Tim240z Posted March 13, 2005 Share Posted March 13, 2005 I want to delete the post Cat O2 sensors on my 99 Suburban motor, but can't see paying the ~$90 for a pair of simulators. Can I just wire in a resistor so that the PCM doesn't set a code? If so, anyone know the correct value resistor? Thanks, Tim Quote Link to comment Share on other sites More sharing options...
Tim240z Posted March 20, 2005 Author Share Posted March 20, 2005 Anyone? Quote Link to comment Share on other sites More sharing options...
Administrators SuperDan Posted March 20, 2005 Administrators Share Posted March 20, 2005 Can't you just disable it in software somehow? Quote Link to comment Share on other sites More sharing options...
Tim240z Posted March 20, 2005 Author Share Posted March 20, 2005 Yes, but I will have to send the PCM to the programmer...it isn't expensive for the reprogram ($35), but I wanted to see if I could get away without having to reprogram until the conversion is done. i don't want to wonder if the engine isn't starting because of bad wiring on my part, or bad programming..... Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 20, 2005 Share Posted March 20, 2005 Oxygen sensors actually output a voltage. They don't vary in resistance, so I couldn't imagine a simple resistor working. But I am only speculating..... Quote Link to comment Share on other sites More sharing options...
Tim240z Posted March 20, 2005 Author Share Posted March 20, 2005 This is the way I understand it (not saying it is correct, though): The PCM sends a 5Volt signal to the O2 sensor. The amount of oxygen passing over the sensor changes the resistance of the current and sends a voltage signal back to the PCM......so, if I was to just force that resistance with an inline resistor, wouldn't that fool the PCM into thinking that the Cats were there and operating correctly? Isn't that how the aftermarket O2 sensor simulators work? Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 21, 2005 Share Posted March 21, 2005 Well I may be displaying my own ignorance, but I know to test them you get them hot, hook up a digital multi meter and look for a half volt output. Tells me they are more like a thermocouple than a variable resistor. I thought the only power that went to them was a heater? Hopefully someone who has actually done it will chime in. But I would venture to guess the computer will compare the after cat sensors with the before cat ones (or else how could it tell if the catalytic converter was doing anything to the exhaust?). So if the after cat sensors don't vary with the before cat, I don't know if that will be enough to fool the computer. Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted March 22, 2005 Share Posted March 22, 2005 summit catalog has some gizmos that plug into o2 sensor plugs that put out a correct signal to keep ecm happy.made by caspers electronics.guys use these on mustangs &camaros,vettes with the (off road) exhaust systems.if you have a obd2 reader you can check the output of the rear 02 sensors.if you feed that to ecm you will fool it into thinking cats are installed.i think its easier to avoid the radio shack thing and just buy the caspers part. Quote Link to comment Share on other sites More sharing options...
Tim240z Posted March 22, 2005 Author Share Posted March 22, 2005 I know...I've seen those, but I wanted to cheap out. I'm sure all those have is a resistor of the correct value in them..... Quote Link to comment Share on other sites More sharing options...
wheelman Posted March 23, 2005 Share Posted March 23, 2005 Tim, Why don't you measure the voltage drop across the sensor with the O2 sensors operating in a normal environment then get a resistor of the same value. I think the PCM uses the 5 volt as a reference and then compares the voltage it reads back to determine a value from the O2 sensor. Either that or it measures the current flow which will still be determined by the correct value of the resistor. I also don't think the PCM compares the front and back set of sensors. I think it uses the front to set fuel/air ratios in closed loop mode and the back to determine that the cats are still good. Thats why you can put a sim in and it won't affect the engine operations, only fools the PCM into believing the cats are good. Wheelman Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 23, 2005 Share Posted March 23, 2005 From http://www.bobblick.com/techref/projects/o2sensor/o2sensor.html A ceramic-like material is exposed to the exhaust gas on one side, and to the outside air on the other. A voltage is produced as oxygen travels through the material. A rich mixture will produce almost 900 millivolts. A lean mixture produces about 100 millivolts or less. The sensor does not produce in-between voltages with any regularity. I think the material in the sensor is made to match the optimum air-fuel mixture, and the output of the sensor is pretty much on or off(too rich or too lean). The car's fuel-injection system picks a voltage to compare with, and modifies the mixture to try to maintain an on-off ratio close to 50%, so if you averaged the voltage over time it would be about 400 or 450 mV. The on-off transitions occur about one or two times per second in my car when it's running properly. When the sensor is cold, no voltage is produced, and the sensor is an open circuit with almost infinite resistance. When the sensor heats up, the impedance drops and it can produce a little bit of current. If I had to guess, I'd say that mine has an impedance of about 5000 ohms when hot. The car presents a very high impedance load, in the megohms. Don't confuse impedance with resistance. The device produces voltage but unless you measure it with a high impedance digital multimeter, the current draw will cause a voltage drop inside the device giving a false voltage reading. Still doesn't answer how to fool the computer. This is an excellent article, although it contains so much information it tends to confuse the issue http://www.findarticles.com/p/articles/mi_m0BQA/is_2_80/ai_71847178 Guess some GM cars actually cycle the air fuel mixture and check for time delays in the response from the before and after cat sensors. Perhap this article which shows you how to build your own O2 simulator is of more interest. http://www.freeinfosociety.com/electronics/schematics/sensor-based/oxygensimulator.html This guy claims his $5 solution fixed his eclipse. Looks like you could keep the heater part (black wires) hooked up to the original sensor, but then use the capacitor and 1 megaohm resistor on the sensor part. http://www.vfaq.com/mods/O2bypass.html A guy in this thread says you can just weld an extension onto the rear O2 sensor bung and the cooler air will make the computer think the sensor is downstream of a functional cat http://forums.beyond.ca/showthread/t-76662.html Quote Link to comment Share on other sites More sharing options...
bjhines Posted March 23, 2005 Share Posted March 23, 2005 I see the o2 bypass circuit... i can only assume that it is feeding voltage from a thermocouple sens wire back into the o2 sensor wire....as far as the heater element...that is obvious..although I would use a 50 watt resistor and keep it away from flammable materials. some people seem confused about the operation of these sensors... The actual sensor outputs voltage to the computer...anywhere from 0.01 volts up to about 1.0 volt....the only difference in wideband and narrowband sensors is the range of measurable air/fuel ratios. the wideband picks up a wider range without spikey voltage swings..... as far as the sensor just operating on and off modes in a repeating pattern every second...that is your cars computer in closed loop sensor mode doing that.....the sensor will simply track the actual a/f ratio...the computer is alternately forcing the engine to run rich then lean over and over again....all this happens at idle or part throttle....look up closed loop mode in google. Quote Link to comment Share on other sites More sharing options...
dot Posted March 23, 2005 Share Posted March 23, 2005 I have bench tested a sensor using a propane torch and got it to produce one volt, with nothing electrical attached. It’s just my own looney theory, but you might be better off with a one volt battery than a resistor. I would however hate to be wrong and burn a computer. Quote Link to comment Share on other sites More sharing options...
80LS1T Posted March 23, 2005 Share Posted March 23, 2005 One other thing to consider is with newer OBD2 systems, it will throw a code for "o2 stays at center" so just putting a resistor inline will more than likely not work. I have no idea how those O2 simulators work? Guy Quote Link to comment Share on other sites More sharing options...
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