Tach signal and EFI, Megasquirters and Subaru converters please read... Options

[falcor75]

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?

[falcor75]

The signal is on a dedicated tach output from the ECU, and I can set it to either 12v or 5v with 50% duty cycle or a duty cycle I choose myself.

[stugray]

The signal is on a dedicated tach output from the ECU, and I can set it to either 12v or 5v with 50% duty cycle or a duty cycle I choose myself.

So the ECU reads the actual tach signal from the coil, then 'conditions' it and routes it back out to the TACH as a +12 or +5VDC signal?

That sounds a lot like my setup using the MSD 6AL.
There is a +12V 'TACH' signal that comes out but the Stock TACH could not read it.
So I used the MSD Tach-Adapt like shown above.
HOWEVER - the TACH signal that goes INTO the Tach Adapt has to be capable of relatively high current, so check to be sure the Megasquirt tach signal you intend to use is capable before you try it or you could fry it.

[falcor75]

The signal is on a dedicated tach output from the ECU, and I can set it to either 12v or 5v with 50% duty cycle or a duty cycle I choose myself.

So the ECU reads the actual tach signal from the coil, then 'conditions' it and routes it back out to the TACH as a +12 or +5VDC signal?

That sounds a lot like my setup using the MSD 6AL.
There is a +12V 'TACH' signal that comes out but the Stock TACH could not read it.
So I used the MSD Tach-Adapt like shown above.
HOWEVER - the TACH signal that goes INTO the Tach Adapt has to be capable of relatively high current, so check to be sure the Megasquirt tach signal you intend to use is capable before you try it or you could fry it.

Oops, I'm sorry I misunderstood you. My ecu gets the crank speed signal from the crank position sensor and a 36-1 tooth wheel mounted behind the fan.

The ecu can then send this signal out as either a 5v or 12v signal depend what I select in the setup.

I guess I'll need to build an amplifier circuit since I dont really want to open up the tach to modify it.