1970 Merc 800 - Proper tach

[Moody Blue]

Want to install a tach for the above 4 cyl motor. Sticker on engine cowling says to "use only the tachometer specified for this ignition system" or something to that effect.

What should I be looking for? Can I use any of the current marine tachs (Faria, Teleflex, Quicksilver etc)?

Thanks

 

[Laddies]

Re: 1970 Merc 800 - Proper tach

Yes you can use any alternator driven tach but before hooking it to the existing wiring harness you need to remove the brown wire from the starboard side of the switch box to one of the rectifier yellow treminals.

 

[Moody Blue]

Re: 1970 Merc 800 - Proper tach

Thanks for the info.

To be clear, I need to remove the brown wire from the starboard side of the switchbox and reconnect it to either of the yellow/red stator terminals on the rectifier? This is no doubt the tach signal wire to the control box.

What is the function of the brown wire on the switchbox?

What is an alternator driven tach? How can I identify such a beast?

 

[Laddies]

Re: 1970 Merc 800 - Proper tach

At the time your engine was built the brown wire at the switch box was the sender wire for a tach. because of the problens they had with ign. powered tachs shorting the ign. systems out the manufacturers started building alternator powered tacs in the early 80s thats why yon need to change the sender wire. Heres a link that should help. http://www.downloadingfiles.co.uk/teleflexfiles/library/instructionmanuals/99279_R4.pdf

 

[LORDY611]

Re: 1970 Merc 800 - Proper tach

I just installed a Tiny Tach on my 4cyl 2 stroke Mercury. I couldnt believe how simple it was. One ground wire to the head somewhere, and literally wrap the signal pickup wire around a spark plug wire, then mount the readout somewhere. I didnt realize that the method of getting the pulse from the engine was basically an antenna, no electrical connection other than the ground.
Mine also has a resettable hours meter, all for about $40 shipped.

 

[Moody Blue]

Re: 1970 Merc 800 - Proper tach

Gentlemen, thank you for all this info.

I now know what direction to take and the proper way to do it.

 

[Moody Blue]

Re: 1970 Merc 800 - Proper tach

Well I bought a new-old-stock Quicksilver tach that the sales guy said would work in my motor.

Selector switch has 4 options 1-4cyl 2-6cyl 3-8cyl and 4-12pole alternator. Does anyone know if the 12 pole alternator setting will work on my motor?

 

[Laddies]

Re: 1970 Merc 800 - Proper tach

Thats the right position