DIY O2 Sensor MIL Eliminator
Republished from: http://www.paladinmicro.com
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NOTE (07/17/2007): The
circuit presented here is identical (give or take some component
values) to every commercial MIL
eliminator I have examined--at least a dozen. However, you
may find that it does not work with newer (2001-1/2+) cars because
of advances made in both engine control software and processing
power.
Specifically, many newer ECUs (PCMs or whatever) no
longer monitor the rear O2 sensors for only reduced voltage and
phase difference (see the oscilloscope traces below), but also for
switching frequency ratios between the two (front and rear)
sensors. The simple passive RC network used in
MIL eliminators cannot simulate the
required switching frequency ratios and may not work properly with
newer cars. In this event you will need to use a custom ECU program
("tune") to switch off the rear sensor(s) (catalyst monitoring
function) to block the
MIL.
Click here for a
'99 to '04 Mustang V8 specific wiring diagram...
Commercially available generic MIL Eliminators1 range
from reasonably priced ($12 each) to absurdly priced ($40 each),
however they all have one thing in common--inside there's only two
passive electronic components worth a couple bucks or so at Radio
Shack, specifically:
RS p/n 272-1434 -- 1.0uF 20% dipped tantalum
capacitor ($1.59 each as of this writing)
RS p/n 271-1356 -- 1.0 M Ohm 1/4 Watt 5%
carbon film resistor ($0.99 for 5 as of this writing)
-or-
RS p/n 271-1134 -- 1.0 M Ohm 1/2 Watt 5%
carbon film resistor ($0.99 for 5 as of this writing)
The diagram shown below is all there is. The circuit is a very
simple single-pole RC filter with a corner frequency of 0.16 Hz or
so, which when placed in series with the rear sensor's output
attenuates the signal by a variable amount depending on the
frequency of the sensor signal. This is normally 1.0 to 5.0 Hz,
making the filter's output 15dB to 30dB (or 6 to 30 times) less
than the input signal (which without catalytic converters is the
same as the output of the front sensors).
The filter therefore feeds the PCM a line of crap--specifically
that the rear O2 sensor's signal is less than the front sensor's
signal, and that it is changing differently over
time (due phase shift introduced by the filter) than the front
signal--combined this manipulated signal makes the PCM a happy
camper and it keeps the MIL off...
Here's a series of photos showing how I make 'em up, all
connections are just twisted and soldered. You should squirt a glob
of RTV sealant on the whole thing and then wrap it up in
tape, or use heat shrink tubing like the commercial guys usually
do.
And finally, for those who might give a rat's backend, here's a
comparison of the input and output of this circuit when driven by a
0.600 V sine wave at 1.0Hz and 5.0Hz. As you can see the
attenuation values are very close to the theoretical values
presented above (the capacitor is a +/- 20% component, the resistor
+/- 5%). The reduced input voltage measurement at 1.0Hz is due to
the loading effect of the filter as the input frequency approaches
the turnover frequency (I should have used a 50 Ohm source).
Clearly seen are both the amplitude and phase changes as the
input frequency changes, this is what tricks the ECU.
(Instrumentation: Wavetek 1288 function generator;
Lecroy 9450A digital oscilloscope)