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MG MGA - Oxygen Sensor Polarity

Has anyone out there installed a rich/lean oxygen sensor set up on a positive ground MG? I bought the Edelbrock unit and their techies say it can't be done. No further explanation....
Thsnks,
Doug
D Sjostrom

Since you are apparently not wedded to concours, why not change the car to neg earth? No parts involved, (unless you have a pos earth radio without a polarity switch, or an electronic pos earth SU pump), much more flexibility re modern accessories. Most modern electronic whatzits can only be switched by isolating the unit from the car, and wiring backwards - very iffy and likely dangerous.Don't know if you could do that with the O2 unit, since the sensor is of necessity grounded by its body.
FRM
FR Millmore

Sounds like the best solution. I have a non-electronic SU and will probably need to reverse the diode.
Thanks,
Doug
D Sjostrom

I don't believe your o2 sensor cares. It generates a millivolt signal when heated. The signal is the measured between the housing and the signal wire, which has nothing to do with the battery polairity. I have one on my TR4, positive earth, along with a throttle positon sensor ( radio shack pot) both signal go to a minature data logger which is powered by a laptop. lots of tuning info is generated. good luck. Bill
w.g cook

As long as the car remains positive earth, the guys at Edelbrock are correct. A 1-wire sensor will produce a positive voltage with respect to chassis (earth) regardless of the chassis polarity. IIRC, about 0.5V positive with respect to the exhaust pipe. Although the "box" could possibly be isolated and made to run on a positive chassis car, this type of sensor will be incompatible with the capability of the box.

There maybe 2 wire (non heated) O2 sensore that are not electrically tied to the exhaust pipe in which this may work.

The best solution will be to change to negative earth.

no fault.... no foul.....

Chuck (speaking from the electronics side, not the O2 sensor side)
Chuck Schaefer

I am intrigued. What does this oxygen sensor do? Never heard of it before.

Steve
Steve Gyles

O2 sensor tells you the fuel/air mixture, sort of. There are two kinds, narrow and wide band. Narrow band is cheap, but only tells you rich, just right, lean. The readout is a voltmeter, and just right is 0.45V. Many Gauges that purport to do the job consist of a bunch of LEDs that light up as voltage increases from 0.1 to 0.9V, but this is misleading, since the sensor only responds accurately at the stoichiometric or "just right" range. Can still be useful.
Wide-band sensors are much more expensive, but give you actual Lambda, or air/fuel ratio, over the entire range. These are getting cheaper by the day; a few years ago the sensor alone was about $500, now down to as low as $30. The electronics are $2-600. Good info and source for DIY electronics and WB unit is:
http://www.techedge.com.au/vehicle/wbo2/default.htm
The cat's whiskers for tuning freaks.
WB would likely require neg earth car, but that is up to the designer of the unit.

The one wire sensor may in fact work, since you are just reading voltage from sensor to ground. But there should be no electrical connections to the car's system. Two-wire likewise.
Doug, is the Edelbrock unit WB or the misleading many-LED (or calibrated voltmeter) type?
FRM
FR Millmore

Poly LED i'm afraid, but better than Kentucky windage.
Negative ground it will be. Thanks to all for the information.
Doug
D Sjostrom

This thread was discussed between 28/07/2005 and 31/07/2005

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