Unplug the sensor cable from the PLS 2000 unit.
Measure the voltage at E4 (red cable) coming off the board with respect to GND .

 

Unplug the sensor cable from the PLS 2000 unit.
Measure the voltage at E4 (red cable) coming off the board with respect to GND .

E4 red probe:

315 mV VAC stable
3.2 mV VDC and dropping

Black probe on white supply as before.

 

I cannot see the tracks on the PLS 2000 board clearly.
We want to see how E4 receives power. Trace E4 back and tell me to what component it is connected.
Or better still, post photos of both sides of the board with proper lighting so that we can see the tracks.

 

attached

 

Thanks for the photos.

Unplug power and sensor cable.
With your DMM set to measure resistance, measure the resistance of this resistor.
It appears to be 20kΩ.

Edit: We already tested the voltage on both legs of this resistor. My verdict is this resistor is bad and needs to be replaced.

 

Without the sensor plugged in, but with power supply plugged in:

the resistor (“first stop”) has a resistance oL (I think that means error).
The voltage, with black DMM lead hooked up to white supply, is 102 VDC on the top side of resistor, and 2 mVDC (and dropping) on the bottom side.

should i run this with the Sensor plugged in… just realising that would make sense, eh ..

 

Without the sensor plugged in, but with power supply plugged in:

the resistor (“first stop”) has a resistance oL (I think that means error).
The voltage, with black DMM lead hooked up to white supply, is 102 VDC on the top side of resistor, and 2 mVDC (and dropping) on the bottom side.

should i run this with the Sensor plugged in… just realising that would make sense, eh ..

No. Stop where you are.

Just to be certain. Unplug power.
WITH POWER DISCONNECTED,
What was the range setting on the DMM resistance measurement?
Set it to 2M.
Touch the meter leads together and check that it reads 0.
Measure the resistance of that suspect resistor again.

If the reading is OL, replace the resistor. If you can see RED, BLACK, ORANGE, GOLD, it is a 20kΩ 5%.
It looks like a ½W resistor. Replace it with 20kΩ 1W resistor.

After you have replaced the resistor, plug in the sensor.
Measure the DC voltage at E4.

 

No. Stop where you are.

Just to be certain. Unplug power.
WITH POWER DISCONNECTED,
What was the range setting on the DMM resistance measurement?
Set it to 2M.
Touch the meter leads together and check that it reads 0.
Measure the resistance of that suspect resistor again.

If the reading is OL, replace the resistor. If you can see RED, BLACK, ORANGE, GOLD, it is a 20kΩ 5%.
It looks like a ½W resistor. Replace it with 20kΩ 1W resistor.

After you have replaced the resistor, plug in the sensor.
Measure the DC voltage at E4.

it ain’t pretty but it should do.
DC voltage at E4 is now 5.35 VDC

 

Does the switch now function properly when you put everything back together?