Sounds like your are on the right track. Which model computer is it?
This is from the Raymarine
site for testing a compass
Multi-meter should be set to:
(a) Resistance scale
(b) 200 ohm scale
The following procedure should then be followed:
1. Disconnect the transducer from the system.
2. Connect one multi-meter lead to the red wire and the other lead to the green wire and a reading
as indicated in the test data table should be present.
3. Follow the above procedure for all the color combinations indicated in the test data table.
4. If all readings match those indicated assume the compass transducer is functioning correctly.
Compass Test Data:
Red to Green 4 ohms
Red to Yellow 4 ohms
Green to Yellow 8 ohms
to Blue 8 ohms
All other combinations Open Circuit
You are looking for a short or open. A +or - 2 Ohm reading is allowed.