Assuming the solder jumpers are open, when you measure across the test points, you are actually measuring the resistance of the LDR in the optocoupler. When the LED is off the resistance should be very high (practically open) and when the LED is on the resistance should be relatively low depending on how bright the LED is (which is determined by the trim pot). If the resistance won’t get above 1K then this tells me there is most likely something wrong in the optocoupler circuit. Make sure everything is installed correctly and all the solder joints are very solid (and noting is open or shorted) in that circuit. Make sure the jumpers are open too, that would really screw up your readings. It is possible that the trim pot is bad, but that is not likely. If everything else looks good then you can check your trim pot by measuring the resistance across it’s outer most pins while the power is off. You can check your readings against the LOW trim pot resistance measurements to see if they are similar.
Let me know what you find.
-Brach