LM311 Op Amp as a differential amplifier to drive a solar tracking motor
As explained in the comments, the LM311 is a comparitor, not an op-amp.



Figure 1. The LM311 internals.

Note also that its output is an open collector arrangement. While you have correctly grounded the EMIT OUT, pin 1, you have ommitted the required pull-up resistor on COL OUT, pin 7. Even if you were using it as a comparitor your output voltage would fail to swing high due to this omission.



Thinking about your question today I suspect that your choice of chip may be appropriate but your description of it as an op-amp is the problem and confusing your readers. A comparator is designed somewhat differently to an op-amp but their schematic symbols are often the same or very similar. Let's see ...


  I am trying to use a change in sensor voltage of about 0.015 V (between pins 2 and 3) to operate the Op-Amp comparator with a sensor voltage range of about 2.5 to 3.5 volts. The output of the Op Amp comparator will turn on a Mosfet that triggers at about 3.8V to power a relay to the drive motor.


OK. The comparator output will be either at V+ or 0 V so this should switch your MOSFET cleanly.


  Pins 1 [EMIT OUT] and 4 [V-] are grounded, pin 8 [V+] is 16.5 volt supply and pin 7 is the output. 


Yes, but pin 7 is open collector.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. A way of visualising the transistor output.

When you connect the emitter to GND you need a pull-up resistor on the collector.


  I have a voltage divider network on the inputs to reduce gain to 379 = (49300/130) * (.015). Pins 5 and 6 are not used.


If R1 is 130 Ω and R2 is 49 kΩ then the division ratio will be \$ \frac {49k}{49k + 130} \$ so there will be very little attenuation. I suspect you have the divider the wrong way around.


  My problem is that the op Amp is saturated to rail voltage no matter what the input differential voltage is. 


That's probably caused by the missing pull-up resistor and maybe the incorrect divider rations.

There's a good article on TI's LM311 datasheet which includes the following explanation of hysteresis.



Figure 3. Hysteresis setup.

The datasheet is generic for a range of comparators and Figure 17 shows how to use the pull-up resistor for the open-collector type. Read the preceding paragraph to the figure.