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Recently we started an LS7 powered 69 Camaro. The LS7 was off of a 2010 ZO6 and the harness is from Speartech. We just installed Dakota Digital gauges, connecting the Tach white wire to the Dakota module, all working except for the TACH!!!!!
I've done everything that can be thought of!
1. Tried all the setup scenarios (4-8 cylinder - 5-12V).
2. Checked the tach wire continuity, its ok.
3. Replaced CPU with an LS3 CPU from Speartech.
4. Replaced Crank Sensor.
5. Made a direct Tach wire bypass to the Dakota module.
6. Replaced the Tach gauge and it's wire that connects between the two gauges.
The odd thing is that although I did all of the above, I'm still getting 0.25V in the Tach wire when checked by a Multimeter (AC). I should be getting 2-3V instead.
The last thing to do is to check with an OBD of the Tach signal is disabled, but note that I used two CPUs!!! It would be odd that both of them had the TACH signal disabled!!!
Recently we started an LS7 powered 69 Camaro. The LS7 was off of a 2010 ZO6 and the harness is from Speartech. We just installed Dakota Digital gauges, connecting the Tach white wire to the Dakota module, all working except for the TACH!!!!!
I've done everything that can be thought of!
1. Tried all the setup scenarios (4-8 cylinder - 5-12V).
2. Checked the tach wire continuity, its ok.
3. Replaced CPU with an LS3 CPU from Speartech.
4. Replaced Crank Sensor.
5. Made a direct Tach wire bypass to the Dakota module.
6. Replaced the Tach gauge and it's wire that connects between the two gauges.
The odd thing is that although I did all of the above, I'm still getting 0.25V in the Tach wire when checked by a Multimeter (AC). I should be getting 2-3V instead.
The last thing to do is to check with an OBD of the Tach signal is disabled, but note that I used two CPUs!!! It would be odd that both of them had the TACH signal disabled!!!
Any thoughts!
Thanks in advance.
I think the tach signal is an open collector output, with the voltage pull-up provided by the instrument cluster. Something in your setup needs to provide the pull-up.
If you disconnect the tach wire, what is the DC voltage at the Dakota gizmo's tach input? If it's around 0V, it's not providing a pull-up.
You can provide one by attaching a 5VDC reference to the tach signal wire through, say, a 1K ohm resistor.
I think the tach signal is an open collector output, with the voltage pull-up provided by the instrument cluster. Something in your setup needs to provide the pull-up.
If you disconnect the tach wire, what is the DC voltage at the Dakota gizmo's tach input? If it's around 0V, it's not providing a pull-up.
You can provide one by attaching a 5VDC reference to the tach signal wire through, say, a 1K ohm resistor.
Let me get this straight, we checked the white wire (Tach) from the harness while it is disconnected from Dakota gauge thru AC, but now we can check the same wire thru DC, is that it? Otherwise how can I check the Dakota gizmo input voltage wile wire is disconnected?
Is attaching the resistor going to boost the signal?
Let me get this straight, we checked the white wire (Tach) from the harness while it is disconnected from Dakota gauge thru AC, but now we can check the same wire thru DC, is that it? Otherwise how can I check the Dakota gizmo input voltage wile wire is disconnected?
Is attaching the resistor going to boost the signal?
If the tach signal provided by your ECM has an open collector output w/ no internal pull-up, you can think of it as providing an intermittent connection to ground that turns on and off at engine speed.
Something needs to provide a reference voltage, through a resistor, in order to get a usable waveform out of such an output (again, presuming that's what we're dealing with).
While the ECM is not "pulling down" (grounding) the signal you just see the reference voltage. While it is, the voltage goes near ground. This happens every time the engine goes around and you get a square wave for the tach.
I was curious if the Dakota thing provided this pull-up, which you would see as a DC voltage at its tach input with the tach disconnected. If it doesn't, and the ECM works this way, you need to provide the pull up yourself.
If the tach signal provided by your ECM has an open collector output w/ no internal pull-up, you can think of it as providing an intermittent connection to ground that turns on and off at engine speed.
Something needs to provide a reference voltage, through a resistor, in order to get a usable waveform out of such an output (again, presuming that's what we're dealing with).
While the ECM is not "pulling down" (grounding) the signal you just see the reference voltage. While it is, the voltage goes near ground. This happens every time the engine goes around and you get a square wave for the tach.
I was curious if the Dakota thing provided this pull-up, which you would see as a DC voltage at its tach input with the tach disconnected. If it doesn't, and the ECM works this way, you need to provide the pull up yourself.
Speartech already provides the resistor built in the harness. The only thing I didn’t try was to assume that this built-in resistor is damaged somehow. Tomorrow I’ll install another resistor on the Tach wire and check and confirm.
So, we dug thru the harness and turned out that the built-in resistor was the problem. The resistor was a 4.7K Ohm resistor with half volts! I’m not sure on the “half volt” part though. We replaced it and everything works
see pictures below...
Last edited by 71&64Vettes; Apr 8, 2018 at 03:12 PM.
Dakota Digital Tach not Working
