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.

Click to view attachment

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?

I'm going to say a high-resistance terminal is the problem. Some of the stock tachs work with modern ECUs, but many require an amplifier. The computer signal is really a bit weak to run an old tach. The easiest solution is to grab an Autometer tach from your local store. I have found a few wraps of gaffer's tape will create a nice friction-fit.

Otherwise, the installation of a signal amplifier is also easy.

except about the Autometer Tach. Works but it's not original.

Yeah I want to use the orginal tach. Anyone know if a signal amplifier that works? I googled some homebuilt stuff but I'd rather buy something off the shelf.

Google pull up resistor

Although I built my own here is a commercial option:
http://www.casperselectronics.com/cart/ind...&search=Vdo

http://www.jegs.com/i/MSD-Ignition/121/8920/10002/-1



Wiring--
Red: +12v
Black: Ground
White: Black/Purple wire from engine/coil
Violet: To tachometer, where the black/purple used to go.

You can install this behind the gauge and use all the wires off the tach for connections.

or read Timothy's great writeup on how to convert your old tach guts to new tach guts.

Where are you grabbing the tach signal?

If it is from the coil (points) then it is not just 12Volts, but more like +/-200.
I have a homebuilt datalogger (ECU) and I pull the signal from the optical sensor in my Mallory Unilite.
It IS a nice clean 0-12VDC signal, but unlikely it is 50% Duty Cycle.

Can you put an oscope on the signal and look at it?
If you dont have access to an oscope, you could use one of these for $26 US:
http://www.banggood.com/DSO138-DIY-Digital...-p-1051616.html

But if you are not sure of the signal amplitude you would use an attenuator (variable resistor) to first check the signal to avoid blowing up your scope.

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.

You can modify the stock tach to work without a signal amp. I do not recall the specifics off the top of my head but the info is out there if you search for it, its a very simple mod. I did that and the mod to eliminate the tach bounce about 10 years ago and its been fine triggering off my Megasquirt+MSD setup ever since.

[size=5]I am using a cool adapter called tach adapt with a Suby controlled by a link computer on my car. It converts a modern low voltage signal to what your old tach needs. Also will convert 4,6,or 8 cyl signal to 4,6,or 8 tachs. I somehow stumbled onto the website and mine works great. Pelican sells them or you can buy direct. AshlockTech.com Good luck.

Found my notes, fix was to jumper over the primary input resistor, super easy single wire addition in the tach.

Did some testing today...

First i borrowed an Autometer SportComp tach from friend and hooked that up to the ECU output cable. The autometer tach is spot on what the ECU is sending out.

Popped the VDO tach from the cluster and pulled the signal wire from it, it still shows 3000 rpm! So, electronics isnt my strong suite but I'm guessing one of the internal components of the tach has failed somehow?

Like I said, you can throw an Autometer tach in there and gain perfect (not bouncy) function plus a shift light if you like.

A /4 VDO tach is nothing special, except that it looks like it goes there.

Is this really that different from a stock tach?

It has several cool functions and works perfectly with your ECU.

Yeah I know it would work but its ugly and looks like it should be in a NOS'd Honda rather than a seventies Porsche....

I bet North Hollywood or Palo Alto Speedometer can put modern guts in an old tach.

That would be the perfect solution.