Strange behavior from o2 sensor

[usagoal]

MS-II, V3.0 2.891 code, Ford 351w
I started up my boat project from last spring and left it with a non working o2 sensor. I bought a new Bosch NB one wire and hooked it up, but will not get a signal. MS works on the stim and even in another project of mine with a WB - so I believe the circuitry is correct. The only time I get a signal is by holding the sensor wire with one hand and grounding myself to the motor!?? I figured I may have had a bad ground, but checked all the obvious ground locations and even ran an extra ground from MS back to battery negative with no luck. Body of the sensor is also grounded well. Would it be a good idea to calibrate the AFR table (VOLTOXTarget) first, even though I already had an o2 sensor in place? I forgot to do this while working on it and wanted to get some feedback/advice before going back to where this boat is being stored. I've never had such a problem with a sensor like this before. Thanks!

[Matt Cramer]

Are you able to get a voltage signal from the sensor if you unplug it from the MS, using an MS ground for reference? Or does it still exhibit odd grounding behavior?

[usagoal]

I did not use a MS referenced ground....just engine block ground. I will check that and get back to you.

[trakkies]

I did not use a MS referenced ground....just engine block ground. I will check that and get back to you.

All sensor grounds must be run back to the same point on the MS as per the wiring diagram. You are in effect measuring very small currents from them with the MS, and different ground paths will upset this.

[usagoal]

trakkies, I'm slightly confused by your response. I understand that all sensor grounds go back to MS. And for all sensors (except o2 sensor) I have used grounds that run back to MS, which is then grounded to battery negative terminal AND engine block. Are you saying I should have a separate ground from where my 02 sensor plugs into the exhaust back to MS ground? (don't forget, this is a one wire NB sensor and ground is the body of the sensor) If that's the case, I can do that. I have yet to check the ground at MS while measuring voltage on the o2 sensor itself as advised by Matt. I will try to get this done soon, I'm curious about what I find.

[usagoal]

Matt,
I tried unplugging the o2 sensor from MS and using a MS ground as reference, but still no luck. The only time I did get a reading while using the MS ground (or block ground) is when I hold the sensor wire in one hand and touch ground with the other. I know the human body is conductive and can act like a resistor or capacitor in some situations, is it possible C10 in the o2 circuit has gone bad and my body is acting like a capacitor?? I may take MS back out and re-test on the stim or my other MS project.

[trakkies]

trakkies, I'm slightly confused by your response. I understand that all sensor grounds go back to MS. And for all sensors (except o2 sensor) I have used grounds that run back to MS, which is then grounded to battery negative terminal AND engine block. Are you saying I should have a separate ground from where my 02 sensor plugs into the exhaust back to MS ground? (don't forget, this is a one wire NB sensor and ground is the body of the sensor) If that's the case, I can do that. I have yet to check the ground at MS while measuring voltage on the o2 sensor itself as advised by Matt. I will try to get this done soon, I'm curious about what I find.

Ah - sorry. I've not come across a one wire O2 sensor.

[Bernard Fife]

usagoal,

O2 sensors need to get quite hot to give a signal. Three or four wire O2 sensors have heaters to help with this, one wire O2 sensors rely on the exhaust heat alone, which may not be enough in an non-OEM set-up. There's more on this here: http://www.megamanual.com/v22manual/mwire.htm#ego

Lance.

[usagoal]

Thanks for the advice Lance. I have my MS unit out and will re-test it on the stim to be sure the circuit works. I do have some 3 and 4 wire o2 sensors laying around our shop that I could try. I have been concerned about the one wire sensor not reaching a working temp since starting this boat project. The exhaust manifolds have a water jacket that keeps them very cool (and therefore the sensor too). I will just have to keep testing and figure out what I need to do in this case. Thanks again.

[trakkies]

Thanks for the advice Lance. I have my MS unit out and will re-test it on the stim to be sure the circuit works. I do have some 3 and 4 wire o2 sensors laying around our shop that I could try. I have been concerned about the one wire sensor not reaching a working temp since starting this boat project. The exhaust manifolds have a water jacket that keeps them very cool (and therefore the sensor too). I will just have to keep testing and figure out what I need to do in this case. Thanks again.

IIRC, the heater on a sensor is just for warm-up. When the engine is running, the exhaust gasses should soon take over. I'd guess the sensor would have a short life if the heater was on all the time.

I dunno boats, but on a car, the average exhaust would provide a very poor ground, due to the various joints. Indeed it's quite common to use ground straps to it for RFI purposes - to prevent it acting like an aerial.