Re: more boost

[Alan Cordeiro <cordeiro@avlna.com>]

At 10:52 AM 5/14/96 -0400, you wrote:
>Don't put a voltage divider in the output of the pressure sensor. It will
>advance the ignition and cause you problems as the boost comes up. If you
>want to do it at low cost use a 4.3v zener and a 470ohm resistor.
>
>There are 3 wires coming from the pressure sensor, 0v 5v and output. Use
>your meter to see which is which. The output should be about 2-2.5 v with
>the ignition on and the engine off.
>
>Cut the output wire and now we have 4wires. 0v,5v, output from the sensor
>and input into the board.
>
>Connect the 470 ohm resistor to the output from the sensor and to the input
>to the board.
>
>Connect the zener (stripe end) to the input to the board and the other end
to 0v
>
>Now your computer will not retard after 12 - 13 PSI boost and all of your
>safety circuits will still work


***********************************************
WRONG !!!  you loose ALL BOOST PROTECTION !!!!!
***********************************************
this circuit just clamps the sensor amplified output at 4.3+ volts,
approx 1.75 BAR. when your boost pressure goes over this, your
engine controller will NEVER KNOW.

Next comment, if you had a PERFECT ZENER DIODE, this circuit would
not even work. at 1.5 BAR, ( 3.75 volts ) the fuel cutoff would be
triggered.

this circuit only works because the zener diode had 0.5 to 1 mA 
leakage, i.e., you have a VOLTAGE DIVIDER there all the time
because of the 0.5 mA that leaks out through the zener.

At 1.0 BAR, the CPU internals "see" 0.9 bar, at 1.5 it would
see 1.4 bar, if the zener leaked 0.5 mA. I guess you need
a VERY LOW QUALITY HIGH LEAKAGE ZENER DIODE to make this 
circuit work.

According to my calculation the zener leakage must be
at least 1.33 mA for this system to work.

Perhaps it's time for me to make my design public, it
seems like people will just experiment anyway, and
some may even lose engines. I just didn't want people
who were not really familiar with electronics to try
and implement this INCORRECTLY and then lose their 
engines ( or MAC-11s ). 

I need to figure out how to word it. About $10-15
in parts, and a bit of surgery on the main circuit board
( very hard to describe in text ). You do need a stronger
( e.g. Charlie Smith ) spring.

Alan Cordeiro


>
>I'm $350 poorer :)
>
>
> 
>Peter Wales                    pjwales@magicnet.net
>President Superchips Inc
>Chairman Superchips Ltd        "Timing is everything"
>Superchips home page with all the answers http://www.superchips.com
>
>