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Tachometer inop after 70hp 3 cylinder to 90hp V4

S

sjones73

Member
Ok, swap complete. Motor runs great. 90hp V4 really woke up my little 16ft boat. But would like to get the tach working again.

Old motor was a 1983 Johnson 70hp 3 cylinder. New motor is a '87 Johnson 90hp V4. Tach worked great with the 70. But not with the 90,
Is there something that needs to be changed for the tach to work properly with the new V4?

Thank you
 
No, same item. I assume you didn't unplug it from the control box.
New motor probably has a bad rectifier. Verify your system voltage with new motor running.
 
Tilt & trim model? Tilt & trim has higher output stator with regulator/rectifier unit water cooled betwern the cylinder banks. Non tilt models have just a rectifier.

Tach signal comes from rectifier.

12.99 is too low if engine has been running for a while. You will need 13.8 minimum with low 14s better. Yes it will drop a bit after a while if idling even with zero accessories on.
 
Yes it ought to have a water cooled rectifier, however it might not.

Is very easy to spot - partly under the aft side of the flywheel, you will have a 6 bolt cover with wires coming out of it, if so, that's your water cooled rectifier. If it's only a cover, no wires, then you likely have a little metal gadget that's just secured to the block on starboard, that's your smaller non water cooled version.

I didn't go investigate if the number of poles are the same for a 70/3-cylinder versus 90/4-cylinder model, and it may not be. If it's not, I think your tach should operate - just isn't going to give you the right RPM.

So, in short, go see what you got & trouble-shoot, but if you fix it and the tach starts working, make sure they use the same setting as your tach might not display the right RPM.
 
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