Rudder Reference Sensor Issues

[Quitsa]

My 711C autopilot has developed an odd problem. The rudder position suddenly went to a large out of range value. After checking that nothing had come loose in the sensor mounting and that the wire connections at the processor were solid, in desperation I did a factory default reset and redid the installation setup. The rudder position went back to normal values and functioned normally. However, I left the system on and after about 15 minutes, the rudder position was again showing as an out of range value, which doesn't change at all as the wheel is moved.

The autopilot has worked well for three years. I don't really understand how a simple potentiometer like the rudder sensor could fail in this intermittent way. Any thoughts as to what I could check? I tried measuring the resistance of the RRU and it seemed in line with specifications.

 

[Johnny Electron]

You might want to check for any pressure being place on the RRU arm as it will pull the potentiometer wiper away from the coil and be a problem. Most RRU problems relate to the setup or the RRU itself. Unless you suddenly started picking up some noise on the boat the RRU might be having problems while of course the pilot could have a component going bad. The pilot would be my last thought. If intermittent, that will be a bit of a bear to pin down.
Lucky for us, the Furuno FA6112 Rudder Reference Unit (RRU) is a very easy to troubleshoot. For those who are not familiar, here is a brief introduction to the RRU.

The RRU provides the exact rudder angle/position to the pilot. A Potentiometer (POT) is used to vary or “Divide” the voltage supplied by the NavPilot, by using resistance. The position of the RRU POT determines the amount of voltage sent back to the NavPilot and this value is how the pilot knows the rudder position.

The NavPilot sends a fixed 5 vdc using the white and green wires of the RRU. The wiper connections of the POT (yellow and black) reflect voltage changes as the rudder moves. The wiper voltage value from hard over to hard over will be from 0.5 vdc to 4.5 vdc or vice versa because the POT will essentially “Divide” the provided 5 vdc (0.5vdc+4.5vdc=5vdc) by using resistance (2.2k ohm) as the rudder moves port or starboard.

Note that the resistance of the POT itself does not matter in this circuit. If the wiper can turn all the way to the Clockwise and Counter-Clockwise maximum limits of the POT, the same voltage will be seen by the processor (fig 1.1). The NavPilot 300, 500 and 700 series systems can use RRU resistances from 500 ohms to 5,000 ohms without any issues.

Testing the RRU

Disconnect RRU from Autopilot Processor.

• Connect your digital Multi-meter leads to the white and black wires.
• With the RRU centered, you should read about 1,100 Ohms of resistance.
• When the RRU is slowly rotated, you should see a gradual increase or decrease in the Ohms readout from 50 ohm to 2100 ohms or vice versa depending on the direction. If it is erratic, the RRU may be bad.
• You should see the same by checking the yellow and green wires but opposite of white and black. As value for Yellow/Green go up, White/Black go down and vice versa.
• Resistance of white/black will roughly equal yellow/green with the RRU is perfectly centered.

 

[Quitsa]

Thanks. I decided the easiest way to isolate the issue is to order a new RRU and connect it to the processor unit without actually installing it in the boat so I can return it if it turns out that the RRU is not the source of the problem. Otherwise I think I will be chasing my tail given the intermittent fault. When I tried measuring the resistance, the level seemed to bounce around some. That would suggest the RRU has some internal fault.

I had forgotten that when I replaced the Garmin electronics three years ago, the only thing we kept was the RRU unit so it is not a Furuno FA-6112. Maybe we can blame it on them! The new one I ordered is the Furuno part.

In the meantime, I was surprised by how well the autopilot steered using the Fantum feedback setting notwithstanding that my boat has inboard engines. I do like having the rudder position indicator so that I know the rudders are straight when using just the engines for docking, however.

 

[Quitsa]

The problem although intermittent was the rudder position sensor. Replaced with FA6112 and everything working fine. The autopilot seems to steer better than with the old RRU.