Rudder reference test

Faults in the rudder reference can cause the autopilot to make unexpected course changes or not activate at all. Here is a way to test the rudder reference (RR).

Necessary parts
A multimeter that can measure resistance

Getting started

• Disconnect the RR from the autopilot's course computer (CC) or ACU. You need to measure the resistance of the RR separately.
• Set the multimeter to measure resistance. If your multimeter is not auto-ranging, use the 20kOhm scale.

Measurements
Turn the wheel very slowly from lock to lock several times while making the following two measurements where you are expected to get values as below:

• Red to Green: 5k Ohm (5000 ohms, +/- 10%) stable
• Blue to Red: Approximately 1.6k to 3.3k ohms, about 2.5k ohms when the wheel is centered (+/-10%)
• Blue to Green: Approximately 3.3k to 1.6k ohms, about 2.5k ohms when the wheel is centered (+/-10%)

Additional tips and advice

• Carefully observe the indicated resistance throughout the range: you should see the resistance change evenly and smoothly throughout the range. If the resistance at any point seems to jump up or down to something outside this range, you have a fault in the rudder reference. Dead spots can develop on potentiometers at any point through their range of motion.
• If your multimeter can measure maximum and minimum resistance, use this setting. Max/Min should never be outside the above resistance ranges. We use Max/Min measurements because dead spots can be narrow and easy to miss when you sweep past them: a max/min measurement should catch them.
• If possible, have someone else turn the wheel for you so you can concentrate on the measurements.
• Blue/Red and Blue/Green measurements may not go all the way to 1.6K or up to 3.3K, but whatever is at midship, the deviation should be roughly equal in both directions (e.g., if you measure 2.5k at midship and 3.1k at full lock one way, you should see about 1.9k at full lock the other way).
• If you are familiar with potentiometers, this is a nominal 5kohm pot where Red and Green are the ends and Blue is the wiper.
• If you ever see the rudder bar on the autopilot's display flicker, jump, step back, disappear, go hard over, or if you get 'No rud-ref' alarm or similar, the rudder reference is almost certainly the cause of the problem.

If the resistance is okay but the rudder angles are read backwards?
Swap the red and green wires at the CC/ACU to reverse the indicated direction of the rudder reference. Red and Green are the potentiometer's 'ends', Blue is the 'wiper'.

Next steps
If the resistance measurements for the rudder reference sensor are not as specified above, the rudder reference sensor should be replaced: it cannot be repaired. Raymarine's part number for its rotary rudder reference sensor is M81105. Parts for Raymarine products can be purchased from an authorized Raymarine dealer, see a link here for an example on our website: Dealer Locator. M81105 is a suitable replacement for the rotary rudder reference in all Raytheon or Raymarine autopilot systems. Unfortunately, we no longer stock the obsolete linear feedback unit, as modern autopilot systems no longer require it.