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MG MGB GT V8 Factory Originals Technical - Wiring an oxygen sensor?

I'm in the process of putting in an EFI system in my Rover 3.9. So far so good, but I don't have a complete EFI wiring harness, so I have to go it alone as far as wiring up a couple of things.

I don't understand how to wire in the oxygen sensor. I got a standard Bosch 3-wire heated unit (I've heard that heated ones are better since they get you into 'closed loop' mode a bit more quickly). There is one black wire and there are two identical white wires. Clearly, black is ground. I can handle that. One of the whites is hot, and one of the whites is the signal from the O2 sensor back to the brain; but which is which? Can't Fritz put a different color wire on there for us Yankee dunces?

Anyone who can explain, or point me to an URL that will shed light on this matter, please speak up. I don't want to wreck this $80 sensor by hooking it up the wrong way!
Tim Haskell

Just a thought; use an ohm meter to see wich white hooks-up to the ground wire. I do not know the expected resistance, but it should not matter, only that it goes through a "heater" coil and out to black.

Pete Plouf

Pete, thanks ... however, I'm told that you have to be _really_ careful applying volt/resistance meters to O2 sensors, or you can irrevocably bugger 'em, so I'm hesitant to do that. I can tell you that the junior high school dropout at the Pep Boys counter where I bought it had no freaking clue ... only the question, "what kind of car is this for?"* (As anyone in their right mind -- whether or not they're EFI experts -- knows, all Bosch 3-wire O2 sensors are exactly the same thing, with just the wiring end-plug changed to mate with the plug in different cars.)

Here's what I think now: the two whites are the heater -- it doesn't really matter which is hot and which is ground -- and the black one's the O2 sensor itself. I think I am just going to say a quick Hail Mary and rig it up that way!

* How many times will this irrelevant question be asked during my ownership of my V8??? I'm sure most of you share my view that this is a most tiresome question when you're talking MGB V8. I just roll my eyes now.
Tim Haskell


With a digital multimeter, I do not think you will damage an O2 sensor. The current put out by these things is miniscule. I was more concerned by trying to use a multimeter to read the O2 sensor as it was sending a signal to the computer. This too was unfounded due to the impedance of the meter. If you guess, do you not stand a 1 in 3 chance of firing 12 volts into the computer? No good would come of that! Another option is too point your computer in the direction of BOSH's website and ask them. Who knows, they might even have an answer.

Pete's right on reading with a DIGITAL meter. A good digital meter doesn't use voltage to calculate resistance.

most o2 sensors wire as follows:

heater circuit resitance: 13 ohms warm, 3 ohms cold

black wire is ground, one of the white wires is +5vdc from computer and the other is return signal

output voltage is .5vdc to 1.0 vdc

I don't know however if the white wires can be reversed as one takes in the voltage and the other sends it back. You could test it with a 9 volt battery and try it both ways to see if it still outputs the same readings.

good luck,
Sean Squires


So the O2 sensor heats up with only 5 volts, this to be supplied by the computer? Does the computer shut down the heater when the sensor reaches a certain temp? I spend time around the Miata BBS and the 1 wire to 3 wire swap is quite popular. My impresion was that they were using 12 volts to supply the heater. I guess I could go and re-read some of those postings. Thanks for the info.


You are right about it being a 12volt supply, I was thinking of a different engine computer setup.
good luck !
Sean Squires


It sounds like you know your stuff, so I fear to tread ... but can I super-humbly note that I'm pretty doggone sure the black wire on a Bosch 3-wire unit is not the unit's ground but is rather the feed to the computer, and the white wires are the heater wires (the heater works whichever + or - way you hook them up). IOW, if you just hooked up the black wire to the computer and let the white ones dangle, you'd have yourself an expensive one-wire unheated O2 sensor. Tim, you'll rightly be miffed if I am wrong, so do confirm!!

Harry has the answer; put it in and let the car warm-up. The wire putting out a low voltage signal is the computer signal. The heater won't care which way it is wired.

Sounds quite possible Harry, I'm going off of a cheat sheet from a seminar I was sent to some years back. I may be reading my own notes wrong! Hope I didn't cause any problems for ya, feels like I put my foot in my mouth through this thread.
Sean Squires

There are many detail variations of sensor so I would suggest you identify which one you have from the markings on the side.

The basic principle from Bosch is that the passage of oxygen molecules through the ceramic material above about 300 degrees C will cause the generation of a small current which will be fed to the ECU. Typically this will be around 0.6 to 0.8 volts, although in operation I read fluctuations between 0 and 1.6 volts on ECU interface units.

The way I would read this three wire sensor as being one white for the heater feed, black for the heater earth and the other white for ECU monitoring of valtage fluctuations at the sensor. I suggest the digital meter will help ID which of the two whites does what.

Roger Parker

This thread was discussed between 02/09/2000 and 19/09/2000

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