LS3 tacho - can't get mine to work
Discussion
Okay, I'm at my wits end. I have an e-rod LS3 (6.2) which I've installed into my Rolls but for some reason I can't get the tacho to work. I've tried various values of pull up resistors as recommended and still nothing. I've tried two different tachos so its not the device itself. Is there something stupid I've missed?
stevieturbo said:
From the link he posted, their instructions state this.
So fatbutt, are you saying you have tried this and it does not work ? Can you confirm with a scope or similar you are getting a pulsed signal of some type from the ecu output intended for a tacho ?
Yes, that's the problem: I've installed as per the instruction and getting nothing. I then installed a variable resistor as I had got from other LS3 forums that the value could be higher. So, I tweaked the resistor and while I did get a few blips it still won't work consistently. Its a low voltage square wave directly from the ECU - the e-rod harness comes with a specific output that you are supposed to just connect directly to and voila. I don't have a scope as I didn't think I'd need one given the simplicity of the instructions. I might just go get one of them and stump up the cash.So fatbutt, are you saying you have tried this and it does not work ? Can you confirm with a scope or similar you are getting a pulsed signal of some type from the ecu output intended for a tacho ?
I'm tending to think the ECU has issues but the e-rod one is unique to this package so I'd rather not go buy a $500+ item on a hunch.
stevieturbo said:
As they dont know what rev counter you are using...they cant say a generic solution will or wont work.
What rev counter are you using ? And if OEM, what was the makeup of the ignition system that used to trigger it ?
Whilst you can use a typical 12v relay to generate a higher voltage to fire to the tacho, you can also buy such devices
http://www.emeraldm3d.com/sensors-and-relays/tacho...
http://www.extraefi.co.uk/Drawings/PDF_Files/tacho...
The problem with older tacho's not working with modern low voltage outputs is fairly common.
Cheers. Both tachos I bought are modern units, one from Autometer and one from Stewart Warner.What rev counter are you using ? And if OEM, what was the makeup of the ignition system that used to trigger it ?
Whilst you can use a typical 12v relay to generate a higher voltage to fire to the tacho, you can also buy such devices
http://www.emeraldm3d.com/sensors-and-relays/tacho...
http://www.extraefi.co.uk/Drawings/PDF_Files/tacho...
The problem with older tacho's not working with modern low voltage outputs is fairly common.
I'll take a look at the links, thanks
Max_Torque said:
I'm going to assume that the ECU has internally what is called an "open collector" switch. This is effectively a switch to ground potential, that is closed by the ecu for a certain time period at some point during each engine revolution.
The tachometer has an input that requires a certain voltage level to trigger it, and it may trigger on the voltage going "low->high" or "High->low" across that threshold. The tacho may also include filter circuitry to try to block and noise or spurious pulses from causing false readings.
If you disconnect the tacho wire from the ecu, connect a basic volt meter to the ecu tacho wire, and measure the voltage on that wire with the ignition on, but engine off, and with the engine running.
Now connect the tacho to the ecu, without any externally added pull up resistor and repeat measurements.
If there is no "pull up resistor" in the ecu, then you probably won't see any voltage out of that pin, and your external pull up resistor will need to be a lower value to supply enough current to trigger the tacho. Generally, you should be able to use quite a low resistance, but i'd avoid anything lower than about 1Kohms. (ideally, something like a 4.7KOhm resistor should work).
So, add external resistor and repeat.
Let us know the results and i'll advise!
(a basic DC voltmeter will not be able to see the individual pulses on the tacho wire, but it should report the "average" voltage, which i would expect to increase slightly with engine speed (as there will be more "on" pulses per second). If the average voltage is less than say 1 or 2Vdc then this is probably too low to trigger the tacho.)
Top smart, I'll give that a go. I did something similar when I started but I don't remember the values so I'll do it again. Note: Amazon should have my scope software and adapter here tomorrow so I should be good for some screen grabs.The tachometer has an input that requires a certain voltage level to trigger it, and it may trigger on the voltage going "low->high" or "High->low" across that threshold. The tacho may also include filter circuitry to try to block and noise or spurious pulses from causing false readings.
If you disconnect the tacho wire from the ecu, connect a basic volt meter to the ecu tacho wire, and measure the voltage on that wire with the ignition on, but engine off, and with the engine running.
Now connect the tacho to the ecu, without any externally added pull up resistor and repeat measurements.
If there is no "pull up resistor" in the ecu, then you probably won't see any voltage out of that pin, and your external pull up resistor will need to be a lower value to supply enough current to trigger the tacho. Generally, you should be able to use quite a low resistance, but i'd avoid anything lower than about 1Kohms. (ideally, something like a 4.7KOhm resistor should work).
So, add external resistor and repeat.
Let us know the results and i'll advise!
(a basic DC voltmeter will not be able to see the individual pulses on the tacho wire, but it should report the "average" voltage, which i would expect to increase slightly with engine speed (as there will be more "on" pulses per second). If the average voltage is less than say 1 or 2Vdc then this is probably too low to trigger the tacho.)
Note: I did try at first with both tachos and no resistors. Didn't get a flicker.
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