Tacho output

[M800]

200 Hz is 6000 rpm on a 4 cylinder tacho and 3000 rpm on an 8 cylinder tacho.
This 4 cylinder may spin to 9500 rpm on the stock (MX5) tacho.

Thanks

My question was do the ign outputs have the 200 Hz cap?

[NathanS]

Wolf_Tm250 wrote:
how could i drive it with a low side signal, if it needs positive 20v?

The 4.7K ohm resistor that Martin mentioned effectively reverses the voltage of the signal from the output that you are using, providing a pulsed high signal to the tacho.

The ignition and injector outputs on an Mxx0 ECU have a max freq of 5000Hz so either would be suitable for running a tacho.

[M800]

Thanks Nathan, I just wanted to be sure

Another way of wiring the tacho is, instead of a resistor, put a contactless relay between the output and 12v. Does the same thing as the resistor. The inductance in the coil changes depending on the load so it's always got the correct resistance for the tacho

Cheers
Bazz

[Mazi]

I think you can use INJ8 on your M400 configured as knock window output with resistor coil or tungsten lamp connected to +12V

Thank you Motec

[MarkMc]

Just be a bit careful,
Depending on the age of the car the Tacho may be a coil negative drive tacho which means it works on the big back EMF voltages you get when the coil fires. If it is a coil negative tacho the simple pull-up to 12v may not work. You may need to add an inductive coil like the control side of a relay (see attached drawing).

If you use a realy you will be best to get the metal bodied ones where you can remove the body to rip off the high current switch otherwise the relay buzzes away at the frequencey of the tacho.

[Wolf_Tm250]

Just be a bit careful,
Depending on the age of the car the Tacho may be a coil negative drive tacho which means it works on the big back EMF voltages you get when the coil fires. If it is a coil negative tacho the simple pull-up to 12v may not work. You may need to add an inductive coil like the control side of a relay (see attached drawing).

If you use a realy you will be best to get the metal bodied ones where you can remove the body to rip off the high current switch otherwise the relay buzzes away at the frequencey of the tacho.

Hello Mark,

I tried the relay but it doesn't work on my 1995 Toyota Celica.
Could you please tell me how you would resolve it ?

This pic is my igniter wiring diagram, the "TAC" grey (GR) wire (red mark) is the output to the gauge:




This pic is my gauge diagram:

[Wolf_Tm250]

I removed the relay option and put the resistor:
even in this way, TESTING THE OUTPUT, it didn't work...

Maybe it doesn't work, in either way, because, testing the output, the speed sensor is not working ?

Thank you.

[SprinterTRD]

The relay trick should work on most coil neg style tacho circuits, are you sure the relay was a non suppressed type?
Some relays have diodes or resistors across the coil to reduce the voltage spikes.

Also the relay trick wont work on some of the ECU outputs as they have inbuilt protection diodes.

[Wolf_Tm250]

The relay trick should work on most coil neg style tacho circuits, are you sure the relay was a non suppressed type?
Some relays have diodes or resistors across the coil to reduce the voltage spikes.

Really don't know... but it seems to me to be nothing on these relays I have used, except the coil... please have a look:

Also the relay trick wont work on some of the ECU outputs as they have inbuilt protection diodes.

Do you think M400 has this protection ?

I tested relay output before and after the coil, during output test, but the voltage is always 12v,
so it seems the voltage is not raised up by the coil...

where am I wrong ?

Thanks in advance.

[SprinterTRD]

No supressors in the relay so thats ok. The relay trick will only work with AUX 3,4 and unused injector and ignition outputs.