Tachometer pull up resister??

markr9909
Posts: 1
Joined: Sun Aug 16, 2015 8:26 pm
Location: Kent

Tachometer pull up resister??

Postby markr9909 » Mon Aug 17, 2015 9:55 pm

Hi all,

I'm reasonably new to the forum in terms of being a member, although i have surf for information many times in the past and found it to be a lot of help.

I'm currently fitting a Dash2 in to my AE86 corolla which is running a Nissan sr20det engine with the standard nissan ECU
I have all of the dash up and running fine but am having a bit of trouble with the tachometer.

After doing a bit of surfing for info and using the search engine I think I'm right in saying that the Nissan tachometer runs an open collector signal,
And will need a pull up resister to 5v to work??

I believe someone on this site had success using a 10k pull up resister to 5v.

I'm hoping for clarification that I'm heading in the right direction and if so can someone please give me info on how I go about doing this?

A wiring diagram would be a big help if at all possible??

A quick sketch would be more then

Cheers for your time reading this and look forward to any help or advice that can be offered..

Regards Mark...

Support (D)
Posts: 74
Joined: Mon Aug 17, 2015 9:36 am

Postby Support (D) » Tue Aug 18, 2015 9:25 am

Hi Mark,

Yes you are heading in the right direction.

Try connecting a 1K pull up to the signal wire and 5V.

Best regards,

Support (D)

Aandreo92
Posts: 5
Joined: Mon Dec 14, 2015 11:18 am
Location: Russia

Tachometer pull up resister

Postby Aandreo92 » Thu Dec 17, 2015 9:29 am

Is there bedding for the pull out in the room prior to arrival, or does it need to be requested?
Thanks

dwarak17
Posts: 1
Joined: Wed Mar 09, 2016 5:49 am
Contact:

Postby dwarak17 » Mon Mar 14, 2016 11:17 am

A pull up resistor is merely a resistor - I'd suggest a 10kOhm 1/2W carbon film or similar (not wirewound - if they still exist!).

It should be connected between the tacho's signal wire and its +12V supply.

It sounds like your ECU has an open collector output for the tacho, and your tacho does not have an internal pull up resistor (why should they - they attach to ignition coils!).

The pull-up resistor pulls the open (collector) output up to +12V.
When the output transistor turns on, its collector terminal is connected to ground (close to 0V - maybe 0.2-1V?).
Hence the output is a square wave from near-ground to near +12V.

They should have provided a suggested value, but 10k should be safe.
I'd ask the before trying 1k.
Either value is low power but 1/2Watt is a common size that is reasonably robust. But 1/4W is still ample.



The FYI ramble follows:

"Open Collector" outputs are typically used in logic circuits - eg, CPUs, electronic controls/switches etc.

It simply means that the output is either earthed/grounded else is floating.
IE - a transistor is turned on to connect the output to 0V (zero volts) or it is turned off (high impedance) so the output "floats".

Why do this? Because you can then interconnect systems of different voltages - ie, 12V, 24V, 5V, 3.3V etc. (Not their power supplies - just their inputs & outputs.

Ignition points are an equivalent example - the points are either closed and shorted to chassis/ground, else open and floating. (It's the same as most ignitors, HEIs etc; though not CDI.)
The points are pulled up by the ignition coil.

For open collector outputs, there is nothing to pull the output up from ground.
Hence the pull-up resistor.
Most logic pull-up resistors are 10k Ohm (10,000 Ohms), but they can be much higher to reduce power consumption.
They can be as low as 1k Ohm, but that can be too low for many circuits, and 100 Ohm is very likely to burn out logic & CPU open collector outputs.
(Ignition coil ignitor open collector outputs are a special case that can handle say 12A switching - that's 1 Ohm at 12 Volts.)

Kamalesha
Posts: 2
Joined: Thu Mar 24, 2016 4:17 am

Re: Tachometer pull up resister??

Postby Kamalesha » Tue Mar 29, 2016 5:33 am

Some sensors can connect the info line to the bottom once activated....but they are doing NOT connect the info line to the +12v once NOT activated. The aim of the electrical device is to forever connect the signal line to the +12v at a restricted current. Once the detector connects the info line to the bottom, it doesn't have a electrical device, therefore has rather more current to ground, and "wins" the "fight" with the electrical device bridging the info line to the positive aspect, pull the info line down close to zero volts. However once the detector isn't activated, the electrical device pulls the signal/data line up to shut to +12v. It's vital that the electrical device has enough ohms so you're not passing an excessive amount of current from +12v to the homepage. IMHO your 10K Ohm electrical device ought to have lots of ohms and will not "fight" too onerous.

oceansoftwares
Posts: 1
Joined: Tue Feb 26, 2019 7:03 am

Re: Tachometer pull up resister??

Postby oceansoftwares » Tue Feb 26, 2019 9:04 am

I've been talking to my ECU suppliers who provide me with a custom ECU to run my engine. I am having trouble getting the car's original rev counter to show any reading.

If I put the voltmeter onto the pin comming out of the ECU which should go to the rev-counter then I can measure a pulse.

I've tried connecting a spare rev counter to the ECU to eliminate the rev counter. So it's not the rev counter causing the problem. The rev counter was also working in the past with the previous ECU. I seem to get a little blip when the engine first starts.

While on the phone to the guys from the ECU company we decided the voltage comming from the ECU might not be high enough to run the rev counter. The guy from the ECU company said put in a 'pull-up' resistor into the lead between the ECU and the rev counter.

I was wondering what a 'pull-up' resistor is and where I might be able to buy one. Are they usually called pull-up resistors and can they be bought from normal electronics shops
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