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1994 Mazda B4000 4.0 O2 Sensor issue


mattcarlson7

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Vehicle Year
1994
Make / Model
Mazda
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Automatic
Hello,

I bought a 1994 Mazda B4000 for cheap and am trying to bring it back to 100%. It runs foul in open loop, when the computer should be controlling everything. When we put the computer on it, back probe the pins on #2 side and run a 12v test for the O2 sensors we get a solid 1.22v output and 0 like we should. When we back probe the #1 side the voltage only goes up to .5 and bounces when 12v is applied and is a constant 0 when it is put on negative. We pulled the #43 and #44 pins, the input from the O2 sensors into the computer, and shot 12v straight into the computer. There was no change. Initial thought was the computer was bad, its sending bad signal or changing signal wrong. however we changed the computer and all the outputs were still the same. We are absolutely lost. We checked for grounds, bad pin connections, everything. HELP...
 


RonD

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canada
Vehicle Year
1994
Make / Model
Ford
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Manual
O2 sensors are Heated, that's what 2 of the wires are used for, 12v and Ground for heater inside sensor, heater comes on with key on, and it is usually Grounded via computer terminal so computer can "see" each O2 heater is getting 12volts, if it doesn't "see" the 12v then it will set a code for no O2 heater, 1994 doesn't have this
Light Blue/orange stripe is 12v wire(from fuse #18)
Black/white stripe is the Ground wire(to computer), shares, pin 40 and 60 Ground
These are Vehicle harness wire colors not O2 sensor wire colors
And this 12v/ground has no internal connection to the Sensor inside the O2, it is just a heater

The other 2 wires on an O2 sensor generate their own voltage, an O2 sensor only works when it is heated above 650degF, this is the reason for Open Loop, computer has to wait for O2s to heat up before it can use them, and its also why O2s are heated, so they can be used ASAP.

O2s output .1-.9volts, .9 is Rich(low oxygen), .1 is Lean(high oxygen), but they only work after they are heated above 650degF

This is important
On your V6 engine there will be TWO O2 sensors, #1 on Passenger side, and #2 on Drivers side, and computer MUST KNOW which is which
Both will have a common Orange wire, reference ground wire

Passenger side uses Grey/light blue stripe, Bank 1
Drivers side uses Red/black stripe, Bank 2

Since all O2s are the same, work the same(not wide band), their connectors are universal, so can be plugged in to vehicle wiring harness reversed.

If that happens then when in Closed Loop computer adjusts mix on passenger side injectors but it doesn't respond on passenger side O2 sensor, so it keeps adjusting and engine starts to run like crap and CEL comes on.
And visa versa on Drivers side.

This has happened to me, so check the wire colors, there is only 1 wire color that is different for #1 and #2, on the vehicle wiring side, not the O2 sensor wire colors, they are universal.
So find the O2 connectors and make sure they are on correct "bank"


Also your computer pin/terminal numbers are incorrect

You are using a EEC-IV computer in 1994, 1995 and up Rangers used EEC-V, next and current generation computer
Pins 29 and 39 are the .1-.9volt for O2 signals
29 = Grey/light blue wire, bank 1
39 = red/black wire, bank 2

O2s common Orange wires are pins 46 and 49 although they will be Grey/red stripe wires at computer end since this reference ground is shared with other sensors

The heaters 12v and ground are not connected to computer in 1994, ground pins 40 and 60 do share the same ground point as O2 heater grounds
 
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