Tach question for gaugeguy

[Boomyal]

I'm going to be installing an MSD capacitive discharge ignition system in my boat. Speculation is that my Teleflex tach will not work with the ignition. MSD has a seperate tach lead that puts out a 12 volt square wave signal with a 20% duty cycle. My current Teleflex tach is a apparently a voltage trigger tach as it hooks to the negative terminal of my coil.<br /><br />Do you know of a marine tach (approx std 3 inch) that will work with the ignition. I might as well by a new tach as opposed to buying a 'tach adaptor'. <br /><br />The Teleflex is a replacement for my OEM tach that crapped out a few years ago. I never liked it anyway. It alwas sits at about 1200 rpm with the key off.

 

[gaugeguy]

Re: Tach question for gaugeguy

Hey Boom,<br />The Teleflex tach still may work. The way it is wired now(off the negative side of the coil), it is actually frequency triggered by the ignition pulses. A 12V square wave with 20% duty cycle is just about a perfect signal for a tach to pick up.<br /><br />The only problem that may arise is how Teleflex filters out the junk on their tachs. If they filter out anything 12V or higher, for noise suppression (ignition pulses have a very high amplitude) then it may not work. <br /><br />The tachs that we build here, we filter out noise below 6V of amplitude and have had very good success running standard ignition tachs off of the MSD ignition set up. Before you go spending money on a new tach, try the Teleflex, you may be pleasantly suprised.<br /><br />Quite a few tachs nowadays do not return to zero when power is removed. As you know, in the marine industry, boat builders beat you up over dimes. Most manufactures have removed the RTZ feature to save a few pennies. We removed it as well but still offer it as an "add on" feature.

 

[amirm]

Re: Tach question for gaugeguy

Curious as to what keeps an analog guage from returning to zero. Is it the capacitance on the input and as such, a standard bleeder resistor is all that it takes for it to return to zero on power off? Or are you talking about a mechanical system that would return the needle back to zero?<br /><br />Amir

 

[Boomyal]

Re: Tach question for gaugeguy

Thanks Guageguy I'll give it a whirl.

 

[gaugeguy]

Re: Tach question for gaugeguy

There are two ways a tach pointer returns to zero. One is by using a coilspring on the pointer shaft to mechanically drive it back to zero. That is the more expensive way to do it.<br /><br />The way most gauge manufacturers do it is by a return to zero magnet. If you have ever opened up a gauge, the pointer is attatched to a rotor and then to a circular magnet. That magnet is then charged and given it's pole locations. An external magnet is stuck to the movement assembly and then when the gauge is at rest, the pointer magnet's N pole will align itself with the external magnet's S pole, returning the pointer to zero.<br /><br />Simple as pie