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So I have a '86 nissan S12 with a ctsv ls2 swapped in it. I used a gmpp engine controller kit with it.
The whole dash work except for the tachometer. I expected that I would need to make a pull up circuit. I tried 4.7kohm as GM suggest, I then tried various resistance down to 670ohm. The needle didn't move at all.
I'm pretty sure the wiring is good. Ground to the tach is good, +12v to the tack is good and I ran a dedicated wire for the pull up from the ignition switch to the resistor then T'ed in the tach signal wire.
From my understanding, the E67 ecm output (white wire) is a 5v that is grounded to a frequency that correspond to a typical 4cyl engine. The purpose of the pull-up is to raise that 5v to around 12v. When the ECM ground the signal, the 12v drop near 0v so all the tach see is a 12v-0v square signal at a frequency of 2cycle/rev.
I managed to borrow an oscilloscope for the weekend. I want to make sure that my understanding of what the tach is looking for is correct.
So just the ecm white wire alone should output a 5v-0v square wave and with the pull up I should get something close 12v-0v?
I included the wiring diagram from the S12 fsm. Does it seem like a standard tach setup?
A lot of people will use a typical car relay to create the emf spike which would normally trigger the tacho, more akin to what happens in the ignition coil.
No doubt similar to what some commercially available "tach adaptor" type devices will do.
As for your pulse count that can sometimes be accommodated by adding a trim pot to the power coil of the tacho itself, or some may have an adjustable pot on their circuit board already.
Or Jaycar make this speedo corrector kit, which is essentially a pulse modifier which can go +/- 100% vs the input.
I'm almost sure it can do a different output vs input as far as VR copy or digital hall effect too. So it might even help getting the tacho to work.
Thanks for the reply, I'll definitely try the transitor method.
I can't say what the tacho resistor is, but on the first swap, I tapped the signal straight on the - ve of the coil and it worked...so I guess it was a way to filter the signal a bit but not really necessary.
I took the tach out of a spare cluster I have, just out of curiosity. The description in the mustang forum seem pretty spot on with what I have
I tried building my own tacho booster with a relay coil before using this one and it didn't worked. This one have an additionnal connection and I didn't bother opening it to see how it's wired in. The important part is that it work!!
So if you have an old car that took it signal from the coil for the car and nothing seem to work (dakota, pull up, etc.) this gadget is really worth the try!
On the E67 we just add a pin (J1-25) to use the built in pull up resistor.
So tach output J1-24 and the pin next to it are tied together and then to the tach input
. Steve
Yeah I guess I could have pull up the signal with an additionnal pin, although in the instruction that comes with the GMPP controller kit, they explain how to do the pull up circuit with a resistor.... maybe the GMPP E67 don't have that pull up built in??? Can't see a reason why they would remove it.
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Need help figuring my tach problem 
