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Recently my Honda GL1200 started to exhibit a peculiar problem:

- When riding, the speedometer on the digital instrument panel displays numbers that jump erratically rather than smoothly, never reporting accurately the correct speed.
- While in neutral with the bike stationary, after about 15-20 seconds, if I quickly rev the engine, it will show erratically show some digits on the speedometer.

http://spaznet.gotdns.com/media/gl1200_speedometer.avi (a video of the problem in action)

- After I've reved the engine, if I let it sit and idle, the speedometer may do some miscellaneous digits.
- If I disconnect the speed sensor, at the wiring connector, the speedometer only shows '0' (which one would assume it should since there is no speed sensor connected)

I followed the manual's directions to test the speed sensor (connect a multimeter's probes to the blk/br and bls/y wires while the speed sensor wiring is still connected, the key is at the on position, and turn the speed sensor clockwise slowly, getting a reading that goes from 0 then to 5 volts approx 8 times). When I did this, my speed sensor fluctuated from .15v to 2.0~2.1 volts, about 8 times. I separated the wiring connection, turned the key to the on position and read the voltages from the speed sensor's connector harness (multimeter's black probe to ground and red probe to each of the 3 connectors)

blk/br wire : 10 volts
blk/y wire: 4.5 volts
blk/grn wire: 0 volts

So, is the speed sensor bad? or maybe I have a short that is somehow causing the speedometer to get false reading? Any ideas would be great as I'd hate to sink the money into a new sensor, only to find out that it's something else!
 

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Sounds to me that either your speed sensor or the wiring is defective. I'd disconnect the speed sensor and chase out the wiring to make sure there's continuity all the way through to the display and no shorts to ground. If that checked okay then it's time to risk some dollars on a sensor.
 

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My '83 Aspencade had the same issues with the speedo, and it turned out to be a bad sensor. Finally just gave up the ghost altogether.
 

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My 1200 just started to do the very same thing. Also... the auto volume control on the radio goes up and down eradically. Gonna check everything over tomorrow, but I think I am in the same boat as well.............
 

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The auto volume on my Gl1200 seems to be working ok, but let me know how things work out with your speedo issue. I'd be interested to know if your diagnostics (ie: voltages, speedo sensor results) are the same as mine.
 

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Odds are pretty good, it's the speed sensor. Had to change them out on both my 1200's, doing the same thing. Always hunted down used, off fleabay, but if I were to do it again, I would buy new for about $100, especially if I were planning on keeping the bike awhile.
 

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I had the same problem with my '85 1200A. Replaced the sensor and the problem is gone. I constantly get a speedometer error of about +10% when compared to my handheld GPS. I have heard that this is not uncommon.
 

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For some reason the 1200s seem to have the most speedometer error of all the 'Wing models. 10% isn't unusual.
 

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Yeah, mine is the same way on my 85. Speedo is definitely an "estimate". Ran some tests with a cop friend using a Ladar and it was around 10% across the board, so the faster you went, the more error was evident in the speedometer.
 
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