autopilot st4000+ problem with the sensor

[cmedsailor]

Although the actual sensor is position correctly in the center and makes a correct mechanical movement during sterring, the sensor bar sometimes correctly shows that the wheel is in the middle and suddenly shows (without moving the wheel) full bar to the left (like steering full to port). What is going on? Could it be some wire connections? Problem is that autopilot cannot work like this. Thanks

 

[hightech]

The arm should be directly opposite the cable when your steering isat midships. I have seen these units be taken past their endstop and give the problem you have. The unit has a potentioneter inside it,so basicallya big resistorwith a wiper picking off a varying resistance. My guess is that the potis upone end and going across the gap,hence the jump.

You can check the resistance and watch the variable value going up and down, disconnect from the pilot. Cannot remember which way you connect, but it is the coloured leads, not the screen. Should see a fixed resistance across one pair of wires, 10k from memory and a varying resistance to the other wire as you move the arm.

If ithas gone past the stop and the arm is no longer over the wireat midships, you can turn the arm clockwise untilresistance is felt, then keepgoing and it should click back to thecorrect place.

 

[alastair]

I think Geoffs answer is probably spot on for this fault.
A problem Ive found is that the rudder angle indicator occasionally moves eratically. A short term fix was to open the sender (I think the back was glued on) and sqirt some WD 40 on the potentiometer (pot) . The pot isnt a sealed unit, and some wd40 finds its way inside to the wiper contact.
Reseal the back cover and all was well for a tear or so, thes some slight eratic movement started again, so I ended up replacing the pot with a matched unit from RS for about 2 quid. Dont buy a complete new sender unit - they cost a fortune
Good luck