Tach on 77 merc 70hp

[kylehartig]

I have a 77 merc 700 with thunderbolt ignition. I have a tach connection on the front of the mercontrol i haver heard on this site that you should be able to set the tach to 5pole and use that tach connection. i am just double checking that this is the correct way and pole setting for my specific engine.

Thanks in advance

 

[Barnacle_Bill]

Re: Tach on 77 merc 70hp

Those connections should work but the tach should be set to 6 pole.

 

[Texasmark]

Re: Tach on 77 merc 70hp

I have a '02 90 hp and just answered a question couple of days ago on a Merc 25.

My service manual which includes Merc 75 hp back thru '94 says the engine has an 8 pole alternator (shows pictures of 8 poles also) and the tach is to be set to 4P. They show a picture of it and dedicate a page to the subject. It may be that they are referring to a Quicksilver tach.......because.......

The teleflex website has how to do it instructions (go look it up) and the last page lists engines and their settings. According to this sheet, the 25 Merc also has an 8 pole alternator and is to be set to 4P.

The teleflex web site also says that anything (Merc/Mar) of 2 cycle of over 40 hp is set to 6P like bhile said...........but it also says that they are 12P alternators and that's not true or my service manual lied to me in half a dozen places which I seriously doubt. , it may be 4P like mine.

I think the proof of the puddin is that it is either 4 or 6 and you can determine the correct setting 2 ways. If idle speed, in the water in F gear gives you around 700 rpm's and WOT is around 5000 to 6000 you are there. If idle is different or WOT is off the scale or way low, change to the other position.

Also, the pointer has to be exactly on the number to get the reading.....seems a little moving back and forth to find the sweet spot (where it reads best) is the way to set it.

HTH.

Mark

 

[kylehartig]

Re: Tach on 77 merc 70hp

Thanks for the response i will test the 4pole and 6pole settings

 

[Scaaty]

Re: Tach on 77 merc 70hp

You can flip it around and check..won't hurt anything. Just have the motor off when flipping

 

[ricksrster]

Re: Tach on 77 merc 70hp

If you are talking about a 70hp then it is 12 pole/6 pulse

 

[Texasmark]

Re: Tach on 77 merc 70hp

Not sure it matters whether or not the engine is running when flipping. Having dissected a QS week ago, the only difference in settings is resistor values leading into the tach chip. But if you don't have to why do it. I'm trained to have power off when you connect/disconnect equipment due to possible transient spikes. It is the best policy.

Mark

 

[Sig_Mech]

Re: Tach on 77 merc 70hp

I just installed a tach for my '74 Thunderbolt 500 and I can help you with some possible problems you will encounter. If you are using the tach harness fron the front of your controls and you are connecting to a stock Merc tach for that motor then you will be fine. If you are using an aftermarket tach (Faria, Teleflex, etc) then it will be a little more involved. Make all electrical connections to the tach and set the adjustment switch to 6p. The tach will use pulses from the stator poles to operate.
Try this and see if it works. If you still have problems (as I did), then you will have to move the tach sensing wire on your engines electrical harness from the switchbox to one side of your rectifier unit. I can help you with this if you need it.

 

[ricksrster]

Re: Tach on 77 merc 70hp

To clarify what Sig said: Connect to either yellow wire on the rectifier.

 

[kylehartig]

Re: Tach on 77 merc 70hp

so what you and sig are saying is that because i do have a aftermarket tach i need to move the tach sensing wire on the switch box to either yellow wire on the rect. what color on the switchbox would it be and it dosnt mess with how the boat runs right.

Thanks

 

[Barnacle_Bill]

Re: Tach on 77 merc 70hp

According to the wiring diagram for that motor you shouldn't have to change anything. The tach leads at the remote are Black-ground, White-12v, Brown-Sensor.

 

[Texasmark]

Re: Tach on 77 merc 70hp

So I just got tired of this "Flim Flam Man" stuff. I took my 6'5", 250 lb. frame and dove under the dash of my current (little) boat this afternoon (since I couldn't bale hay with all the rain and all).

With the (my) '02 90 Merc 3 cyl. which the manual says that the 4P position is where you are supposed to be, as I mentioned yesterday or day B4, I went a lookin. Lo and Behold, the selector switch on the rear was sitting on 6P and I guarantee you the tach is working correctly.

So I guess Teleflex is right after all. Besides, the instruments on this boat, I guarantee you aren't Merc,. QS is nowhere to be seen on any of them. So the question is, is/are Teleflex and Quicksilver on different sheets of music?

So, like I said yesterday. Move the switch around till it makes sense.

HTH

Why does life have to be complicated all the time.....why can't it be simple for a retired guy who is trying to despool from all the technical crap. Maybe I just need to get a cane pole, a glob of worms, a six pack of Coors....grin, and sit on the bank.

Mark

 

[Sig_Mech]

Re: Tach on 77 merc 70hp

Sorry for the mis-information, bhile is correct. I checked the wiring diagram for your motor and it should be ready to rock. Just connect up your tach to the harness, set it to the 6p position and it should be good to go. I was using my experience with a '74 500 and the wiring is a little different. We learn something new everyday.

 

[Texasmark]

Re: Tach on 77 merc 70hp

Not familiar with your yearmodel. Later on, Merc used Gray for the tach drive signal. Purple was 12V power and black was ground.

Mark