early 911 tach compatible with MSD system

[Nivag]

I am running an early 911 VDO 8000rpm(6cyl) tach that I am having converted over to run as a 4cly tach. I am running a MSD billet distributor and digital 6 ign box.

Does anyone know if an early 911 tachometer will work with a MSD ignition system or does the curcuit board in the tach need to be changed?

Thanks for any info, Gavin

[Mo_Gearhead]

I'm running an MSD setup in my car and my tach. works fine (once I got the wiring sorted out).

What confuses me is your comment about converting it to 4 cyls.? Using it in a 912 ...or???

[304065]

You don't say how early. In general, a 64-71 tach won't work at all as it uses a low impedance 24V square wave driven from the coil. A later tach uses a high impedance 11V square wave but the duty cycle is wrong for MSD

You should send it to NH Speedo or Palo Alto to be converted to MSD, this is not expensive. Search here for more information, there are dozens of posts on this.

[Nivag]

Thanx for the replies...

John, the tach is dated 10/66 so thanks for the info. I did speak to Hartmut at NH and he was the one that originally suggested I might have to change but he wasnt sure. So I thought I would double check here before having them upgrade.

thx,Gavin

[pope]

My 1970 tach works with the MSD tach adapter in place.

I think it's more bouncy than it was before the MSD install but when it settles its reading is more or less correct. I don't know what is normal though. Someone told me that old 911 tachs always bounce.

[RWebb]

Originally Posted by john_cramer

A later tach uses a high impedance 11V square wave but the duty cycle is wrong for MSD

John, I know you've pulled these thinga apart and analyzed the circuitry, but my 197.5 tach works fine with my bare (no adapter) msd 6AL...

[304065]

Originally Posted by RWebb

John, I know you've pulled these thinga apart and analyzed the circuitry, but my 197.5 tach works fine with my bare (no adapter) msd 6AL...

no it runs at the wrong rpm check with external tach like the one on a timing light

[RWebb]

are you saying my tach is reading high? or low?

by how much?

[pope]

I'm following this with great interest

[pwd72s]

Another possible solution would be to have a VDO tach from a later model, one that works with MSD, refaced to look like the old '66 Tach...dunno if the dia. would be the same, tho...

[304065]

NH or Palo alto can easily silkscreen a different face on the tach and change the needle to one appropriate for the year, this isn't difficult at all.

Randy, for a while I used a mid-year tach in my car, the one with the pointy needle (not the blunt SC needle) with black center (not silver center). While the needle moved when driven by the MSD's 12V square wave with 20% duty cycle, the RPM was wrong compared to a dash-mounted digital tach/shift light. If I recall correctly the tach read too HIGH and it was by a couple thousand RPM at the high end of the range.

I ended up sourcing a good 912 tach of the same vintage and using that, the entire circuit board gets changed and only the movement remains.

Thinking about the design of the ancient tach circuits it shouldn't make a difference and here's why--

a 20% (MSD) vs a 63% (911) vs a 50% (912) duty cycle shouldn't make a difference, because it's the RISING edge of the signal from the points that triggers the tach. Remember that the monostable multivibrator circuit's output is stable in the LOW state and when an input pulse arrives, this turns OFF Q1 and ON Q2, causing the circuit's capacitor to charge and the needle to deflect. The pulse width of the input pulse doesn't matter- because as soon as Q1 turns off Q2 is held on without regard to the presence of the input signal.

Nor should input voltage matter that much, as the circuit includes a voltage regulator. The capacitor must be charged with exactly the same voltage, as the charge time determines the duration that the needle is deflected. Higher voltage charges the cap faster and also moves the needle higher so you get an unpredictable result. Otherwise your tach reading would fluctuate all over the place under varying electrical loads (e.g. read lower with the headlights on). But it doesn't.

So. . .in review-- trigger of the circuit should be independent of pulse width; needle deflection should be independent of system voltage or input signal voltage. . . .however like most things in the real world it doesn't operate as it SHOULD. . . try the external tach and see if it's right, I've been wrong before!

[RWebb]

Thanks MUCH!

"Remember that the monostable multivibrator circuit's output is stable in the LOW state and when an input pulse arrives, this turns OFF Q1 and ON Q2..."

- You are making me travel back thru many decades of cob webs, here John - I am getting all choked up from the dust and detritus in the old neural circuits. Very much like the Doonesbury series Inside Reagan's Brain...

For those who never encountered this stuff at all, the bottom line is the same -- get a good tach and test yours.