914 Tach signal??

[Fast951]

I converted my 2.0L 914 to use a newer Motronic (944) ECU running a wasted spark system. Prior to removing the distributor, I did not take a snapshot of the tach signal. The original 914 tach is not reading the tach signal as generated by the 944 DME.

I was wondering if anyone has a snapshot of the 914 tach signal as generated by the distributor (a scope is needed for this). Or if anyone else a solution to interface to the tach.

TIA

[914GT]

John, I recall the waveform at the coil -terminal as a negative-going pulse of at least 100V peak with a lot of ringing. Pretty typical looking for a points or transistor ignition. The 914 tach needs this high-voltage ratty-looking waveform to trigger, since it has some R-C filtering on the input to deal with it. I think most of the ECUs put out a nice clean low-voltage (logic level) square wave and the old tach won't respond to it. There are some tach converters that are supposed to convert the low voltage signal to something close to the stock ignition. I built a tach calibrator a few years ago and used a transistor to switch the secondary of a small transformer (a common 120V - 12V step down transformer hooked in reverse as a step up transformer). This worked well to get a high voltage ringing waveform from a 5V logic level clock signal.

[Fast951]

Thank you for the information. The tach output from the DME is a 12V square wave. If I have to build a interface between the DME and the Tach, then I'll do it. However, due to time constraints, I rather get something off the shelf to move forward with the project.
Any idea who might sell such a interface?

[914GT]

This looks to be an adapter that converts from a 12V pulse to a signal similar to that from a conventional coil/ignition. I don't know if this particular product will work with the 914 stock tach however. The old VDO tach is somewhat finicky with what it wants to work correctly. I have seen some folks posting having trouble with their tachs with only their points being worn or improperly adjusted.


http://race-mart.storesecured.com/items/products/auto-meter/AUM9117-detail.htm

[Fast951]

I'll check it out. Thanks.

[MURF]

You will need msd tach adapter to use original 914 tach with dme.or electromotive...i forget the model # but call msd.or clewitt engineering

[Fast951]

Murf, Thank you for teh info. I did a search, it appears that MSD Tach Adapter PN 8920 might work.

I'll let you know how it goes when I try it.

[MURF]

You could fit an autometer 5" 'tach in the 914 hole we hot glue guned ours in..we changed to get higher rpm capability and memory

[904svo]

Heres a home made tach adapter that will drive the VDO tach


These parts were from Radio Shack and should cost less than $15

[MURF]

The autometer is much better tach.better response time and no vague bounce higher rpm capabilities and doesn't require a tach adapter.and has memory!! I bought mine used at swap meet for $50..cheaper than tach adapter from msd

[SirAndy]

Originally Posted by Fast951

I rather get something off the shelf to move forward with the project.

Not to state the obvious, but wouldn't it be much easier to just get a Tach with more modern internals?
Like, you know, maybe out of a 944?
Andy

[Fast951]

A local friend has the MSD Tach Adapter PN 8920. I connected it to the tach signal out of the DME and the output went to the 914 tach signal. It works fine, problem solved.

Thanks.