As I'm learning how to program microprocessors, I've decided to do a project for fun to help me get a little more experience. Here's my project... I'm going to build a shift light which is basically a bar meter which has bi-color LEDs. It will sense when the car is at idle and run random pretty patterns. Once the car is above idle it will act as a bar meter style tachometer and will start flashing at the programmed shift point.

My problem is I'm not quite sure what the tach sense line looks like and I don't have a scope to look at it with. Does anyone know off the top of their heads or can someone look for me? I think it's a square wave, but I also need to know the voltage levels at high and low. I also need to know how many pulses there are per rpm. I'm not sure if there's 1 pulse per rpm, or 1 pulse per cylinder firing.

Thanks in advance guys.

 

I'm looking.

EDIT:
Found this:


It's by the stock airbox, under the cover with the relays.

If I had to guess, I'd say square wave, 0-5v, one pulse per revolution.

If I get motivated, I could hook up my scope and find out, but it's dark out now.

 

Thanks, I know theres a wire somewhere beneath the dashboard, because I've used it for a tach sense line for a remote starter I installed several years ago. I will find it pretty quick once I get to that stage.

I'm going to take your word on the 5v and skip designing any protection. If it does become a problem, these parts are cheap.

Also, I was thinking that there would be more than one pulse per revolution since after market shift lights and tach meters require you to program in how many cylinders your car has.

 

I was guessing one pulse per revolution based solely on this being the point you'd hook an external tach to if you wanted to have one under the hood with you while you were working on the engine. The one on your dash might get a different signal.

 

Alright, now I feel like an idiot. From what I remember, you find the tach sense line by probing around with a multimeter and looking for a wire who's voltage varies from 1-5v when you hit the gas. I tried last night and couldn't find it. I went through every wire measuring both AC and DC voltage. After I got frustrated enough I went to the location you mentioned above and tried it. It's voltage didn't vary at all for me. Then I remembered having a printout of the terminal block coming out of the ECU behind the glove box. Green/white, pin 87 tach sense wire. I tried it. still no luck.

I've got my project built to the point where I can feed a signal into it from a function generator and it writes the frequency it reads to an LCD. It does so very accurately. So I plugged it into pin 87 of the ECU. Frequency bounces all over the place. I found I can only get a good reading if the signal is clean otherwise it registers a pulse every time the voltage rises just a little bit.

This is giving me a bigger headache than I thought it would.

 

Update: This is slow going because I only work on it when I'm bored. My program reads a signal fine from a function generator, but when I wire it to the car it goes all over the place. I hoping this is because the line from the car is a dirty signal. The processor I'm using picks up interrupts at every rising edge of the signal and I think I'm getting interrupts from the noise too. I'm in the process of getting parts to filter the signal. Hopefully that will work.

 

You're going to make me break out the O-scope and test it, aren't you?

 

I guess you don't have to worry about it. That is, assuming this IS a square wave. After I run it through an inverter, everything SHOULD work. If it doesn't work, then take out the O-scope.

 

if it has the same wiring on the multimeter display as my evo IX MR, it is the solid green with silver strip wire at the back of the tach, speedometer display.

https://www.evolutionm.net/forums/sh...d.php?t=235939

 

Cool

 

I hooked it up last night and it works great. I was reading double the actual rpm so it looks like there are two pulses per rpm. This was both at the point by the relay box and the tach sense line under the dashboard. Now I just have to finish the program, layout a board and install everything. That might take a few months. I'll post a video when I'm done.

 

first dibs on your shift light!

 

Update:

http://s38.photobucket.com/albums/e1...t=MVI_0943.flv

I'm running the function generator to simulate the tach line. There is an LCD on the board which displays the actual rpms the unit is reading. During the demo, the function generator cannot go far past the 1k rpm range so I have to turn it back down then change the range. As you can see it does the pretty pattern below 1k rpms. I have the shift point programmed to 6.5k rpms. This is encoded into the program. Eventually I will learn how to read/write to EEPROM and have two buttons so that the user can program his or her own shift point.

Video quality sucks but you get the idea. If a bunch of people want, I can do a higher res one. Now I just have to get it on a circuit board and into my car.

 

That will look awesome!!!

 

nice i'm in