Two different circuits..it is actually 2 seperate sensors , but they share the same physical case ....just picture/draw a 'Y" with the bottom leg being grounded and each of the upper legs being a resistor. That is what the internals look like on a wire schematic of the sensor. So, you can see that from the tip of one leg to ground you have one resistor, but if you go across the tips of the upper legs, you are measuring both resistors in series
[ resulting in R1 + R2 for value.]
But b/c each has it's own circuit, that 2R value is never used by any inputs....
Get it ??????
I can tell from your reply that you do, so.....just FYI , et al...........
The ONLY difference between the 4 pin and 2 pin sensors for this chassis is the 2 pin uses the sensors case for ground of the two resistors to complete each circuit to ground.
Whereas , the 4 pin does not use a ground, but rather has each resistor individually wired across 2 pins, making each resistor completely seperate from one another electrically..they still share the same phyical casing, but that's all. The circuits on 4 pin are completed by power going in one pin, through the resistor , and back out the other pin to complete the circuit.
You can not get a R1+R2 value reading on this sensor b/c they are not joined electrically. This chage was made b/c there was an electrical design change made in the chassis that required isolation of the sensors ..... both styles still have the same values and they both have 2 sensors in the single case.....it is just how they are wired back to the the controls that is different. The 4 pin has IN/OUT connections/pins, the 2 pin has IN only, getting the completion of the circuit to ground through the case [ thus needing no OUT terminal]
In other words , if you unscrewed the 2 pin from the engine , eliminating the ground, their would be an OPEN sensor circuit...but if you did the same on the 4 pin, you would still have a completed circuit...