After intermittently showing high revs for short spells, my tach started to continuously read almost double. Drove me mad. After checking out the coils and wiring I concluded the tach itself had to be the problem. So, out came the cluster and the tach mechanism itself, after getting hold of a working tach from an old 82 Seca 650 for cheap. After locating the corresponding wiring and setting up a fan to keep the engine temp manageable I started and warmed it up, set the engine speed to 3000 rpm on the Seca tach then shut it off and removed that tach. On the FJ tach mechanism, top right, there is a rectangular white plastic resistor block with a round, slotted adjuster in the centre. I hooked the FJ tach up (holding it in my hand), started it up again, adjusted the resistor with a small screwdriver until the tach read 3000 and shut it off and on a few times to ensure it was consistent, which it was. Cluster was reassembled and reinstalled, and it has been behaving itself for over a month. No ideas to offer on why a resistor should behave like that, but as long as it's working, who cares.
Quote from: FJ4ever on July 24, 2017, 11:40:10 AM
After intermittently showing high revs for short spells, my tach started to continuously read almost double. Drove me mad. After checking out the coils and wiring I concluded the tach itself had to be the problem. So, out came the cluster and the tach mechanism itself, after getting hold of a working tach from an old 82 Seca 650 for cheap. After locating the corresponding wiring and setting up a fan to keep the engine temp manageable I started and warmed it up, set the engine speed to 3000 rpm on the Seca tach then shut it off and removed that tach. On the FJ tach mechanism, top right, there is a rectangular white plastic resistor block with a round, slotted adjuster in the centre. I hooked the FJ tach up (holding it in my hand), started it up again, adjusted the resistor with a small screwdriver until the tach read 3000 and shut it off and on a few times to ensure it was consistent, which it was. Cluster was reassembled and reinstalled, and it has been behaving itself for over a month. No ideas to offer on why a resistor should behave like that, but as long as it's working, who cares.
Follow-up Dec 5/17 - the fix worked perfectly for the rest of the season, with no further tall tach tales.