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Old 01-15-2019, 01:53 PM
  #1  
umc276
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'83 928s
I posted this question in a different form last week but got no responses, except for one private one.
Should the large male connector coming from the computer that attaches to the O2 sensor, with the key on,
with the sensor disconnected, show any voltage.
I think it should be 0 volts, mine shows 0.5 volts.
Logical for the O2 heater circuit to show voltage but not this one.
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Old 01-15-2019, 06:26 PM
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GregBBRD

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I generally skip over these questions, letting the real experts chime in....but since you got no responce, I'll throw in what little I know:
Completely normal. The brain supplies .5 volts to that circuit (which is half way in the operating range), until the O2 sensor takes control.

Think about it:
Any other voltage....or no voltage.....would force the brain to incorrectly compensate. Starting off dead centered in the range allows the car to run without compensation, until the O2 sensor takes over, relying on the AFM and the temperature, only.

Worth noting, Bosch eventually added electrically heated O2 sensors that could "take over" quicker.


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Old 01-16-2019, 01:04 PM
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umc276
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Thanks for replying.
If I understand, the computer should stop supplying O.5 volts once the O2 sensor starts sending voltage.
I'll take some readings today and report back.
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Old 01-17-2019, 01:26 PM
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Your answer seems to be correct. Thanks.

Results at idle with sensor disconnected from computer:
........28lbs fuel pressure - 0.3-0.4 volts
........36lbs fuel pressure - 0.7-0.8 volts.
connected to computer
.......both around 0.5 volts.
So computer seems to be responding to any voltage from sensor and correcting mixture despite
initially supplying 0.5 volts.
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Old 01-17-2019, 04:17 PM
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Originally Posted by umc276 View Post
Your answer seems to be correct. Thanks.

Results at idle with sensor disconnected from computer:
........28lbs fuel pressure - 0.3-0.4 volts
........36lbs fuel pressure - 0.7-0.8 volts.
connected to computer
.......both around 0.5 volts.
So computer seems to be responding to any voltage from sensor and correcting mixture despite
initially supplying 0.5 volts.
Fuel pressure should not be affected. The O2 sensor corrects the injection pulse duration.
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Old 01-18-2019, 12:04 AM
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No, No.
I changed the fuel pressure by removing vacuum from regulators - no vacuum 36lbs, with vacuum 28lbs.
This tests the O2 sensor with computer disconnected, which is as it should be.
I was concerned because I stumbled on that 0.5 volt reading from the computer without the sensor connected.
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