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Thread: O2 ?

  1. #16
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    Is the 4 wire O2 just have an extra chassis ground? So its not using the exhaust to ground the sensor?

  2. #17
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    The 4-wire has a sensor element negative or return wire that runs directly to the ECU so that it doesn't rely on the exhaust as the signal path.

  3. #18
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    Ok so where does the 3 wire O2 ground? ECM??

  4. #19
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    So far as I know it grounds the same as a one wire. case,exhaust,motor and then where ever the ecm grounds, usually the block.
    6395, BHDF, 7.4 BBC lightly modded now 6395 BMHM back to BHDF

  5. #20
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    Both have a connection between the negative of the element and the case of the sensor.

    A 3-wire used the case for the signal path. The case connects to the exhaust, the exhaust connects to the head, the head connects to the intake and then the intake is connected to the ECM via a wire.

    The 4-wire setup runs the wire that went to the intake directly to the sensor instead. It eliminates the possibility of a bad electrical connection in the exhaust pipes from causing false O2 readings.

  6. #21
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    What’s the best way to add a ground to the 3 wire O2?? Ground the exhaust somehow to the frame?

  7. #22
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    Change it to a 4-wire sensor. Then, find the negative sensor input wire and remove it from the engine ground and take it to the 4-wire sensor plug instead.

    It would be pointless connecting the exhaust to the frame, since the frame isn't directly connected to the ECU.

  8. #23
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    Guys over thinking

    one wire O2 goes to ECM

    Three wire O2 one goes to ECM black or tan or purple
    Two white wires one goes to hot in run 12V+ the other to engine ground

    Four wire O2 one goes to ECM black or tan or purple or yellow
    The other black or tan or purple or yellow goes to ground wire at back of head or on corvette the intake manifold. The ECM return path is a pass through that goes to the head on LT-1 and Corvette LT-1/lt-4 it goes to the intake next to distributor. On TBI the same circuit goes to back of head on Crossfire TBI it goes to the intake manifold.
    The power wires one is grounded to engine block the other is hot in run 12V+

    Besides I am thinking he needs a fuel pump.

    The only reason for the difference on the Corvettes was the fear of electrolysis at the head gasket he first year we ran aluminum heads.
    The corrosion engineer was a little paranoid about having constant current through the head to the block even though his fix did the same thing it was next to a water jacket and he thought it was better. On production the noise engineer moved the O2 return off the engine and ran it to the ECM 1992 in 1991 the loom goes to the head.
    Last edited by daveosx; 01-07-2018 at 11:58 PM.
    Avatar is my motor 800 RPM 184ft Lbs Torque 18 inches x 18 inches x 9 inches thick external combustion engine.

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    95 Suburban 6 inch drop, Zoomies, Tri-Y Headers, Pioneer DVD in Dash (Soon to Be LT-4).
    95 Caprice 9C1 Fleetwood Brougham Interior, LT-4 (retired)(Engine Donor for new truck)
    2002 Seville STS

  9. #24
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    Thanks Dave!!

  10. #25
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    Why would you take the 4-wire sensor element ground or negative wire to the engine block? Since It can be connected directly to the ECM O2 input circuit, it should be. This is a separate wire from any other ECM ground wires, typically tan colored. If you don't re-wire to connect the 2 together then you might as well just use a 3-wire sensor.

  11. #26
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    Yep on older cars the provision for sensor return from the O2 is not present.
    On the ECM circuit board all the grounds are tied together and tied to the case through the bolts.
    On the next group of ECMS the grounds were separated to two groups sensor return and chassis.
    These circuits are C429 C452 --- C429 is the TPS Coolant sensor Ground and C452 was all other sensors ground.
    Starting in 1981 they moved the EST and HEI modules to C486.
    In 1991 they started using the O2 sensor return.

    On SBC GEN I in all GM vehicles there was one ground point on the rear of the cylinder head.
    On the next series GEN 1.5 (one Piece Rear Main) GEN 2 there was the head ground and the Intake manifold ground.
    On Later models there was the head, Intake and battery ground.

    Internals of the sheet metal GM ECMs all had a common chassis buss and intake buss some had a ground lead for HUD dash.
    The cast aluminum ECM cased boards actually had better noise control on the electronics.
    Some ECM circuits used a difference amplifier to increase O2 sensor signal reliability it was only then in 1996 that the GM 4 wire sensor could not be substituted with a three wire.
    pre 1996 four wire sensors can be substituted with a three wire.

    But as far as sensor return lines on pre 1996 GM vehicles the return wires are all tied to ground just some do not share the same physical ground path.
    There is an effect called loop induced current that will induce a voltage on a intended shield line or a secondary ground loop.
    Electrical engineers at GM had seniority and the electronic engineers worked under them.
    The radio engineers worked for AC Delco not GM proper.
    So where I or any modern electronics engineer or technician would use shields and ground distribution blocks in a car they did not.

    Two areas that will benefit from using shielded cable
    The Reference line from a HEI7-8 to the ECM with the distributor side ground only to the shield
    Knock sensor with the EST module side of the shield grounded.

    pre 1996 The rest of the returns and other grounds will function fine if grounded to the same point.
    Last edited by daveosx; 01-08-2018 at 12:03 PM.
    Avatar is my motor 800 RPM 184ft Lbs Torque 18 inches x 18 inches x 9 inches thick external combustion engine.

    98 WS-6 full tube chassis rally car Aluminum Block LS2 (soon to be LSX 376)(in Storage)
    95 Suburban 6 inch drop, Zoomies, Tri-Y Headers, Pioneer DVD in Dash (Soon to Be LT-4).
    95 Caprice 9C1 Fleetwood Brougham Interior, LT-4 (retired)(Engine Donor for new truck)
    2002 Seville STS

  12. #27
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    Quote Originally Posted by daveosx View Post
    Yep on older cars the provision for sensor return from the O2 is not present.
    On the ECM circuit board all the grounds are tied together and tied to the case through the bolts.
    On the next group of ECMS the grounds were separated to two groups sensor return and chassis.

    Every ECM I have ever looked at (for example '7749 or '7747 from 1987 or newer) has an O2 return terminal on the PCM and a separate dedicated wire from this terminal to the block. If I'm recalling right, it is typically a tan wire. Every one of these ECUs has had a special O2 sensor IC for the O2 sensor. To switch to a 4-wire sensor you disconnect this wire from the block and wire it to the O2 sensor instead.

    I have never seen a ECU that uses a common O2 negative combined with the other sensor negative wires. I have never seen one with combined ECU grounds and sensor grounds. The ground circuits are typically black with black/white for sensor negative. Sensor negatives are never grounded to the engine or chassis.

    Maybe something quite old isn't this way, but it must only be with a bit of the early stuff.
    Last edited by lionelhutz; 01-08-2018 at 06:14 PM.

  13. #28
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    The harness for the 7747 I am familiar with.
    The black and white wire can be either circuit 451 or 450 on all GM up to 95 on cars 99 on trucks.
    circuit 451 is serial data Assembly line link data second pin top row from left of OBDI connector.
    The tan wire is circuit 413 called O2 sensor low it is just another ground at G161 it is combined with 450 there. On the back of drivers side head top bolt all GM.
    Disconnecting this and connecting to the 4th wire will make the O2 sense extremely noisy the output from the Operational Amplifier attached at 412 before the ADC amplifies the signal from a scale of .1 to .9 mapped to 0 to 5V this 5 volt signal is referenced to that ground at 413 and the common 450 ground level the following stage is the ADC that provides a 7 bit resolution.
    Circuit 450 wires are tied together at splice S450 in the harness behind the distributor the ground wire is attached on non corvettes at the passenger side head rear top bolt on corvettes this is attached at the intake manifold. This is the location of G160.
    Circuit 450 is a ground the ECM MOTHER BOARD ties all of the returns together internally there is NO differential amplifier on the O2 sensor on any of the GM sheet metal ECMs.

    With a meter you can verify this for yourself unplug the ECM touch the negative lead to pin 13 of the 24 pin connector and continuity test pins 3, 6, 11, 12 on the 24 pin and
    3,11 and 15 on the 22 pin connector.

    Checking a GM harness at the same plugs and pins by the Service Bulletin the OHM reading was less than 1 ohm or repair harness at G160 and G161 if the repair did not provide result than the splice at S450 was inspected and resoldered. If that provided no result than the whole harness was replaced.
    I replaced 4 corvette harnesses in 1991 due to corroded 450 wires.

    There is also circuit 452 black wire at coolant sensor and TPS that is grounded in the ECM.

    So on cars or truck with a single wire purple circuit 412 you can use a three wire black to circuit 412 or you can use a four wire one side of sensor to 412 the other to ground.

    On Cars and trucks with sheet metal cased ECMs there is no differential amplifier on the 02 sensor the input circuit to the controller is just a comparator set at .7 V and an ADC that provides basic diagnostics by giving a 7bit digital approximation of the analog signal,

    On 91 Lumina Corvette Cadilac and Some models of Fbodies there was a change to the GM66ECU with the cast aluminum case up to 1995 they were called OBD1.5 because they are not compliant with OBDII connectors or function.
    The same ECMS are used in 1996 with different software to enable OBDII.
    On these ECMS there is a differential amplifier connected to the O2 sensor and the ADC counts are used for closed loop operation.
    So on cars with the cast aluminum ECM there are two four wire O2 sensor that has 412 413 terminated at the front left connector 414 415 at the front right
    In 1996 the numbering scheme also changed and to post cat O2 sensors were added.

    On 1995 Bodies and Corvette the was a optional performance switch that changed the spark and fuel maps on the fly some ECMs have the switch pined out on Caprice Impala ECMs the 3 gear selector switch triggers the second map. There is also a highway mode stored in the State trooper cars providing a third fuel and spark map location if I think of it later today I will post the part number for the three way ECM.

    Thought you might like this
    Screen Shot 2018-01-09 at 5.37.08 AM.jpg
    Last edited by daveosx; 01-09-2018 at 03:38 AM.
    Avatar is my motor 800 RPM 184ft Lbs Torque 18 inches x 18 inches x 9 inches thick external combustion engine.

    98 WS-6 full tube chassis rally car Aluminum Block LS2 (soon to be LSX 376)(in Storage)
    95 Suburban 6 inch drop, Zoomies, Tri-Y Headers, Pioneer DVD in Dash (Soon to Be LT-4).
    95 Caprice 9C1 Fleetwood Brougham Interior, LT-4 (retired)(Engine Donor for new truck)
    2002 Seville STS

  14. #29
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    Dave do you have a pin out pic like above for a 6395 eco that’s in my truck? Or where can I find one..
    Thanks

  15. #30
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    Quote Originally Posted by daveosx View Post
    The tan wire is circuit 413 called O2 sensor low it is just another ground at G161 it is combined with 450 there. On the back of drivers side head top bolt all GM.
    Disconnecting this and connecting to the 4th wire will make the O2 sense extremely noisy
    The tan is a completely different circuit than the other grounds. On the GM's I have worked on it's a separate wire to a separate ring terminal attached to the engine. The ECM terminal connects to a lambda sensor interface amplifier chip. Here is an example of this chip, different pinout than the '7747 ECM but the same pins and same functionality.

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

    4-wire O2 sensors for use in GM's have the negative of the sensor element grounded to the case. I just double checked this and confirmed it is true due to another thread here. So, the wire is not left floating when it runs directly to the sensor.

    I have no idea what pins you're describing on the '7747 ECM??? The pins go A1-A12, B1-B12, C1-C16 and D1-D16.


    Quote Originally Posted by daveosx View Post
    On Cars and trucks with sheet metal cased ECMs there is no differential amplifier on the 02 sensor the input circuit to the controller is just a comparator set at .7 V and an ADC that provides basic diagnostics by giving a 7bit digital approximation of the analog signal,
    It definitely is not a hard wired comparator in the '7747. You can most definitely set the O2 switching voltage threshold limits.
    Last edited by lionelhutz; 01-09-2018 at 11:47 AM.

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