Tach for 1975 Mercury 500 thunderbolt

[oba97]

I have done a lot of reading and am still confused on what a original tach looks like and how it connects. I found a old tach (might not even work as it's stuck on 2000) but it has the right plug to connect into the mercontrol. On my outboard it says to only use a quicksilver tach for the thunderbolt ignition. I have no clue if this is it or not?? Here are some pics...the number on the tach is 5657523 and my motor serial number is 4284039.

 

[oba97]

Anyone????

 

[clueless75]

They say to only use a specific tach, but as long as there are the three terminals (ignition, send, ground), you should be fine. Also, if you plug in your tach and turn the key to the "ON" position, the tach should move from 2000 to 0 (or close to it). I have a 1975 merc 850 with a brand new tach and works fine

 

[oba97]

No Title

I believe this is my wiring diagram. If you follow from the brown wire at the controls you will see the tan jumper at the motor harness that leads to the switch box. From there a brown wire leads to the mercury switch and a blue wire to the ignition driver. What is the ignition driver? Is the what the tach gets the signal from?

 

[clueless75]

I believe your diagram is wrong.. The "ignition driver" is the distributor, and if I'm correct your distributor shouldn't look like that, although I'm just basing that on my 1975 85hp merc. In your motor, the brown wire should be coming right out of the internal wiring harness and connected to one of the poles on the rectifier, as that is where the tach signal comes from on that year of motor. If you could send pics of your actual motor and its wiring harness, that would help.

 

[clueless75]

Does this look a bit more familiar? You can see that the brown wire connects to a terminal on the switch box, which in turn connects to the rectifier. You can also simply bypass the switch box and connect the brown wire directly to a rectifier terminal (connected to a terminal with a stator wire connected).

 

[oba97]

No Title

so I think what is going on is ours our different (at least from what I can tell from my repair manual). I found your motor and compared with mine and they are not the same. I've added a couple of pics from the book I am referencing.I did notice a difference in that the jumper wire is orange not tan.

 

[clueless75]

Ah, my bad, they are really different systems. Normally the tach signal comes from the rectifier, but this one seems to come from the distributor. I'm not familiar with this style of wiring but as long as everything is hooked up the way it shows on the diagram, it should work. Are you not getting a tach signal?

 

[oba97]

I haven't tried it other than turning on the the power and as you said the needle did drop to 0 as well as the light coming on. I read somewhere that you could potentially hurt the motors electrical system of you don't hook up the right tach?? Not sure if it's true but has me gunshy

 

[clueless75]

I've never heard that you can harm the electrical system by hooking up an improper tach, although I do know you can harm the tach if you hook that up wrong, but really it should be fine. Turn the motor over and if there's a tach signal, then you're good to go! If no signal, then diagnose and see what isn't right. If there's a big spark, then you hooked something up wrong hahah. You should be good though, give er a try!

 

[oba97]

No Title

I'll give it a shot. From doing some reading I think I might need something like in the attached picture. It does something to the signal??? Thanks for the help

 

[clueless75]

Honestly, I have no clue. I've never seen such a thing, maybe someone else knows what it does? Maybe do some more research and hopefully someone who knows more about this than I can help