Are Tach's engine specific?

[Uncivil]

I have a 89 Hewescraft with an 88 70 HP Johnson with a dead tach. When I looked into it at the local marine repair shop they wanted $150 cdn to supply a new tach plus installation. I can handle the installation myslef but I am wondering how to select the right tach. So what I have not been able to findout is, is there a right tach for a particluar powerhead? My johnson is a three cylinder so do I have to purchase a particluar tach or does it matter. Thanks for your advice.

 

[Texasmark]

Re: Are Tach's engine specific?

Yes. There are different sense systems so the engine has to supply the type trigger (input signal) that the tach is designed to recognize and second, it has to be calibrated. Calibration, as far as this subject is concerned, means that the tach needs to show the correct number on the dial for the number of pulses (or what ever) that are received.

Some current tach's available have selector switches on the rear which allow them to be used with 2 or 4 stroke engines of numerous cylinder values. Archives herein should have info on Teledyne tach's for example in what switch position relates to how many cylinders and the type of engine (2-4 cyl).

Doesn't say what you have won't work, you just have to find out.

Mark

 

[Silvertip]

Re: Are Tach's engine specific?

Your engine can use any of the generic tachometers that have a 6P (6 pulse) setting which is required with a 12 pole alternator. However, if and engine has a system check (low oil, no oil, overheat) indicator built into it, that would need to be taken into consideration. Yours apparently does not so any of the generic Teleflex/Faria tachs would work. Number of cylinders has nothing to do with the pulse count.

 

[Barnacle_Bill]

Re: Are Tach's engine specific?

Are you sure the tach is dead? There are other things that can cause a problem too. Like no power to it, bad connection, or bad rectifier.

 

[Uncivil]

Re: Are Tach's engine specific?

Thanks for the input. As far as I can tell the tack is dead. I am a bit clueless around electrics so I could have been misled. When w first bought the boat we thought the tach was ok but we had trouble with the VRO motor dying after it warmed up. we thought itm ight have been the stator unit or the related things which name escapes me right now!!!! Turns out it was the VRO pump so we removed it and the engine runs ok. After that point the tach has stopped working. So i am still a bit confused.

 

[Texasmark]

Re: Are Tach's engine specific?

Number of cylinders has nothing to do with the pulse count.

Maybe not in today's distributorless 4 cycles, but back when I worked on cars, the distributor was hooked to the cam and the cam turned half as fast as the crank (because the cam gear was twice the circumference of the crank drive gear) meaning that the distributor fired every other rev or half the frequency that it would in a 2 cycle for a given rpm.......did I forget something over the years?

Mark

 

[Mark_VTfisherman]

Re: Are Tach's engine specific?

.......did I forget something over the years? Mark

No. But add-on tachs for cars and motorcycles read spark plug wire pulses by induction pickup or are mechanical. Boat motors use the pulses of magnetos or alternators which are mechanically a part of the flywheels.

This is probably not the most scientifically correct description but I think it answers your question.