O2 ?

[C2500]

Ok so where does the 3 wire O2 ground? ECM??

[myburb]

So far as I know it grounds the same as a one wire. case,exhaust,motor and then where ever the ecm grounds, usually the block.

[lionelhutz]

Both have a connection between the negative of the element and the case of the sensor.

A 3-wire used the case for the signal path. The case connects to the exhaust, the exhaust connects to the head, the head connects to the intake and then the intake is connected to the ECM via a wire.

The 4-wire setup runs the wire that went to the intake directly to the sensor instead. It eliminates the possibility of a bad electrical connection in the exhaust pipes from causing false O2 readings.

[C2500]

What’s the best way to add a ground to the 3 wire O2?? Ground the exhaust somehow to the frame?

[lionelhutz]

Change it to a 4-wire sensor. Then, find the negative sensor input wire and remove it from the engine ground and take it to the 4-wire sensor plug instead.

It would be pointless connecting the exhaust to the frame, since the frame isn't directly connected to the ECU.

[daveosx]

Guys over thinking

one wire O2 goes to ECM

Three wire O2 one goes to ECM black or tan or purple
Two white wires one goes to hot in run 12V+ the other to engine ground

Four wire O2 one goes to ECM black or tan or purple or yellow
The other black or tan or purple or yellow goes to ground wire at back of head or on corvette the intake manifold. The ECM return path is a pass through that goes to the head on LT-1 and Corvette LT-1/lt-4 it goes to the intake next to distributor. On TBI the same circuit goes to back of head on Crossfire TBI it goes to the intake manifold.
The power wires one is grounded to engine block the other is hot in run 12V+

Besides I am thinking he needs a fuel pump.

The only reason for the difference on the Corvettes was the fear of electrolysis at the head gasket he first year we ran aluminum heads.
The corrosion engineer was a little paranoid about having constant current through the head to the block even though his fix did the same thing it was next to a water jacket and he thought it was better. On production the noise engineer moved the O2 return off the engine and ran it to the ECM 1992 in 1991 the loom goes to the head.

[C2500]

Thanks Dave!!

[lionelhutz]

Why would you take the 4-wire sensor element ground or negative wire to the engine block? Since It can be connected directly to the ECM O2 input circuit, it should be. This is a separate wire from any other ECM ground wires, typically tan colored. If you don't re-wire to connect the 2 together then you might as well just use a 3-wire sensor.

[daveosx]

Yep on older cars the provision for sensor return from the O2 is not present.
On the ECM circuit board all the grounds are tied together and tied to the case through the bolts.
On the next group of ECMS the grounds were separated to two groups sensor return and chassis.
These circuits are C429 C452 --- C429 is the TPS Coolant sensor Ground and C452 was all other sensors ground.
Starting in 1981 they moved the EST and HEI modules to C486.
In 1991 they started using the O2 sensor return.

On SBC GEN I in all GM vehicles there was one ground point on the rear of the cylinder head.
On the next series GEN 1.5 (one Piece Rear Main) GEN 2 there was the head ground and the Intake manifold ground.
On Later models there was the head, Intake and battery ground.

Internals of the sheet metal GM ECMs all had a common chassis buss and intake buss some had a ground lead for HUD dash.
The cast aluminum ECM cased boards actually had better noise control on the electronics.
Some ECM circuits used a difference amplifier to increase O2 sensor signal reliability it was only then in 1996 that the GM 4 wire sensor could not be substituted with a three wire.
pre 1996 four wire sensors can be substituted with a three wire.

But as far as sensor return lines on pre 1996 GM vehicles the return wires are all tied to ground just some do not share the same physical ground path.
There is an effect called loop induced current that will induce a voltage on a intended shield line or a secondary ground loop.
Electrical engineers at GM had seniority and the electronic engineers worked under them.
The radio engineers worked for AC Delco not GM proper.
So where I or any modern electronics engineer or technician would use shields and ground distribution blocks in a car they did not.

Two areas that will benefit from using shielded cable
The Reference line from a HEI7-8 to the ECM with the distributor side ground only to the shield
Knock sensor with the EST module side of the shield grounded.

pre 1996 The rest of the returns and other grounds will function fine if grounded to the same point.