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oxygen sensors after cats

This is a discussion on oxygen sensors after cats within the General Help forums, part of the LSx Technical Help Section category; i am currently running no cat's on my trans am. but due to the engine swap, i havent turned the ...

  1. #1
    386HP 373TQ jwindbigler's Avatar
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    oxygen sensors after cats

    i am currently running no cat's on my trans am. but due to the engine swap, i havent turned the engine over yet. its just taking a while to put the new engine in. my question is if you dont have the post cat oxygen sensors installed, will the car run rough or at all. If so, is there any way to fool the computer to think that they are there? thanks

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    99PontTA
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    it will run fine without the rear o2 sensors,,,but the check engine light will be on

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    Furthermore if you want to permanently get rid of the rear o2's you can either purchase simulators, or have someone tune them out of the computer. Both ways will save you from the annoying SES light.

  4. #4
    99PontTA
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    there is another way to beat the pcm's input from the rear o2's...it invovles installing a diode in series with the output of the o2 to drop the voltage output from the o2 in half.Cheaper than buying the sims and this will do exactly what the sims do.

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    Quote Originally Posted by 99PontTA
    there is another way to beat the pcm's input from the rear o2's...it invovles installing a diode in series with the output of the o2 to drop the voltage output from the o2 in half.Cheaper than buying the sims and this will do exactly what the sims do.
    Not anymore, the computer requires a varying voltage signal that a single diode can not provide. You can however get a setup for $25 from www.bakerelectronix.com that will provide the right varying signal and has outputs for both sides so you only need one. Much cheaper than buying two O2 sims for sure. It's just 4 wires and it's really easy, and I've never had any trouble out of mine.

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    99PontTA
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    it will stilll vary...you use the diode WITH the o2...works just fine

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    SUPREME member-oderator oneBADDz's Avatar
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    I can see that working in theory, it makes sense if you are using it with the O2 since the O2 will still be sending it's varying sig. What res value do you use do you know?

  8. #8
    99PontTA
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    The first mistake people often make is trying to trick the PCM by using a resistor inline on the O2 sensor. After all, the post cat O2 sensors are now outputting 0.95 volts instead of an expected 0.1 to 0.8. A resistor of value X, where value X is whatever your friends brothers dog walkerís nephew runs on his Honda with 100% success. The problem is our PCM uses a comparator circuit mega-ohm measurement device, meaning just about no matter what kind of resistance you put in, the full amount of voltage will still be seen. Remember, resistance does not drop voltage, it limits current. If the PCM were to apply a load, then resistance would inadvertently drop the voltage it saw. So this is the right idea, but the wrong method. Next mistake Iíve seen is a complicated circuit that takes the O2 sensor signals, processes them, and re-transmits them at a lower value. Geeze!!! Talk about overkill. While that will work, how about the most simple and cheap design possible!

    Standard Diode from radio shack, P/N: 276-1141

    The average rectifier diode is a one-way valve. It has a voltage drop when current passes through it, which is usually around 0.5 volts. There are millions of different diodes that can greatly alter that figure, but for our purposes we will use that figure because itís EXACTLY what we need. Your cats arenít burning off any of those unburned hydrocarbons when theyíre laying in a million pieces in the trashcan, so there is plenty of oxygen present as far as the downstream O2 sensors are concerned. Theyíre putting out 0.95 volts. Pass through this lovely $1.29 incarnation, and whoopee! Itís 0.45 volts. What does the PCM expect? 0.45 volts is within perfect specifications.

    You want to place the diode inline with the O2 sensor output. Not the 5V reference voltage, and not the heater circuit. You can find which is which by placing a voltmeter on the O2 sensor wires. The one with a varying voltage is the one you want. On most 4 wire O2s you have two matched color wires(heater circuit) and two variable colors (one is ref, other is sensor output)
    Once you know which wire is your sensor output, remove the O2 Sensor/s and cut the (sensor output) wire, crimp in the diode with the silver bar facing away from the O2 sensor (toward the connector). If itís backwards, your PCM wonít see any voltage at all. So if that happened, now you know why. Make sure you cut enough of the wire to make up for the length of the conglomeration. Itís about 1.5Ē if you use two crimp connectors and cut the diode leads to about 0.5Ē long. Bonus points if you heat-shrink this connection. Double points if you trace this/these wires back to the PCM side and do it there. If done at the PCM, the silver bar faces the PCM. All this allows positive voltage to flow from the sensor through the diode to the PCM. Re-install your O2/s.

    When youíre done, the voltage drop occurs and all is good. Clear those codes and enjoy!
    __________________

    I grabed this off another site that is discussing this the same thing....i'm not sure if the mods here will get pissed if i post a link to it.

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    Quote Originally Posted by nhraformula
    tune it out
    We're talkin about being too cheap for even O2 sims. . .I'll get mine deleted when I'm ready for a full tune

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    or just drop the $50 for the plug and play sims

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    Quote Originally Posted by 99PontTA View Post
    there is another way to beat the pcm's input from the rear o2's...it invovles installing a diode in series with the output of the o2 to drop the voltage output from the o2 in half.Cheaper than buying the sims and this will do exactly what the sims do.
    Do you have the wiring diagram to do this ?????

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    Quote Originally Posted by trans am man 99 View Post
    Do you have the wiring diagram to do this ?????
    look at the post date...also it looks like he is a guest...dont hold your breathe for a reply

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    bringing this one back from the dead. Has anyone actually had success with this? Tried this this weekend and got a constant voltage of 1.2V instead of a slightly fluctuating .8-.9 that was I was seeing, but not the slightly varying 0.45 that was hoping for.

    Quote Originally Posted by 99PontTA View Post
    The first mistake people often make is trying to trick the PCM by using a resistor inline on the O2 sensor. After all, the post cat O2 sensors are now outputting 0.95 volts instead of an expected 0.1 to 0.8. A resistor of value X, where value X is whatever your friends brothers dog walkerís nephew runs on his Honda with 100% success. The problem is our PCM uses a comparator circuit mega-ohm measurement device, meaning just about no matter what kind of resistance you put in, the full amount of voltage will still be seen. Remember, resistance does not drop voltage, it limits current. If the PCM were to apply a load, then resistance would inadvertently drop the voltage it saw. So this is the right idea, but the wrong method. Next mistake Iíve seen is a complicated circuit that takes the O2 sensor signals, processes them, and re-transmits them at a lower value. Geeze!!! Talk about overkill. While that will work, how about the most simple and cheap design possible!

    Standard Diode from radio shack, P/N: 276-1141

    The average rectifier diode is a one-way valve. It has a voltage drop when current passes through it, which is usually around 0.5 volts. There are millions of different diodes that can greatly alter that figure, but for our purposes we will use that figure because itís EXACTLY what we need. Your cats arenít burning off any of those unburned hydrocarbons when theyíre laying in a million pieces in the trashcan, so there is plenty of oxygen present as far as the downstream O2 sensors are concerned. Theyíre putting out 0.95 volts. Pass through this lovely $1.29 incarnation, and whoopee! Itís 0.45 volts. What does the PCM expect? 0.45 volts is within perfect specifications.

    You want to place the diode inline with the O2 sensor output. Not the 5V reference voltage, and not the heater circuit. You can find which is which by placing a voltmeter on the O2 sensor wires. The one with a varying voltage is the one you want. On most 4 wire O2s you have two matched color wires(heater circuit) and two variable colors (one is ref, other is sensor output)
    Once you know which wire is your sensor output, remove the O2 Sensor/s and cut the (sensor output) wire, crimp in the diode with the silver bar facing away from the O2 sensor (toward the connector). If itís backwards, your PCM wonít see any voltage at all. So if that happened, now you know why. Make sure you cut enough of the wire to make up for the length of the conglomeration. Itís about 1.5Ē if you use two crimp connectors and cut the diode leads to about 0.5Ē long. Bonus points if you heat-shrink this connection. Double points if you trace this/these wires back to the PCM side and do it there. If done at the PCM, the silver bar faces the PCM. All this allows positive voltage to flow from the sensor through the diode to the PCM. Re-install your O2/s.

    When youíre done, the voltage drop occurs and all is good. Clear those codes and enjoy!
    __________________

    I grabed this off another site that is discussing this the same thing....i'm not sure if the mods here will get pissed if i post a link to it.

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    I just had my tuned out.

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    Quote Originally Posted by 35th-ANV-SS View Post
    I just had my tuned out.
    this.....keep in mind when this thread was created hptuners had only been out for like 2 years so people were still learning. These days very few would even think twice about trying to wire in diodes and what not to get past the O2 code. Just have it tuned out. You could always go extremely old school and put a couple of spark plug defoulers in each of the O2 bungs and then the O2 sensors into those. You have to use 2 defoulers in each bung and drill the outter most defouler out with a 1/2" bit to clear the sensor tip.
    Quote Originally Posted by Mean Green Z28 View Post
    Don't worry about understanding women. Women understand women, and they hate each other

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