standard ECU

[browny]

re-read my post above.

There is a reason why that wire doesnt read give any reading. It needs to be connected to a voltage source.

Go to an auto electrician that knows what they are measuring......

[hayes_666]

I see what your saying you need a load on the line from the tacho, or a resistor feed with 12 volts going back to pin 7 to allow the safc to read the squarewave output, as there is no high or low just a low and a nothing (open circuit).

So your getting a output of 0volts to ground instead of 12volts to ground

Nathan

[discopotato03]

RSX84 are you doing this with the HR30's std tacho ? I will need to know this too .

Cheers A .

[Sydneykid]

the thing is is that wire from my ecu has no signal at all, its dead, been tested by two auto sparkies now and told no signal....

They are testing it the wrong way, what they are doing is connecting the positive from the multimeter to pin 7 and the negative from the multimeter to earth. This means they are measuring the voltage between earth (ECU) and earth (battery), obviously they will get no reading.

What they should be doing is connecting the positive from the multimeter to positive (battery) and the negative from the multimeter to pin 7. They will get a signal then.

Hope that helps

[Jay95R33]

What I found out about the output of the tacho is that you'll need to use a very high impendence load so you don't drain the signal.

It doesn't take much of a load to make the signal stay below 3v, and 3v won't run the tacho... Also, a quick short of the tacho output will result in popping a resistor in teh ECU (I found this out the hard way !).

So you problem could be one of two things. Someone in the past tried to hook up a after market tacho or shift light and stuffed it up resulting in a blown ECU, or the tacho your running now isn't compatible (is it the factory one ?) and the impedance is too low.

To test the signal you need an oscilloscope, a standard multimeter won't do it Just earth the ground of the oscilloscope to the ground of the ECU and probe the ECU tacho output, you'll see the signal being pulled to ground - just a normal square wave.

I can't help you with where to tap your SAFC into to get another signal

[rsx84]

what your saying is i've paid idiots who didn't take the time to do something right, and all in need is to boost the voltage of the signal! this will fix the problem so i can run the SAFC and tacho....

by the way its a autometer tacho i'm using not the standard one...

thanks to browny and sydneykid and everyone else for your help now i'm off to have a work with an auto sparkie.....

[rsx84]

part 2 of the sarga:

i replaced the ecu and nothing, didnt change anything still no signal, so its not the ecu?

anyone have any idea as to what else it could be?

[MonstaS4]

Like someone said earlier, you need a 10k ohm pull-up resistor from 12v to the tach out wire on the ecu. I'm guessing the factory tach does this internally, whereas the autometer wouldn't.

If you connect an oscilliscope to pin 7 i'm guessing you will see a square wave, but it won't be 0-12v, more like 0-300mV. This is similar to how the signals from the cas arrive at the ecu.

[browny]

In a nutshell you have four options:

* use the factory tacho

* keep trying aftermarket tachos until you find one that works

* Try a pull up resistor

* Get a tacho booster

[rsx84]

i got a tacho booster from the guy someone recomended before i got my hands on the new ecu, it didn't help, but i'll wire it up again with the new ecu just in case.... my old ecu is about to be used to fire up a halfcut i got the new ecu from so i'll see if it runs the factory r32 tacho still apart of the cut, that should give me a better idea of whats going on...

where does the ecu get the signal from the engine as thats my next path, see whats fgoing on there?? thanks guys, i'm just getting a bit f@#ked of with this hole process......

[browny]

Have you talked to Grant from Sixworks?

Possible problems:

* The tacho has had the dick

* The ecu has had the dick

* The booster is wired up incorrectly (mine didn't display the check light as per the instuctions because of the way the Nissan system works - but it works fine with engine running)

* Wrong type of booster for the tacho - there are two types

[browny]

I think the CAS provides the ECU with RPM.

[wideawake]

The tacho signal wire (pin 7) generates a signal by shorting to ground through the ECU.

If you were to connect this wire to an oscilloscope (and nothing else), the voltage waveform would be a flat line at 0V, ie the pin is being switched by the ECU between  ground and 0V.

To get a signal other than a flat line, you need to connect a voltage source to pin 7, so then the ECU is switching pin 7 between (say) 12V and 0V and therfore generating a square wave.

I'm pretty sure the gear that provides a voltage source to pin 7 is integral with the OEM tacho and I think this generates a 0-5V waveform.

Also different tacho's are deisgned to work with different voltage waveforms, some are universal but others are designed to work at say 12V or hundreds of volts (ie direct from the coil).

You can generate a 0-12V square wave by connecting pin 7 to a 12V rail. Because no-one knows the current limit of the circuit for pin 7 a pull up resistor of say 10k ohms is usually connected between the 12V rail and pin 7. This limits to current to a small amount so you don't burn out that part of the ECU.  

The pull up resistor method will only work for tachos which can use a 0-12V signal tho and it won't solve your problems if you need to modify the frequency of the pulses for different cylinder engines. If the tacho doesn't work, you will need a device that can generate a voltage signal that your tacho can read. These are commonly referred to as tacho boosters. Sixworks in Newcastle sell these devices (approx $50 or $100 depending on which you need).

These take about 5 min to install, I can put you in touch if you like.

Thanks for this info. It is proven. I managed to install my Defi D-gauge tachometer using this method.