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Thread: Black box PCM help

  1. #1
    Fuel Injected!
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    Black box PCM help

    Heated%20Oxygen%20Sensors.jpg

    Like the title says ,

    Have a 1999 Blazer , 4.3 , Black box PCM (16263494) , fighting an issue where none of the three O2 sensors shows a return ground , Pin 25, 26 and 7 are supposed to be the O2 sensors return ground , when I check the connectors on the sensors I have Power , ground for the heater and a signal back to the PCM , but no O2 ground return.

    Hope that makes sense .

    I checked the wiring to the sensors and the signal wires all make it to the PCM , (Pins 19,22 and 21) , and all of the return grounds make it back to the PCM (Pins 25,26 and 7) , so that ruled out a wiring problem to the sensors.

    My question is , where does the PCM get the ground to supply to the three pins in question ? (25,26 and7) I can only find two PCM grounds at pin 17&18 , they are both good .

    Anyone mess with the black box anymore that could provide some advice ?

    If I jump grounds to pins 25,26 and 7 the O2 sensors will function normally . Swapped PCM's and same result .



    Thanks

    TOM
    1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
    2006 Forest river-Sierra 37' 5th wheel
    1999 Astro Van, L31 Vortec , 4x4 conversion
    1994 Pontiac Sunbird "Work ride"
    1993 K-3500 "Ex-diesel" 454/4L80E - '7427
    1992 454SS "Eh Stock" , Blower & '7427 swap .
    1992 S-10 434 SBC/Tremec - '7427
    1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
    1980 Z-28 454/700R4
    1977 Olds 98 Regency 403/700R4

  2. #2
    Super Moderator dave w's Avatar
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    I'm thinking that the heater elements get ground on Pin "C ".

    I'm thinking the heater elements get Ignition power on Pin "D".

    I'm thinking Pin "A" is the internal ground or low reference for the O2 signal, similar to the "-" used on a volt meter.

    I'm thinking Pin "B" the actual voltage sent to the PCM from the O2 sensor, similar to the "+" used on a volt meter.

    I'm thinking Pin "B" can vary from 0 volts to about 1 volt depending on the Oxygen in the exhaust.

    I don't have the internal schematics from the "Black Box" PCM, but I'm thinking the O2 Pin "A" reference low have printed circuit board traces that connect to the ground pins, which really makes the reference low a ground. I'm thinking, that in electrical schematics "Reference Low" means connected to ground (aka Battery "-").

    dave w
    Last edited by dave w; 4 Weeks Ago at 11:52 AM.

  3. #3
    Fuel Injected!
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    I would expect the circuit to look like the attached. The attached is from a OBDI ECM schematic and all of them seem to have circuits like this. I doubt the OBDII circuit is any different since the O2 sensors didn't change and this circuit is well proven to work.

    As you can see, the pins are not really grounded, but rather just an input to an IC that is reading the sensors.

    Check the O2 return voltage with the engine running. I expect it has to be >= 0V and <= 3V to work.

    Here is the datasheet for a typical IC used for the O2 sensor circuit.

    http://www.ti.com/lit/ds/symlink/lm9044.pdf
    Attached Images Attached Images

  4. #4
    Fuel Injected!
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    Just in case someone stumbles on this thread and wants an answer .... :

    The grounds to the PCM that I mentioned are to come from the body of the sensor , the body of the sensor grounds through the exhaust and provides a reference ground back to the PCM .

    The brand new O2 sensors I received had no ground from the 4th wire to the body of the sensor , picked up 3 new units locally and the body of the sensor grounded the 4th pin feeding it back to the PCM terminals I mentioned .

    Double check all of the sensors you receive , these were name brand I have used in the past .

    Thanks for all of the help anyways .

    TOM
    1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
    2006 Forest river-Sierra 37' 5th wheel
    1999 Astro Van, L31 Vortec , 4x4 conversion
    1994 Pontiac Sunbird "Work ride"
    1993 K-3500 "Ex-diesel" 454/4L80E - '7427
    1992 454SS "Eh Stock" , Blower & '7427 swap .
    1992 S-10 434 SBC/Tremec - '7427
    1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
    1980 Z-28 454/700R4
    1977 Olds 98 Regency 403/700R4

  5. #5
    Fuel Injected!
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    I checked a sensor I had and sure enough the negative side of the sensor element is grounded to the case. Still, 4-wire sensors have a 4th wire for the negative side of the sensor element directly to the PCM so they don't use the exhaust as part of the signal path.

    The ground is probably just to keep the input voltages to that circuit I posted within the proper range that the chip can read. You could have grounded the wiring anywhere and achieved the same result, as you had originally posted. If you look at your schematic and the circuit I posted you'll see there is no other connection to chassis ground so there is no way the exhaust can be a path for the signal.

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