O2 sensor unplugged voltage readings are still being displayed

[mmigacz]

I'm in the process of converting my 351w to tbi (gm fuel injection). All sensors are in place, the wiring harness and ECM are installed. Because this is being used in a boat, I am not using:

vss
egr
knock
any transmission related ssensors
anything related to air conditioning

I've downloaded tuner pro rt and purchased a serial cable. I've started to datalog and noticed something strange. I'm using a wide band o2 sensor to see my afr. The narrow band o2 sensor is unplugged. I am still seeing o2 crosses and voltage readings while data logging. How can this be, there is no o2 sensor information being sent to the ECM, right? I would also assume a error code would of been tripped.

[RobertISaar]

you're seeing "stray" voltage happening.

due to the way the O2 sensor A/D channel is setup, there is no pull-up or pull-down resistor connecting the O2 sensor's channel up or down to a known value with it disconnected. so, with no sensor connected, you have a small antenna that will show voltage via induction(i think).

due to the sensor's readings going through an amplifier that multiplies the voltage by a factor of 4.5, even very small changes will show up.

if it bothered you enough, connect the circuit to ground and it will stay at/near 0 volts at all times. i don't think you would need to integrate a resistor, but perhaps it wouldn't be a bad idea?

[mmigacz]

Thanks! I'll ground the black o2 wire and see what happens.

[mmigacz]

Thanks! I'll ground the black o2 wire and see what happens.

I gounded out the O2 wire and the O2 volts stayed 4.5mV. This agree's with your above statement. Here is my next question:

The wide band o2 sensor is active in the o2 bung. It is not tied into tunerpro.
The narrow band o2 sensor is laying about 2 feet from from the unplugged o2 socket
The Narrow band 02 socket is not grounded (my next test)
Tunerpro is continously recording a O2 voltage of 438mv to 455mv

Is the mV reading coming from the wide band or the narrow band o2 sensor?

[RobertISaar]

i'm not understanding the question at all? you're seeing under 4.5mv, but also 438 to 455?

[delcowizzid]

0.450 volt reading is normal for no sensor and for sensor thats not upto temp.it doesnt matter what its reading as long as you have closed loop turned off

[mamigacz]

RobertIsaar:

I tried your suggestion of grounding the o2 wire. The datalogger showed 4.5mV. with no fluctuations.

The 438mV to 455mV readings were recorded after I ungrounded the o2 wire. With the above conditions I described.

[RobertISaar]

okay.... so something changed to where that circuit is much less susceptible to picking up EMI now. odd.

[EagleMark]

It's a Ford Motor, converted to GM, in a boat! There's all sorts of things that could go wrong. Look how many GM conversions we've found with EMI RFI do to just that way the wiring is laid out?

Since most boats are wet exhaust... where did you put the O2 sensor or WB? Got any pictures?

[mamigacz]

It's a Ford Motor, converted to GM, in a boat! There's all sorts of things that could go wrong. Look how many GM conversions we've found with EMI RFI do to just that way the wiring is laid out?

Since most boats are wet exhaust... where did you put the O2 sensor or WB? Got any pictures?
5739

This picture was take before I did the TBI conversion. The sensor bung passes through the water jacket, so there is no contact with water. The O2 sensor shown in picture is not what I am currently using. I'm using a 1 wire set up. I'm considering going to a heated o2. Whether the narrow band sensor is plugged in or not, the voltage stays around 450mV. The only time it changed from 450mV is when I grounded the wire (went to 4.5mV). Based on the feedback from Delcowizzid, I'm guessing it is not getting hot enough to work properly.v Do you agree?

[bobdec]

Not sure what you's running in the boat, but Most GM style ECM/PCMs hold the O2 high side at 450 Mv, after O2 starts switching it's generated signal will override the 450 Mv. It's sometimes called bias voltage. Used to detect a open circuit on defective O2. when conditions say it s/b switching. On a setup with O2 heaters you will see voltage at 450 Mv on a cold key on with engine off , then it will go to the 100 Mv range as the heater starts up the O2 in a min or so. That's the O2 overriding the bias voltage. Seeing 450 on a cols O2 is normal, but if heater is working it should move after a while.. l

[mmigacz]

Not sure what you's running in the boat, but Most GM style ECM/PCMs hold the O2 high side at 450 Mv, after O2 starts switching it's generated signal will override the 450 Mv. It's sometimes called bias voltage. Used to detect a open circuit on defective O2. when conditions say it s/b switching. On a setup with O2 heaters you will see voltage at 450 Mv on a cold key on with engine off , then it will go to the 100 Mv range as the heater starts up the O2 in a min or so. That's the O2 overriding the bias voltage. Seeing 450 on a cols O2 is normal, but if heater is working it should move after a while.. l

I really appreciate the feedback and help. I'm a "tuning virgin". I had an interest in converting my boat to efi for some time. A friend of mine, tunes efi engines at a local dyno. He's helping me out along the way, but I feel bad bothering him all the time with questions and problems. These forums are really helping me with figuring things out myself.

[delcowizzid]

you could just do away with the o2 all together i do on most setups.its 1 less thing to fail

[mmigacz]

It's a Ford Motor, converted to GM, in a boat! There's all sorts of things that could go wrong. Look how many GM conversions we've found with EMI RFI do to just that way the wiring is laid out?

Since most boats are wet exhaust... where did you put the O2 sensor or WB? Got any pictures?

I know this is an older post, but I am curious about something. Is there a preferred way to lay the wiring out so EMI RFI doesn't cause issues? My ECM is under the council of the boat. The wiring harness is very long (approximately 25 ft). See the attached sketch.

6059

[EagleMark]

Two way radio antenna can be an issue in a boat.

Normal EFI wiring issues are which EMI/RFI/Noise are picked up is through spark plug wires, coil and distributor as well as alternator. Keep EFI wires clear of these.

[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]

..............

[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]

....

[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]

................