technoversions tachmatch voltage booster

[kully 560]

thanks Zed head, I did read that and that is why I choose the v- booster. on the early 74 260z the tach blue/white wire did connect to the ballast resistor if I remember which limited the voltage. the msd does put out 12v but on an oscilloscope it is only about 3-3.5 volts. that is why when I read the online info you found at the technoversions site about the products, I thought I needed the v-boost, but it does seem to be too much in voltage increase.

[kully 560]

yes, msd makes the 8910 and the 8920. the 8910 is for the early 240z cars with current driven tach, and the 8920 is for the 260z and up with voltage driven tachs. I will try and come off of the msd tach wire and go straight to my tach and see what happens. I do not think it will damage the tach as it is still voltage. if that fails I will have to play around with different value resistors and see how that works out. I will update as this goes on. thanks for all the help

[Zed Head]

This occurred to me - a GM HEI module should work as a voltage increaser.  It's odd that MSD's "12 volt square wave" is not 12 volts. 

But, you should be able to use an inexpensive HEI module as a pseudo amplifier.  Connect some small load to B and C of the module, like a light or a resistor, with a branch off to the tachometer wire, and G and W to the MSD tach output wire.  The G-W circuit will be the trigger, triggered by the MSD wire, to cause the module to pass the 12 volts to the light bulb or resistor, operating the tach through the branch.  

Basically the MSD tach wire acts like the reluctor trigger and the light bulb acts like the coil, and the branch off to the tach is the standard tach driver wire.

Only commenting while it's in my head.  Something to consider if you get stuck.

[Captain Obvious]

2 hours ago, Zed Head said:

I've always wondered about the voltage sensing tachs.  Do they just see the voltage appearing and disappearing as the coil circuit is made and broken?  Or do they need the big "flyback" spike from the coil discharge?

I've wondered the same thing, but I don't have a spare tach here for analysis. If someone wants to send me a later design tach for investigation, I'd be happy to put it on the operating table.   

[Patcon]

I'm leaning in the direction that Steve was leaning. I think you have too much resistance somewhere. Your new wiring is too small, bad connection, corrosion, something. When the load spikes the flow drops momentarily. Then it recovers. I'm no "Sparkie" but that is where I would start digging. I suggest if the Captain or Steve recommends a path, that you take it. It's one of the things they're experts at!

[cgsheen1]

3 hours ago, kully 560 said:

thanks Zed head, I did read that and that is why I choose the v- booster. on the early 74 260z the tach blue/white wire did connect to the ballast resistor if I remember which limited the voltage. the msd does put out 12v but on an oscilloscope it is only about 3-3.5 volts. that is why when I read the online info you found at the technoversions site about the products, I thought I needed the v-boost, but it does seem to be too much in voltage increase.

Nope. The Blue for the tach signal comes straight off the "-" coil terminal. If you're still using a single coil, there should be no need to amplify the signal to the tach - even if you switch electronic ignition modules. YOU DO: need to remove the stock electronic ignition module from the system. And, you need to take the tach signal from the coil, not an artificial signal from an ignition module. In my turbo-swapped 260Z I've used many different Ignitors (ignition modules) with the stock coil and the tach worked with them all (because it gets it's signal from the coil connection).

[Zed Head]

7 hours ago, kully 560 said:

I installed the technoversions tachmatch voltage booster on my early 260z today so I can have the stock oem tach working off of the msd 6a ingition direct.

No offense cg but I think that you have committed the not uncommon error of not reading all of the previous posts in order to grasp the whole of the situation.

kully is using an MSD 6A ignition, using the tach wire (gray wire) from the box to run his tachometer. Tachmatch Voltage Booster, which is inline with his tachometer.  The MSD has multiple spark at low RPM so his tachometer would be all over the place if he used his coil to drive the tachometer.  Apparently the gray wire wire outputs a simple 12 volt square wave to stimulate the tachometer.  It's described in the MSD instructions.

Edited April 1, 2023 by Zed Head

[kully 560]

well, here is my update today. I changed the resistor value from 35.2k ohm value to a 30.7k ohm value and the tach rpm stayed accurate. the tachometer never changed when I did turn on the electric cooling fan which is a good sign. but the msd was causing the tach to bounce a little at idle. now I did just order some more resistors to have a 32k, 33k, 34k ohm values on hand to play around with. it does seem I am getting close to maybe getting it right. I was a little afraid to hook up the msd 6a tach output directly to the tachometer as the pulse at idle was still making the tachometer bounce with the 30.7k ohm resistor. with the 35.2k ohm resistor I had no bounce. I think I need to find the right value between the 30.7k and the 35.2k ohm resistor.

[Zed Head]

Still not clear why you decided to start putting resistance in the MSD tach output circuit.  The circuit already has a 2.1 Kohm resistor in it.  The tachometer is designed to be on the end of a 12 volt line from the coil negative terminal.  It can't be passing much current because it would cause the coil field to stay energized if it did.  Maybe your tach is damaged and your efforts are finding a way to make it work.

[kully 560]

hello Zed head again, I don't know why but I thought with the msd 6a ingition you cannot wire the tach directly to the coil because of the 12v idle pulse? now the reason for the resistors is in the technoversion v booster the signal is a little too high and the tachometer runs about 1500 rpms higher than normal. now with a resistor value of 35.2k ohm the tachometer was spot on with the rpms except for the electric cooling fan surge. that is why I tried just for the hell of it to lower the resistance valve just to see what happens. and bingo at 31.7 k ohms the tachometer was steady when the fan came on except for the little load the alternator creates. I did have just a little at idle rpm tachometer bounce which I think going up just a little in resistance would fix that issue. am I going in the wrong direction? do you think I should wire it direct to the msd with the 12v idle pulse?

[Zed Head]

I had a weird situation on my 76 where the tachometer would not work correctly unless I connected a capacitor, actually a simple RF suppressing "condenser" from an alternator, to the coil negative circuit.  That was after I converted to a GM HEI module ignition system.  I don't understand why it worked but I confirmed it later when the connection broke and the tach went wonky again.  The needle would just sit and quiver at the low end of the scale.  I was doing the same thing you're doing, just guessing based on my own limited knowledge.  My working word was "noise".

Anyway,  just for viewing pleasure, here is the FSM schematic of the tachometer in a 260Z.  I assume that the square with the line through it represents a switch, probably a transistor, that is actuated by the square wave from the coil negative terminal.  You can see that the tachometer just passes current with each pulse from the coil.

[kully 560]

I did read on the net that there was somebody explaining that he removed a resistor from the back of the tachometer which would have lowered the voltage on stock systems and then installed a jumper to bypass the resistor and the tachometer worked great. but my dash cover will not let me remove the tachometer from the front. I do believe that the rpm idle bounce is from either noise or the msd 12v pulse. it just seems crazy to use the voltage booster to increase signal and then use a resistor to lower it somewhere between the two voltage points. if I do find the point where it works perfect with a little change in resistor value, I might leave well enough alone. the tachometer is basically a coil, but what voltage was it designed to operate on?