Technical question about downstream o2 sensor feedback - SRT Forums

Mech_Mat · 03-10-2008, 11:09 PM

1st off. I hope this is the right place for this question.

Does the factory downstream rely on a voltage offset? If so, what is the offset?

I've gotten conflicting answers from different places about the factory voltage offset on the upstream. I was told it was 2.5v, and then i was told 3v.

My goal is to build a circuit that will add the set offset voltage to an input voltage. therefor allowing me to use the 1v output on my wideband setup for the downstream.

Also, what does the computer look for from the downstream. I guess my goal would be to simulate the signal of there being a cat using feedback from my wideband. (i have no cat, and I have a S0, and that light annoys me.) I will buy a S1 to make this go away, but I have a good knowledge of building circuits, and even incorporating logic circuits when needed.

Thanks guys.

duster360 · 03-11-2008, 10:55 AM

If there's any doubt, never listen to or even ask numb nuts on the internet, check out the Factory Service Manual. There's a free .pdf in the "How-to" section.

Section 25-2 of the Service manual is where all your answers are.

Discussion question:
Your circut won't effectively emulate the downstream O2 without input from the upstream O2. Why not?

Mech_Mat · 03-11-2008, 11:43 AM

I have Alldata. I'm pretty sure the diagrams, and specs are the same. I've checked Alldata, and no advanced specs on the downstream o2 are listed, such as a voltage offset.

My plan is to use feedback from my 1v output on my wideband setup to simulate the downstream sensor.

I've discovered that it looks for a slightly leaner mixture then the upstream sensor, and also for testing the cat is looks for a response time. So, I know that I'll have to put a 10ms or so delay on the feedback from my wideband, I also know that I will need to add possibly .1V to the signal to simulate the slightly leaner signal that the sensor expects to see after a cat, but now I need to know a definite voltage offset, and is it a static voltage offset? If I had a garage, or if it were over 40F I would just test away, but if anyone already knows this info, then why should I freeze out there for hours. If I can figure this out I could offer a schematic, and/or a device to correct this issue. :-)

duster360 · 03-11-2008, 01:34 PM

I posted everything i have on actual Narrow-band O2 emulation on your other thread. You really need to just download the Manual. Most ALLData info that i've seen looks similiar to the Factory manual, but I don't know if its exact. I've never compared them that closely.

Quote: Originally Posted by FSM p14-41

The O2 sensors produce a constant 2.5 volts on
NGC vehicles, depending upon the oxygen content of
the exhaust gas......

The sensor input switches from 2.5 to 3.5 volt,
depending upon the oxygen content of the exhaust
gas in the exhaust manifold. When a large amount of
oxygen is present (caused by a lean air-fuel mixture),
the sensor produces voltage as low as 2.5 volt. When
there is a lesser amount of oxygen present (rich airfuel
mixture) the sensor produces a voltage as high
as 3.5 volt. By monitoring the oxygen content and
converting it to electrical voltage, the sensor acts as
a rich-lean switch.

Note: The O2 heater is also monitored. So if you don't emulate this circut too, your MIL will still be on. I plann on just leaving the old sensor plugged in. Kluase, vp of Engineering of Innovate Motorsports, claims a 25w 12v bulb will work too but not be hot enough to burn or melt stuff. His comments are in one of the links in your other post

Quote:

OPERATION—The Oxygen Sensor Heater Monitor
begins after the ignition has been turned OFF
and the O2 sensors have cooled. The PCM sends a 5
volt bias to the oxygen sensor every 1.6 seconds. The
PCM keeps it biased for 35 ms each time. As the sensor
cools down, the resistance increases and the PCM
reads the increase in voltage. Once voltage has
increased to a predetermined amount, higher than
when the test started, the oxygen sensor is cool
enough to test heater operation.
When the oxygen sensor is cool enough, the PCM
energizes the ASD relay. Voltage to the O2 sensor
begins to increase the temperature. As the sensor
temperature increases, the internal resistance
decreases. The PCM continues biasing the 5 volt signal
to the sensor. Each time the signal is biased, the
PCM reads a voltage decrease. When the PCM
detects a voltage decrease of a predetermined value
for several biased pulses, the test passes

ATaylorRacing · 03-11-2008, 02:55 PM

Can you get rid of (or gut) the cat and put a resistor on the downstream O2 sensor of the SRT4 like we do on the non-turbo Neons? Does the stage 2 set up ignore the ds sensor?

Mech_Mat · 03-13-2008, 03:57 PM

the stage 1 kit only ignores the feedback from the downstream sensor. It needs to be plugged in. It seems the stage 1 kit wants to verify that the heating element is working right, and it looks for at least it's offset voltage to come back so that it can verify that a sensor is plugged in.

If a 12v 25w light will fool it, then the right value resistor array can also work. :-)

Well, I'm just going to order the S1 kit, and leave the sensor plugged in. Now that it's been a few days, my cruising a/f ratio is between 13.9-14.1:1, so, mileage is not great. performance is weak, but it's smooth.

Mech_Mat · 03-13-2008, 04:01 PM

heh.. I'm going to test a few things. I'm thinking if I build a resistor array to fool the pcm that a heating element is there, and then just tie the gnd wire (the one with the 2.5v carrier voltage) to the signal wire then the pcm will both: find a load for the heater, and also it will get it's 2.5v back thinking that there is a sensor there. On the S1 if the computer done care what the signal is, then this will work.

duster360 · 03-13-2008, 04:13 PM

Quote: Originally Posted by Mech_Mat

If a 12v 25w light will fool it, then the right value resistor array can also work. :-)

I don't think a resistor will work because the resitance won't change with time like a bulb or the heater curcut does. The ecu uses a PWM 12v signal to heat the sensor up very quicky an then it backs off the duty cycle to maintain the proper temp. It actually monitors the voltage change(caused by the resistance changing with temperature) in the heater circut to judge the heater's performance.

Obviously, if the ecu doesn't think the heater is working, your MIL will light.

Mech_Mat · 03-14-2008, 04:09 PM

yeah.. hmm. maybe just leave the sensor plugged in.. I just ordered a S1 kit from a dodge dealer through ebay. $550, but I know I won't get burned this time.

never2muchboost · 03-16-2008, 08:20 PM

the whle purpose of the downstream o2 sensor is to verify there is a cat in there.on a s0 car sometimes you can get away with using a spark plug non-fouler before the sensor.i forgot what the exact testing parameters are for the downstream but i think its somwhere between 19-55 mph rpm between 1900-2500.i would have to look it up again but when it tests it just looks to see if the downstream switchrate is different from the upstream

ReevesRacing · 07-07-2008, 06:14 PM

Has anymore been done with this? I was going to be building a circuit for a car using a 555 timer chip that modulated the voltage so that the ecu was happy. I was also going to be using a resistor bank (to get the heater 4 ohm load simulated). From what I've read though, it looks like the resistor bank would probably not work, so I would have to have the sensor or a light bulb plugged in anyway.

Do the foulers really work? That may be the easier route.