Tachometer for 1974 135hp

[WinnerCougar74]

The wiring diagram shows the white tachometer wire comes from the CDI box. So can I use a basic automotive tachometer since they normally just connect to the negative terminal on the coil?

 

[Nordin]

First of all automotive tachometer are for four strokers, your engine is a two stroker. The two stroker is firing at each revolution of the crank shaft the four stroker at every second revolution.
If the tacometer you try to use take the signal from the low voltage circuit of the ignition coil it is a 12V signal.
I do not know level of the white wire signal from the CD box but you can measure it with an analog VOM. With an analog VOM you maybe can see the level of the signal. A digital VOM would not show the peak signal level.
If the signal from the CD box is about 12V or a bit more a tachometer for a regular auto system for a 8 cylinder engine might work.

 

[jerryjerry05]

Won't work.
You need a 20 pole tach.
They went from a 20 to a 12 pole in the early 90's
Google 20 pole tach and see what shows.

 

[Nordin]

Jerryjerry the ignition system Winnercougar has in his 135Hp is the battery driven Motorola CD system. The stator is only charging the battery and the white wire that is for the tachometer is a signal from the CD box.
I think it is the same signal as the signal to the ignition coil but do not know the level of it.
The voltage in the prime side of the coil is about 200V or more.
A regular auto tachometer work with 12V signal to the prime side of the coil.

 

[WinnerCougar74]

Here is a pic of the tach I have now. It is likely original to the boat (1974). The center button rotates. When I rotate the center button it changes the RPM's. So now I'm not sure if I have ever known the real RPM's on my motor. So I'm thinking about getting a new one. Seems like getting a cheap automotive just to test it is the only way to know. I have traced the wiring and the white tach wire does go to the CD box like Nordin said. Wow an original tach #71H03 is for sale on ebay for $195. I think it's unlikely it will sell at that price.
I wonder what kind of voltage is at the negative side of the coil?

 

[jerryjerry05]

Get one and try?

 

[WinnerCougar74]

Seems like all the automotive tach instructions say to connect the signal wire to the tach terminal on the ignition box. The wiring diagrams for my motor indicate the only use for the white wire is tach signal so I think I'm on the right track. I will post results in the spring when I install it. Thx dudes for your help.

 

[stevedan]

not sure if any of this helps, but I also was having issues with finding a tach that would work, only to find out I had a shorted wire causing me issues with V.R. but the tach I finally used was a (part# 59703P) tach and was good for 3 pole up to a 20 pole (per what I was told), either way,, it works ! thanks for reading and JERRY is the man to listen to,, LOL,,

 

[WinnerCougar74]

Here is the followup to this original post about tachometers. An automotive tach will work. I installed a $30 tach that was adjustable. I attached it to the normal tach location on the ignition switch and it works. No need for an expensive marine unit. Prolly because my motor has a distributor so it is a lot like a car engine. Hey we are always looking for ways to keep our expenses down and use some off the shelf stuff since OEM is hard to find and expensive.