tachometer compatibility '74 85hp Johnson

[es_bob]

Hey guys,

The tach in my boat did not work when I bought the boat. I was told that it fried about 10 years ago when the voltage rectifier went out. So I found another tach on ebay (same thing) and the guy said it worked 30 years ago when he removed it. lol... Anyway, I installed it and it seemed to work properly but it got to 4000 rpm and stopped there. Then the next time I started the boat it jumped to 5500 and at that point I unhooked the tach wire.

So I'm thinking, instead of trying to find a tach that works from 30 years ago or fixing the one I have, I'll just replace the speedometer and tach (so both match). My question is, can I use any boat tachometer made for an outboard motor? Is there anyway I can measure the voltage coming from the tach wire to make sure that its giving the proper signal to the tach?

Would something like this work?
http://cgi.ebay.com/ebaymotors/NEW-...ryZ50437QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Thanks!
-Bob

 

[iwombat]

Re: tachometer compatibility '74 85hp Johnson

It _might_ work, but the seller hasn't provided enough information. You want one that is set for 6p, or has a dial on the back to switch from various settings including 6p.

As you may have guessed, 6p is the important piece of the puzzle.

 

[es_bob]

Re: tachometer compatibility '74 85hp Johnson

What exactly does 6p mean? Thanks for the info on this, I appreciate it.

 

[es_bob]

Re: tachometer compatibility '74 85hp Johnson

So I did some more reading and this tach is compatible with 12 pole (6 pulse) or 10 pole (5 pulse). Can someone explain the pole / pulse for me please? This tach SHOULD work then... right?

 

[jddenham]

Re: tachometer compatibility '74 85hp Johnson

Mercury motors from the same era also used 12 pole correct?

So they require the same tach design?

 

[F_R]

Re: tachometer compatibility '74 85hp Johnson

Without getting into the engineering, the basics is in the number of electrical pulses per revolution of the crankshaft. Poles equals twice the pulses. Just know the number that you need, and go from there.

 

[jddenham]

Re: tachometer compatibility '74 85hp Johnson

The point I was trying to make, is that if they use the same tach, you could buy a merc tach.

Often times on ebay, the seller doesn't give enough info. This is actually helpful in getting a better deal, as less people will bid.

Let's say for example there is a tach listed 'mercury tachometer'. The pic shows a nice teleflex tach, almost new, for $9.99. You ask the seller what motor it was on and he responds a 1976 Merc 50HP 4-cyl. That is a 12 pole motor, so it will also work on a 1973 Johnson 115HP.

Just some food for thought.

 

[es_bob]

Re: tachometer compatibility '74 85hp Johnson

Well I bought the tach off of ebay, and it has a dial on the back of it for selecting poles. I will post a picture of it tomorrow. I plan on getting that installed this week.

 

[sony224422]

Re: tachometer compatibility '74 85hp Johnson

would a 6p work on a 77-79 (not sure of the exact year) 115?

 

[es_bob]

Re: tachometer compatibility '74 85hp Johnson

So here is a pic of my tach I got. Should I set the switch to #2 or #5? 6 poles or 12 poles? Also, the battery terminal should go to a constant +12volt right?

Thanks a bunch guys!

 

[jddenham]

Re: tachometer compatibility '74 85hp Johnson

Position #5

 

[es_bob]

Re: tachometer compatibility '74 85hp Johnson

Thank you much, I think I'll install it tomorrow.

 

[es_bob]

Re: tachometer compatibility '74 85hp Johnson

Installed the tach and a speedometer to match. Both work great. The only thing I had to do that I wasn't expecting was wire up the tach to a switched 12 volt.

No big deal. The tach works, and looks great. Thanks for the info guys, I couldn't have done it without this place. On to the next project...

 

[es_bob]

Re: tachometer compatibility '74 85hp Johnson

Here's a pic of the finished product. I want to get a voltmeter and a clock in there yet...