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I'm running my 2.0 four cylinder with an aftermarket ECU.

In the settings for the tach signal I've selected 12v, 50% duty and this worked fine for a few months of testing.

The day before my dyno session the tach started behaving wierdly. It would start up fine but after a little bit of running it would settle between 3k-4k rpm. Revving the engine would momentarily get it back to normal but after a few seconds at constant rpm it would return to its new favorite position.

Any ideas what could be causing this? Do I need to get the 12v signal amplied to the higher voltage spike the coil would give or can I modify the tach to accept the 12v or 5v signal my ECU can send?

http://www.msextra.com/doc/pdf/html/MS2 ... e-3.4.html

You need the "high voltage" circuit.

Thanks Pile, I'll give it a go if I can find the components needed.

Well I had some progress today, but not the good kind.

Borrowed an Autometer SportComp tach from a friend and hooked it directly to the ECU, reads the signal from the ecu perfectly and shows the rpm that the ecu sends out.

Pulled the stock VDO tack from the cluster and when I pull the signal wire from it it still shows 3000 rpm. I'm guessing some component inside it has failed.....crap.

http://www.ebay.com/itm/100-Brand-New-5 ... 236cbf4968

Sadly its only available with ricer RPM display range, but set it to 6 cylinder mode, max is 7300ish, keeps the kids guessing.
I haven't gotten around to ripping the thing apart to make a new face with a reasonable scale.

Comes with sensors and the shift light.

Nah I'll get the stock one repaired, dont want a blingy tach since I can get all the engine info wirelessly from the ECU and display it on a tablet on the dash.