Tachometer Adapter

[budvoss]

Originally Posted by Bob Ashlock Generally this works for most motorcycle applications. Thanks. thanks so much, you just made my day.

[al lkosmal]

Bob, I don't know how I missed this thread.....I've emailed you with a few questions regarding a current project.

regards,
al

[lgargour]

Originally Posted by Bob Ashlock

Update: This started off as a fix for my own car, then I made some for other Porsche enthusiasts. I can now produce these units on demand. The design has been refined and the unit is now in a smaller size housing at 2” x 1.5” x 1” .

To summarize, Tach-Adapt locks to the points (or electronic ignition) signal, and then regenerates a new pulse-train at the rate needed to make your tachometer read correctly, even if it was intended for use on an engine with fewer or more cylinders. Tach-Adapt is useful for…

1) Engine swaps: For example, swapping a 911 engine into a 912 body, or vice-versa. Use Tach-Adapt to make the original tachometer indicate correctly with the new engine.

2) Tachometer changes/upgrades: Tach-Adapt allows use of (for example) a 6-cylinder tach on a 4 or 8 cylinder engine.

3) CD ignition upgrades. When adding CDI to a car that had basic Kettering ignition, the original tachometer usually won't respond because the 'new' points signal lacks a high voltage pulse. Tach-Adapt regenerates this pulse to your tach, making it indicate correctly.

Tach-Adapt is a quick, non-invasive, low-cost, solution that can be easily reversed. It’s simple to install and avoids the hassle and higher expense of tachometer removal and re-calibration by an outside service.

hi i have a modified 930 running electronic xap engine management can i get my original tach to work?

[Bob Ashlock]

Does the engine management system provide a tachometer signal?

[JPM]

Hi,

I have just replaced a 3-pin CD ignition with a MSD ignition and Tach Adaptor P/N 8910 on my 69T, but I am getting quite a lot of needle bounce. Is this something your unit can fix or does it sound like the MSD part is defective?

Thanks,

JP

[Bob Ashlock]

Common problem with the MSD adapter driving these old VDO tachs. You should use the discrete tachometer output from the MSD and send that to my TACH-ADAPT unit. It will then convert that low voltage square wave tach signal to one with a narrow high voltage leading edge that is needed to trigger your '69 tachometer.

[JPM]

Thanks Bob. I don't quite understand the point of the msd tach adaptor then. Or was it designed for the later tachs that don't need a high voltage pulse...but if that is the case isn't the low voltage tach signal what the msd generates in the first place? Confusing issue.

In my old system, the tach was driven by the high voltage CD output from terminal A through the Intermediate Unit. I guess you are saying the msd tach output won't work with the Intermediate unit.

Talking about tachs, I have a question on the RPM Transducer which is not used with carbs except for the 911T USA version. I don't have the throttle valve compensator system anymore so the RPM box is not doing anything, right?

JP

[Bob Ashlock]

As I understand it, the MSD adapter is basically is a toggled inductor that puts out a pretty high voltage spike with some undamped resonating pulses. Some tachs respond to those extra pulses and bounce around. The old VDO tachs are sensitive to this. Your 1969 VDO tach wants to see ONE solid high Voltage pulse, greater than around 80V. (Later VDO tachs from August 1971 will respond to the low voltage MSD signal.)

The intermediate unit circuit (which shapes the CDI discharge signal) will not work if it is connected to the coil (as it was from the factory) because the coil signal now has multiple pulses from the MSD.

The unit I make takes the low voltage MSD tach signal and provides a nice single, narrow pulse that the old VDO tach likes.

The RPM transducer (aka speed relay) modulates a fuel control solenoid on the Zenith Carbs. (And the MFI) Essentially, shutting off fuel to the idle circuits when coming of throttle until rpms drop into the lower ranges, somewhere around 1500 (off the top of my head). A good thing to avoid overrun rich mixture that contributes to smog and possibly some popping in the exhaust. If the fuel control solenoid is missing then the transducer is not providing this benefit.

[JPM]

I discovered some interesting things in preparation for installing your system which I ordered from our host. The pro who installed the msd tach adapter had routed the points signal through the Intermediate Unit to the tach wire which had been cut, in other words, open circuit. How the tach worked at all I don't know. I then tried putting the points signal direct to the repaired tach line and it works, but not 100% so I am keen to compare with your TA. I will keep you posted.

JP

[JPM]

Hi Bob,

Well, I am pleased to confirm that my tach is now working correctly. Thank you for providing this valuable product. Don't know what is in the MSD Tach box but it weighs a ton compared to your neat little device. For the record my VDO Tach had a date stamp of 8/67 and a "1" at the tach connection, so definitely one of the older models which need a higher voltage pulse to work.

At the same time I cleaned up the electronics panel now that I don't need the Intermediate Ballast Unit and also the RPM Sensor since the throttle valve compensator disappeared some time ago. Finally figured out what those two modules were for.

Here is pic of installation

I'll be sure to pass on the good word.

JP

[Bob Ashlock]

Thanks JP! Glad to see you got everything working properly. I have over 1000 of these units now in service.

[Speed metal]

Ive had one for a few years. Good stuff!

[7783911]

can you develop one for speedos :-)