4 wire O2 sensor

[RobertISaar]

colors might be different, but for a 94 blazer 5.7:

two white wires are used for the heater circuit, black wire is used for the O2 signal(goes to pin A10), and the ground reference is pin A12(if you were using a 4 wire, you would connect the O2 signal ground there).

[CDeeZ]

So, while a four wire may not be necessary for my application.... It can't hurt to have that extra ground wire could it? I kind of like the idea of a little redundancy, like a fail-safe.

Also, you guys are talking about pins.... Why couldn't you just ground the two ground wires under the hood somewhere rather than running a ground wire all the way to the ECM???

I was just about to order that ACDelco four wire 02 but now you all have me wondering.

that 94 Blazer was obviously a TBI application and if it used a four wire O2, then I would think you could slap a four wire O2 on just about anything TBI you wanted whether it had it or not.

[RobertISaar]

well, think of it this way: with a 4 wire, grounding of the O2 sensor is essentially foolproof. unless the wire is cut somewhere, it can't be effected by bad grounds on the engine/etc, since it's grounded via the ECM.

the 4 wire listed is also used in 96 and earlier 3100 applications, 97 switched to a different style of connector, but same sensing element.

[EagleMark]

Actually yes having an extra ground for the O2 sensor circut could be a bad thing. This is why they are usually run through the ECM/PCM. If the O2 has a differant ground offset then the ECM/PCM then readings will be affected. Grounding the extra wire to same ground lug on engine from ECM/PCM would be the way to do it.

Here's part of the instructions from the LC-1 Wideband O2 sensor explaining grounds...

2.3 Electrical Grounding Concerns

The electrical environment inside a car provides unique challenges, combining high voltages and

currents, low-voltage signals, convoluted signal paths, and variable conditions (i.e., fans turning

on and off, or starter cranking).

When using precision electronics, it is important for ALL electronics to share a common ground.

Remember that “Ground” is more than just the return path for any circuit- it is also the reference

against which any voltage is measured.

Since it is not always practical to ground every device to the exact same location, here are some

tips on grounding:

1. The BEST grounding scheme is all grounds (i.e., ECU, Gauges, LC1 heater, LC1

system, etc.) SOLDERED into a single lug and bolted to the engine block.

2. The next best is all grounds attached to the same source, as close as possible, but on

separate lugs. This is because even the corrosion between lugs can create ground

offset and noise. Incidentally, this is why many ECUs have separate ground wires for

injectors vs. ECU system ground- separating high voltages and low voltages reduces

noise.

3. Grounding to the engine block is usually better than grounding to the frame.

4. Grounding a gauge to the radio is usually bad- ground offset can vary with volume.

5. Grounding to an ECU housing is generally not optimal- housings are strapped to the

frame for shielding, but not necessarily grounded.

6. One of the WORST things to do is to ground most of your electronics to one place (i.e.

the engine block), but ground one device somewhere else (i.e., the frame). Not only can

this result in ground offsets, it can also create a “path of least resistance” for high

currents THROUGH a low-current device. This can result in melted wires and vaporized

diodes, when, for example, starter currents flow through gauges.

[34blazer]

theres one issue i have come across when using the heated O2. they arent as sluggish as compared to the single wire, which seemed to make tighter swingpoints. i had trouble with my car falling out of CL during highway cruising. part of that was my fault due to tightening up the O2 constant tables. even when i returned them to stock values it would still fall out of closed loop. i fixed it by tightening up the O2 voltage qualifier constants.

[dyeager535]

colors might be different, but for a 94 blazer 5.7:

two white wires are used for the heater circuit, black wire is used for the O2 signal(goes to pin A10), and the ground reference is pin A12(if you were using a 4 wire, you would connect the O2 signal ground there).

So was the O2 used dependent on VIN? The '94 full size Blazer I posted the info for was spec'd for a three wire, the AFS-74...

edit: Duh, I get it, wire "out" from the ECM on A12 for ground reference....

[CDeeZ]

Okay after reviewing the 7747 pinout diagram it looks as if there are two
"system grounds" the A12 and also @ D1. I found both of these wires under the hood and I have them grounded to the intake manifold, but at two different places.. Should I put them all at the same location for grounding?

So, if I were to tap into the A12, would I just figure out which one of two wires under the hood with that color scheme (black w/ white stripe) is the A12 or does it even matter since they're both system grounds… and then just strip some insulation off and piggyback the O2 ground wire onto that???

EagleMark, you say it could be a bad thing, but if it's properly grounded it should be just great right?

Okay now I'm really confused, it looks like the pinout shows that the 02 has a dedicated ground… on pin D6 with tan insulation…… If I buy this 4 wire, where do I ground it? To one of the "system grounds" (blk/white) OR, to the "GND O2" (tan)


It sounds like a 3 wire would be just fine, but if I can properly setup a 4 wire O2 I see no reason why not to go for it?

RobertISaar, I see that you said to ground the 02 circuit @ A12, the system ground as my pinout diagram is calling it… Why not use the other wire the tan "GND 02"??

[EagleMark]

Simple answer: Leave the grounds the way they are and add the heated O2 sensor ground to anywhere on engine block. If you go with four wire ground the sensor to same bolt on engine that ECM already uses, either one.

[RobertISaar]

that's why i would run the O2 sensor's signal ground to the ECM.

even if the entire rest of the ground system essentially fails for some reason, with the O2 having it's own ground reference, it will always read 100% correct. if ground isn't really ground, then the O2 voltage will shift, sending a falsely rich or falsely lean signal, depending on the shift.



my 2 cents anyways.

and i'm not sure what ECM the schematic i looked up was for, it may not have been for a 7747, there were at least 2 O2 sensor circuits i could have chosen from, both were at least slightly different.

[EagleMark]

With what Robert has posted I would not run a four wire O2 sensor on a 1227747. I've added three wire O2 sensors to 1227747 before because it is desinged for one wire from sensor going in! Then has a ground wire going out, and it works! Other 2 wires are power and ground for heater.

Use a three wire! It's been done many times with nothing but improvments!

Why try a four wire? The wire coming out of 7747 goes to ground, if you add the fourth wire from a four wire O2 sensor to it you have already passed whatever is inside the ECM like pictured above so might as well be to ground.

[RobertISaar]

technically.... you can connect the O2LO pin directly to a good ground, the 4 wire O2 sensor's signal ground to a good ground, then it's basically a 3 wire.

the only real advantage (and difference between) a 4 wire has over a 3 wire is that it doesn't rely on the body of the sensor(it's threads specifically) for the ground path. ONLY difference.

[damanx]

that's why i would run the O2 sensor's signal ground to the ECM.

even if the entire rest of the ground system essentially fails for some reason, with the O2 having it's own ground reference, it will always read 100% correct. if ground isn't really ground, then the O2 voltage will shift, sending a falsely rich or falsely lean signal, depending on the shift.



my 2 cents anyways.

and i'm not sure what ECM the schematic i looked up was for, it may not have been for a 7747, there were at least 2 O2 sensor circuits i could have chosen from, both were at least slightly different.

This seemed like a good thread to revive from the dead since a related question about the 7747 case ground was in question.

I'm going to add my opinion since having read through this thread, the correct response was kind of revealed in a "beat around the bush" kind of way.

In any case, the above diagram is the correct schematic for the o2 sensor circuit for the 7747 ecm.

Essentially, if you are going to run a 4 wire sensor, AND, you want to strictly run the O2 sensor ground signal to the ECM, then you really want to use a high grade quality shielded wire. Otherwise, as Robert stated, ground the D6 wire from the ecm and then ground the O2 wire for this pin, but make sure you have a really good ground.

The schematic above doesn't really show the true circuit for the o2 sensor, but in any case, it's nothing more than a differential amplifier and the short of it is that you ultimately want the "potential" reading from the o2 sensor to be a stable reading regardless of how the ground is actually applied.

Edit to add:

If that circuit in the ecm is as I suspect, then that circuit does not care about the voltage level of the signal between o2 high and o2 low, it only cares about the absolute value between the 2 points. Maybe that is easier to understand.

[daleulan]

One thing to be cautious of when swapping O2 sensors around - planar 4-wire sensors. They are very nice in that they can light off quickly, they are faster, more accurate, and that's what most OE's are using these days. Also they are almost always ungrounded - they are designed to be tied in to the ECM sensor ground and eliminate the requirement of that (expensive) differential amplifier. The fact that National Semiconductor stopped making one of the more popular O2 sensor amplifier IC's (LM9040) is a pretty good indication that most everybody has switched over to planar sensors. But heating is critical, the heaters are modulated using a 'dewpoint' algorithm which looks at the exhaust energy and predicted exhaust system temperatures (model for exhaust pipe, catalyst, and oxygen sensor tip). The heaters are then controlled at a low duty cycle for a while then ramped up slowly to avoid breaking the sensor due to water impingement. Same applies to the LSF4.x UEGO sensors. You probably will find that aftermarket UEGO kits 'eat' sensors quite often and this is one reason why.