tapping tachometer wire?

**shotgunefx** · 01-29-2006, 09:05 PM

Originally Posted by fixerofallthing

You are right. That particular car is coil on plug though, so there are 6 coils. So my first response was incorrect. I posted a pic of the wiring diagram for that vehicle and the wire mentioned above is correct. It will be a very "clean" signal because it is produced by a control module for the express purpose of running the tachometer.

That's what I'm tapping off on my car. It's a 5volt signal that drives the tach.

**mauri** · 01-30-2006, 06:20 AM

Originally Posted by shotgunefx

I'm doing something similar for my own front end. I'm going to tap off the rpm/tach and indicators, read them with a PIC16877a and send them serially to the pc.

If at all possible, try and hit ebay for the service manuals for your car. I've found them to be invaluable.

Infos, please!

Very interested for your sw frontend and Pic hardware...is it possible to view the schematic?

Tnx

Mauri

**shotgunefx** · 01-30-2006, 07:09 AM

Originally Posted by mauri

Infos, please!

Very interested for your sw frontend and Pic hardware...is it possible to view the schematic?

Tnx

Mauri

I would if I had one.

I've been tied up with other aspects so far, but it's a relatively straight forward project, so outside of deciding what pins are going to go to what indicator lights, there's not much to figure out.

As far as the front end, it's a Perl/SDL app. The skins are specified via HTML. Tabled images generated from Fireworks. Animated aspects are actually animated gifs. How and what the images/frames mean to the renderer is determined by naming convention.

The board I'm using is an Olimex 40 pin proto board I had laying around. It's got the VREG, MAX232, crystal and serial connector already set up.

Seeing this PIC is not going to do much (if anything) else, I'm just going to continuously poll the appropriate registers for changes to the inputs.

For the tach and speedo, timing is a bit more important. Indicator lights are going to be on for a long time (measured in instruction cycles anyway).

The 16F877A has got two CCP (capture/compare/PWM) modules. So I'll use them to capture the timings of the tach and speedo.

There's a nice writeup in the following tutorial on how to use the CCP module. (An an example in it is a fan speedo

)
http://members.cox.net/sonjarob/TUTs/uP4Idiots.html

As I see it, there are only a few decisions to make.

Should I wire straight in or use optoisolators, I think straight in would be fine as the lights, tach and speedo all come off the ECU, so I'm going in assuming they are fairly clean for a car environment.

The only reason not to use optoisolators is space and laziness. I've got 16 inputs from the indicators (OTOH, replacement ECUs are expensive...)

The other is how to transmit the data back (formatting-wise) and selecting an appropriate update rate. For a pic running at 20mhz, I should have plenty of cycles to spare.

I'm by no means an expert in microcontrollers. I've been tinkering for a year or two though and I'll be glad to help if I can.

**DrStein99** · 01-30-2006, 12:24 PM

So I go behind the glove-box to look for the module, and splice of the green-white wire yes?

Any advice on taking apart the dash to get to the glove-box? Taking the center-console out for the head-unit was kinda trickey, I had to find procedures since there was like booby-trapped pegs and stuff holding things in place.

**rando** · 01-31-2006, 01:00 AM

You might want to give some consideration to safety here. On some cars, the injector pulse (tach) signal is active low. If the circuit accidently finds a ground path through these modifications, the injector could end up stuck on (i.e. flowing). Engine damage, fire, and/or death are potential outcomes.

**shotgunefx** · 01-31-2006, 01:32 AM

Originally Posted by rando

You might want to give some consideration to safety here. On some cars, the injector pulse (tach) signal is active low. If the circuit accidently finds a ground path through these modifications, the injector could end up stuck on (i.e. flowing). Engine damage, fire, and/or death are potential outcomes.

If I may ask, is that directed towards him, me or both?

The signal I'm tapping isn't what drives the injector (AFAIK), it's only output for the purpose of driving the gauge cluster.

Though I very well may be missing something, I'm far from an expert in electronics. Though the ability to drive without the gauge cluster plugged in makes me think I'm reading it right.

**xdjxklusivex** · 01-31-2006, 02:03 AM

i want to upgrade my tach sadly i cant find anything that redlines 13.5 ;-)

**DrStein99** · 01-31-2006, 08:02 AM

Originally Posted by rando

You might want to give some consideration to safety here. On some cars, the injector pulse (tach) signal is active low. If the circuit accidently finds a ground path through these modifications, the injector could end up stuck on (i.e. flowing). Engine damage, fire, and/or death are potential outcomes.

Yes. I agree. I do not want to mess with the engine, I simply would like the safest pulse-signal for display, and possible statisical data logging applications.

So that green/white wire from the electronic control module, will probably do the trick.

I wonder if that green/white wire can be accessed from within the steering column, if i take those screws out of the plastic where the ignition is?

**DrStein99** · 01-31-2006, 08:04 AM

Originally Posted by xdjxklusivex

i want to upgrade my tach sadly i cant find anything that redlines 13.5 ;-)

There are motors that can redline and exceed 13.5 - how much money are you willing to spend? Because the sky isn't really the limit, it's your enginuity and sometimes bank-account; when it comes to that.