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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: GasStation on April 29, 2012, 10:45:58 PM
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I took the K75S for a ride tonight and to my great surprise, I noticed the tach needle was floating or stuck at 2000 rpm while at idle. I turned the bike off, tapped the tach window cover and the needle slowly returned to zero.
The tapping appeared to fix the problem but for how long? Has this happened to anyone else?
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Warped face plate?
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I read about that Franken, thanks. Are we talking about micro-warping here and how do confirm that diagnostic? My front window appears pretty much flat.
I read somewhere else that the fuse box on the left could be a culprit and that a thorough cleaning of all the fuses and fuse holders sometimes helps. I have to admit that I do not understand why the fusebox would have an impact on my instrument cluster and tachometer needle.
In the addition, the needle seems happy right now.
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My front window appears pretty much flat.
He's not talking about the clear face plate, but the black one BEHIND the needle. It can tend to warp and catch the needle from the back side.
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Look at the cluster from the side to see if the face plate is warped.
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Have the same issue on one of my bikes, but only when the instrument house gets very hot in direct sunlight.
Then the needle gets stuck at 4K rpm, the distance between needle and face plate....far from each other.
I`ll guess it is (too) tight tolerances at the moving coil elements in the bottom end of the needle shaft.
Inge K.
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Iv'e heard of people who live in hot climates putting a towel over the cluster when parked. The flat black face plates are ideal conductors of solar energy which is why they can get hot enough to warp.
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To close to the north pole to get that hot here in Norway, and the face plate definitely isn`t warped.
Inge K.
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My tach does this intermittently as well. (1993 K75RT) Have not yet figured it out. The back panel does not appear to be warped but can't be absolutely certain. Happens mostly on sunny/warm days but occasionally other times as well. If I figure it out I will post!
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I don't know if this helps, but I've just solved a similar issue. I recently purchased a 88 k75s. The tach would get stuck if the bike was idling. If I revved the motor the tach would get unstuck and go up to about 5-6000 rpm (wherever I probably revved the engine up to). When I left off the throttle quickly it would stay at 5000rpm.
Last week, with a poor forecast on the horizon, I took off the instrument panel and went to investigate further. I figured I had nothing to lose so I dove right in. I carefully used two forks to pry the needle off of tach shaft, unscrewed the black faceplate, removed the gpi (gear position indicator), and then removed the tach assembly. It turned out the round magnet guide that the tach shaft spins on was a bit corroded. I carefully took apart the tach assembly (being very careful of the copper coils) and cleaned the guide magnet. It was significantly better, but it was still jamming a bit. I then used a small piece of casing from 22gauge wire to raise the shaft a bit. I reassembled the tach and it was spinning smoothly. To test I hooked up a 9v to the two pins on the tach assembly. You want the positive lead attached to the left pin, negative to the right. If it works, great, if not try hooking the positive lead to the right and negative to the left. If that works, then you installed the magnet upside down - fix that.
Before you reassemble the tach, hook the instrument cluser to the bike as is, start it up and measure the voltage across the two connector for the tach on the tach circuit board . You should be getting voltage around 9v when you rev it, it should get higher like 10-12v. If that's good, Reassemble and hopefully you are good to go! I'm kicking myself for not taking pics. Anyways I just typed this on my phone, so let me know if/where you might need clarification
UPDATE: everything still appears to be working. The tach is a little bouncy but is still accurate.
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Might also be a loose ground somewhere.