Hello Guys, I want to make a Tachometer to count the RPM of my car with an Arduino Board and OLED Display, My car is 26 years old and it has no ECU or OBD connector, so I'm looking to get the RPM signal from the ignition coil negative side it's a single coil and it's connected to distributor with ignition module and I already checked the wiring diagrams and also I found the signal wire which I'm looking for, Is there anyone who already make this project before or similar one? and how to connect it that wire to Arduino Board, will I need any other components between the two of them? any help will be appreciated 
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You might search on the internet, using the words "ignition coil pickup circuit", or not.
@Idahowalker, thanks but I already found the signal wire and it's a negative
I did that, picking up the signal from the ignition coil, 30 years ago. That onboard computer (a Z80) ran for 10 years without failure.
If I remember correctly I pick up the kick back spikes on the positiva side, linited the spike and sent it through an opto coupler that was connected to the digitals.
can you help me with the wiring, do i just need to connect an optocoupler??
Nearly. After all this time... The spike was high, like some 50 volt. I think I used a serial resistor close to the ignition coil, then a Zener diode to GND and a serial resistor to the anode of the OPTO.
but in my case the signal is negative, how do i get this negative signal to Arduino??
Your car runs on -12V?
Show a schematic.
Make, model, year of car?
There are older British cars that used positive of battery to the car battery as "ground". My mother had a 1966 MGB convertible wired up that way. Fun car to drive, sat so low to the ground that you felt like you were flying just tooling around at 30 mph. Dual carbs were a pain to keep tuned. Could get rubber in all 4 gears tho.
starwalker90:
but in my case the signal is negative, how do i get this negative signal to Arduino??
How can that be? Is it an English car? Can You draw a diagram on paper and show? I used the kick back on the positive side of the ignition coil.
Idahowalker:
Your car runs on -12V?Show a schematic.
Make, model, year of car?
Idahowalker:
Your car runs on -12V?Show a schematic.
Make, model, year of car?
My car isn't run on negative current, I mean when I checked the repair manual and the wiring diagrams I found the signal wire in the ignition coil and by the way it's pre-equipped in the car, iI mean it's ready to connect if I changed my cluster instrument with a one that has an rpm guage, and it's only a single wire, the car is Peugeot 405 Phase 2 1994
Do you mean the wire is normally positive and switches to ground when active? What is the voltage on this wire?
I tested the wire with a multimeter i connected the (+) to the battery and the (-) to the signal wire, and I got about 8 to 8.5 volts
Why not measure the singnal wire to ground?
8Vvolts, I bet it's meant to be a ignition feedback signal giving out a stream of pulses for the analog rpm gauge.
You can run it through an optocoupler, like how they show in the internet, hook it to a MCU (like how they show in the internet), count the pulses (like how they show in the internet), and divide by the number of cylinders (like how they show in the internet).
Year make model of the car so I can look it up myself.
To be sure I'd put an O-scope on wire 112, connection block 110-c.
My assumption. Wire 112 is the output pulse train for a dash tachometer. I'd tap into the wire 112 put the signal into a optocoupler to buffer the signal and bring it down to 5V pulses. Have a MCU count the pulses and divide the pulses by the number of cylinders. Just like the circuits that can be found through an internet search.
Sorry but it was my cheap multimeter fault ;D ;D , I tested the wire again with an automotive circuit tester and the signal wire is positive (+) and it's giving about 13v, so I'm going to use an optocoupler with something to reduce the voltage and see what happens
Have a MCU count the pulses and divide the pulses by the number of cylinders.
Then multiply by 2 because a 4-stroke engine fires once every two revolutions. 
Sorry but it was my cheap multimeter fault
Meters don't measure pulses accurately. An analog electro-mechanical meter will show the average because the inertia smooths-out the movement but with a digital meter you usually don't know what it's doing. It might show the peak, the average, or it might just jump-around randomly. There are digital meters that will show the true-RMS.
BTW - You do get a negative kickback voltage ([u]Back EMF[/u]) when the contacts open, but that may not be where you are connected.