speedo signal specs

[ext-peter.betts at nokia.com]

I can only speak for Toyota cars but..

They produce 4 pulses per revolution of the driven wheel. This is meant to
be a 0->5V signal but sometimes can be nearly 0->6V

I've designed speedo converters for Japanese imported cars into the UK (KPH
to MPH conversion). The circuit uses a FET transistor to condition the
signal comming in with a zener diode to protect against any signal going
above say 10V. The source of the FET is connected to a regulated 5V supply
rail which drives my little 12C508 PIC chip (Where the duty cycle or period
is modified etc, for duty you might need a bigger PIC like a 16C84). 

The Drain of the FET goes to 0V and the Gate is connected to the input
signal via a 10K resistor.

At the junction of the 10K resistor and the gate of the FET is a 100K
resistor to 0V and in parallel a 10V zener diode.

This work brilliantly. Accepting signals down as low as about 0->2V to
0->12V (clipped at 10V due to zener). It always produces a 0->5V signal at
the source of the FET which is connected to my PIC chip input.


Pete


> -----Original Message-----
> From: EXT Jeff Bromberger [mailto:blownz at home.com]
> Sent: Monday, October 09, 2000 1:12 AM
> To: diy_efi at diy-efi.org
> Subject: speedo signal specs
> 
> 
> I've got a '94 Z28 w/6-speed. the pcm is calibrated for 3.42 
> gears and i
> have 3.73s. i'm wanting to convert the signal from the pcm to 
> the dash to
> compensate for the gears. I'm planning on using the 
> freq-to-volt change volt
> volt-to-freq type of circuit. my question is, has anyone 
> scoped this signal?
> i know the pcm puts out 4000 pulses per mile but is it a 0 - 
> 5 volt square
> wave? 0-12V? My car isn't driveable right now so i can't 
> check it out on
> mine. How sensitive is the speedo to its input? does the duty 
> cycle of the
> wave matter or is it just triggering off of rising edges per second?
> 
> thanks!
> jeff
> 
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