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O2 Sensor Testing Questions

Discussion in 'The Garage' started by jim burke, May 11, 2005.

  1. jim burke

    jim burke Registered Member

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    I have a few questions about testing an O2 sensor ('89 350 TBI):

    -Should I use a jumper (a wire from the O2 sensor to the positive test lead of my voltmeter and then from + test lead to the wire going to the ECM)? Or, just hook up my voltmeter to the O2 sensor wire and do nothing w/ the wire going to the ECM? (In both cases I'd obviously ground the negative test lead of the voltmeter).

    -Can I tell if it's going closed loop by grounding two terminals of the ALDL connection? If so, do I ground the same two terminals I'd use to pull the codes (make the SES light flash)?

    -Any idea why my voltmeter will read the output voltage as say, .500 when I'm in vDC mode, but when I switch to mVDC it goes "OL" (overload) instead of displaying 500 (mVDC)? It does this w/ two different meters, so I don't think it's a bad meter.

    TIA,

    Jim
     
  2. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Don't put a volt meter on ECM terminals or sensors. They work at such a low voltage you will fry them. You can check on open/ closed loop thru the A+B terminals that you use to pull codes. Just run the engine with the jumper wire in there. If it flashes 2 1/2 times a second it's in open loop. Once a sec is closed loop.
     
  3. jim burke

    jim burke Registered Member

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    Not to get off-topic, but isn't putting a jumper wire "in line" the only way to test the TPS (which has a 5 volt reference signal)? The 5 volt reference signal (from the ECM?) goes into the sensor and the voltage going back to the ECM is then measured. That's the way I've been testing the various sensors, hopefully I didn't screw up anything :dunno: :whistle: .

    I'll go ahead and test the O2 sensor without hooking up to the wire back to the ECM. It kinda makes sense that it would work fine that way since there's no voltage being fed *into* the O2 sensor :waytogo: .

    BTW, does anyone know how the O2 sensor generates voltage?
     
  4. spearchucker

    spearchucker 1/2 ton status

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    I've never heard of a voltmeter ruining any component. A voltmeter doen't send out any current so how could it damage anything? It determines the voltage by how much current goes through a very large resistor (a known value) inside the meter. Please explain your statement.:confused:
     
  5. spearchucker

    spearchucker 1/2 ton status

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    Probably because the meter has an internal resistance that is relatively close to what the 02 sensor has and is affecting the reading.
     
  6. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    No attempt should be made to measure the output voltage of the sensor. The current drain of any conventional voltmeter would be enough to permanently damage the sensor. No jumpers, test leads or other electrical connections should ever be made to the sensor. Use these tools ONLY on the ECM side of the harness connector AFTER the oxygen sensor has been disconnected.


    Quoted from Chiltern,s manual.
    Of course they could be wrong in which case I appolagize for the mis-information!
     
  7. spearchucker

    spearchucker 1/2 ton status

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    Quote from the GM service manual:
    "The oxygen sensor voltage output can be measured with a digital voltmeter having at least a 10 megaohms input impedance. Use of a standard shop type voltmeter will result in an inaccurate reading."
    Someone at Chilton's should go back to school.:doah:

    It also states that normal scan voltages should be between 100mV to 1V when you check it.
     
  8. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Do you suppose the guys at Chiltern started the 'weapons of mass distruction' debate? :haha:
    I'm real glad I bought such an informative and helpful manual.


    POST SCRIPT: ignore my earlier post. It was genuine b/s! :grin: :doah:
     
  9. spearchucker

    spearchucker 1/2 ton status

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    You might be on to something there. :thinking:


    .
     

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