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PostPosted: Wed Jul 01, 2015 6:34 pm 
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Joined: Tue Jan 15, 2013 4:41 pm
Posts: 6
Hello everyone,

yes the tacho is supposed to be suited to 5v ECU output.

i have been trying to get a new tacho working with the SR ECU.
i have joined the TACHO out put from the ECU to the green on the Tacho and nothing.
can i seek signal from Positive on the coilpack or any other source.

my auto electrician sad that the signal required a pull down resistor. i will try but have little faith with this suggestion.

this is supposed to be simple my god. anyone else feeling my pain or have solutions.

Cheers,


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PostPosted: Wed Jul 01, 2015 7:48 pm 
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Joined: Tue Aug 26, 2003 10:25 pm
Posts: 40
Hey mate
Wire in a resistor between ign 12v wire and signal wire from Ecu.


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PostPosted: Thu Jul 02, 2015 10:45 am 
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Guru
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Joined: Tue Mar 04, 2003 10:09 am
Posts: 2596
Location: Lonsdale, South Australia
1.2k to 1.5K 1/4 watt will do the job.
Works for us.
BTW, usually called a pull up.

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PostPosted: Thu Jul 02, 2015 11:19 am 
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Joined: Tue Jan 15, 2013 4:41 pm
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Baz wrote:
1.2k to 1.5K 1/4 watt will do the job.
Works for us.
BTW, usually called a pull up.
sorry guru,
Is this what you mean?
i dont see why i need to pull up as the tacho is supposed to be 5v and i thought the ECU output 5v also.


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PostPosted: Thu Jul 02, 2015 11:20 am 
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Joined: Tue Jan 15, 2013 4:41 pm
Posts: 6
danielmcdonald wrote:
Hey mate
Wire in a resistor between ign 12v wire and signal wire from Ecu.
hello Dan,
thank you. do you mean as per my reply to GURU?


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PostPosted: Thu Jul 02, 2015 7:53 pm 
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Joined: Tue Aug 07, 2012 5:44 pm
Posts: 171
Location: Northern NSW / Queensland
Go with the pull-up resistor. A range of values will work, it's not to critical. 1.2k ohms to 4.7k ohms.

The reason is your tacho is expecting to see a signal pulsing from 0volts to 5 or 12volts, the tacho has strictly an input only. The ECU tacho output is "open collector" - it switches a voltage to ground (0 volts) and doesn't "output" a voltage. So all your tacho sees is 0volts the whole time. The pull up resistor puts a voltage on the tacho signal wire for the ECU to pull down to 0 volts periodically. Too small a value resistor and the transistor in the ECU will be trying to switch amps instead of milliamps and will blow.


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PostPosted: Fri Jul 03, 2015 11:21 am 
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Joined: Tue Jan 15, 2013 4:41 pm
Posts: 6
Ok randomly this worked.

i found this by fluke


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