O2 sensor unplugged voltage readings are still being displayed

[lionelhutz]

The '7747 ECU has the O2 ground or negative signal wire at pin D6. In stock form, this O2 negative wire generally went to a loop installed around one of the rear intake manifold bolts and used the engine block and exhaust manifold to complete the negative side of the signal path to the sensor.

If you switch to a 4-wire O2 sensor you can remove the wire connection from the engine block and connect the wire directly to the O2 sensor. If you want to help with noise then twist the positive and negative O2 signal wires as they run between the sensor and the ECU.

FYI, the O2 input on the ECU is a differential circuit which reads the voltage between the 2 input pins D6 and D7.

[mmigacz]

The '7747 ECU has the O2 ground or negative signal wire at pin D6. In stock form, this O2 negative wire generally went to a loop installed around one of the rear intake manifold bolts and used the engine block and exhaust manifold to complete the negative side of the signal path to the sensor.

If you switch to a 4-wire O2 sensor you can remove the wire connection from the engine block and connect the wire directly to the O2 sensor. If you want to help with noise then twist the positive and negative O2 signal wires as they run between the sensor and the ECU.

FYI, the O2 input on the ECU is a differential circuit which reads the voltage between the 2 input pins D6 and D7.

lionelhutz:

Is it important that the O2 ground wire (connected to D6) is grounded at the engine block? Currently, I have it connected to a ground near the ECM (under the control consul). I assumed ground is ground. Bad assumption? It will be easy enough to run another wire from D6 to the engine block, if its recommended.

Eaglemark:

I do not have a two way radio. The only thing I could see that might give some form of interference is the depth finder wiring. Should I be concerned with EFI wires running next to the depth finder wires?

How far should the EFI wires be from the alternator, distributor, coil and spark plug wires? Currently, the closest my EFI harness gets to any of the listed components is 4 inches.

[EagleMark]

Not sure on depth finder, I've never O scoped one so don't know how much noise they put out or wiring?

4 inches of clearance to EFI wiring should do the trick.

O2 ground needs to be to block.

[lionelhutz]

If you are going to stick with the 1-wire O2 sensor then you need the O2 ground wire run separately in the harness and connected to the block. Actually, right to the manifold would be even better. The idea is to connect it as close to the O2 as possible.

If you do switch to a heated O2 sensor then connect it directly to the O2 sensor.

[mmigacz]

If you are going to stick with the 1-wire O2 sensor then you need the O2 ground wire run separately in the harness and connected to the block. Actually, right to the manifold would be even better. The idea is to connect it as close to the O2 as possible.

If you do switch to a heated O2 sensor then connect it directly to the O2 sensor.

I am switching to a three wire, heated o2 sensor.

[lionelhutz]

If you haven't bought the 3-wire yet then I'd recommend you use a 4-wire. When you extend the D7 wire to the engine you can then put it directly to the sensor so you know it's making a good connection.

[mmigacz]

If you haven't bought the 3-wire yet then I'd recommend you use a 4-wire. When you extend the D7 wire to the engine you can then put it directly to the sensor so you know it's making a good connection.

Too late, I bought the 3 wire already.

[billygraves]

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[lionelhutz]

O2 Ground at the Block is not where I would place it in your application.

I can't agree with that. The O2 ground has to be connected to a good grounding location as close to the manifold as possible and 100% never connected to another wire that provides the grounds for any other component. There are good reasons why there is a separate O2 ground going directly to an intake manifold bolt from the factory.

I'm not sure where you were reading your 0.7V, but reading 0.7V between the battery negative terminal and the block means nothing to the EFI when the EFI is using the block for it's power and signal grounds.

I heard the O2 heaters do not need to be continually ON and in OEM is switched off after a time and temp.

All the older GM stuff just has the heaters connected to key-on ignition power. You don't want to turn off the heater on a long tube header install. The O2 can cool enough at idle to start throwing a false signal. The same will apply to this application with the cooler running engine and the water jacketed manifold.

[billygraves]

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[lionelhutz]

The stuff I call older goes back before 2005. I really haven't looked at schematics for stuff 2005 or newer enough to comment on the O2 heaters being shut off. It could happen.

Next time I recommend you measure the voltage between the PCM/ECM O2 negative connector and the shell of the O2 sensor. The 1-wire and 3-wire O2 sensors uses the O2 sensor shell -> manifold -> block -> head and -> O2 negative wire -> PCM as the negative half of their signal path. Putting the meter between the O2 shell and the PCM connector will directly check the complete negative side circuit.

[brian617]

The stuff I call older goes back before 2005. I really haven't looked at schematics for stuff 2005 or newer enough to comment on the O2 heaters being shut off. It could happen.

A lot of your late model 4 wire pre cat o2's vary heater voltage. It can be watched on graphing scan tool. I use a Snap-On Modis here at work.

[billygraves]

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