring easily haha thanks as well for the help
 

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Hotrod758

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merc850 said:
The signal wire runs from ONE of the YELLOW wires on the rectifier, if that's a new tach it probably means 8 poles which will work. What is the serial nr. of your motor? Did you find the tach with the nr?
Click to expand...
4233250 the tach is a 80s us marine tach idk what a nr is
 
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Chris1956

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A typical tachometer would have several settings. It is powered by the stator pulses. Outboard motors use a common stator, so a common tach can be used as well. Merc's generally uses the 6 cyl/12 pole setting. In your case, you will need to see if it works properly with your motor.

One more thing. 1973 and earlier Merc 850 motors powered the tach off the switchbox, versus the rectifier. To use the modern tachometers, the brown wire must be moved from the switchbox to one of the yellow wires on the the rectifier. Your wiring diagram shows an earlier Merc, which would need the wiring change. Is that what you have?
 
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Hotrod758

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Chris1956 said:
A typical tachometer would have several settings. It is powered by the stator pulses. Outboard motors use a common stator, so a common tach can be used as well. Merc's generally uses the 6 cyl/12 pole setting. In your case, you will need to see if it works properly with your motor.

One more thing. 1973 and earlier Merc 850 motors powered the tach off the switchbox, versus the rectifier. To use the modern tachometers, the brown wire must be moved from the switchbox to one of the yellow wires on the the rectifier. Your wiring diagram shows an earlier Merc, which would need the wiring change. Is that what you have?
Click to expand...
Yes serial 4233250 so I would have to move the brown wire for the tach to work so white would be ign and then black to ground and the brown would be moved to a yellow off the rectifier that would be my signal ? Why does a tach specify a certain cylinder?
 
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Chris1956

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Of course, a 4 cylinder 2 cycle motor's ignition pulses can mimic an 8 cylinder 4 cycle engine's ign pulses. However, there is no way to know if that tacho can read the OB's battery powered CDI ign pulses, as it is likely for a points/coil ignition.
 
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Hotrod758

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So I wired up rhe tach and cranking engine over it does move cranking shows just shy if 2k so I'm assuming this should work then or else it would register anything correct ? And there are adjustments on the back so i can tune it to match what should wot be on this so once I'm out on lake I can see if it's accurate ? I also have a rpm gun so I can compare actual reading to the gauge
 
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Hotrod758

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Chris1956 said:
A typical tachometer would have several settings. It is powered by the stator pulses. Outboard motors use a common stator, so a common tach can be used as well. Merc's generally uses the 6 cyl/12 pole setting. In your case, you will need to see if it works properly with your motor.

One more thing. 1973 and earlier Merc 850 motors powered the tach off the switchbox, versus the rectifier. To use the modern tachometers, the brown wire must be moved from the switchbox to one of the yellow wires on the the rectifier. Your wiring diagram shows an earlier Merc, which would need the wiring change. Is that what you have?
Click to expand...
So I wired up the tach and when I turn the engine over it reads rpms on the gauge but I didn't move that brown wire if uts showing its reading would I have to move it ? Not sure how all that works
 
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Chris1956

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OK, if you have an automotive style tach, it may work just fine with the pulses from the switchbox, as you have it wired now. No way to know, but try it. Let her idle and see if you believe the reading.

if the motor is tuned correctly, you motor should idle at 850-1000RPM on the flusher. The cold start lever should raise the RPM up to 1200 or so.

If you do not believe the tach readings, you might switch the brown wire to the rectifier and see if you believe the readings than. Again, no way to tell, since the tach is of unknown compatibility.
 
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