Well I guess I will jump in. Great forum, lots of great information. I do not yet own one of these things but I am looking very seriously at a 284LE to tame my 7 1/2 acre jungle. The posts here and some of the other sites have been very helpfull in making this decision.
Since I do not yet own one, I am not intimately familliar with the electrical setup, so I can offer no specific help. My field of expertise however is electronics and I have a fair ammount of experience with speed sensors and generator governors.
Since this was described as a single wire magnetic pickup, the other half of the sensors electrical circuit must pass through the engine/transmission case and frame and up the ground wiring to the gauge. If I were investigating this myself, I would look to see how the other half of the sensor signal makes its way through the ground wiring to the gauge. I am still reading all the posts in this chinese tractor forum but I recall some posts regarding the tach and proper grounding of the electrical instrunments(this will apply to the accuracy of the other gauges also). From what I have read here, paint/corosion under ground wiring terminal connections and poor electrical connectons are not uncommon on these things. I would seperate and re-connect ALL the gauge wiring connections and all the engine/chassis ground straps. Inspect all the crimped electrical connections and terminal lug crimps and make sure all screwed down terminals have clean brite and shiny metal to metal contact and are screwed down tight. The signal from that magnetic pickoff is small to begin with. Add in a poor ground path and the signal will be too weak to run the gauge. Your moving the sensor closer upped the signal output and may have overcome this to show some meter movement.
Magnetic pickoff sensors are simple and are typically either good or they are not. Since you are able to get some life out of the tach by moving the sensor in close(touching) to the flywheel ring gear, I would guess that it is probably OK and that the gauge/wiring is at fault. As for setup of the sensor, in my experience with other magnetic sensors, it should be no closer to the teeth on the ring gear than about the thickness of 2 stacked business cards and as far away as will reliably operate the meter.
Hope this helps.
