Erratic speedo on '98 VFR800FI

[bob prohaska]

On a leisurely ride the speedo on my '98 VFR800FI started dropping
out between 35-45 mph. Not to zero, but bouncing down erratically
when speed got near 40 MPH. There's an LED voltmeter with good
transient response, it looked normal, so there was no indication
of low voltage problems. Other instruments and clock were normal.

After stopping for gas and resetting the tripmeter to zero I rode
home, noting that only 0.4 miles were recorded on a journey that
normally registers 1.2 miles. Being in traffic, I didn't note the
speedo reading, but it was a less-than 25 mph trip.

Back home I ran the bike on the centerstand, same story. At about
40 mph the speedo would momentarily drop to 30-35 mph and come back
up. Watched the battery voltage with a meter, at least 13.6 volts
at the battery, LED voltmeter showed no hint of low voltage
transients under close observation. Ran it long enough to make
the fan cycle on, no obvious change.

After re-seating the speed sensor connection the tests were repeated,
same result. Next, I tried spinning the sensor with an electric drill
while the ignition was on but the engine not running. The speedo ran
up to 120 mph with absolutely no hesitation.

For the moment I'm tempted to think it's a faulty ground. Unfortunately,
that amounts to a long and winding road with a poor view of the scenery.

If anybody's been down this road before and knows a shortcut, please
post!

Thanks for reading,

bob prohaska

[bob prohaska]

Post by bob prohaska
On a leisurely ride the speedo on my '98 VFR800FI started dropping
out between 35-45 mph. Not to zero, but bouncing down erratically
when speed got near 40 MPH. There's an LED voltmeter with good
transient response, it looked normal, so there was no indication
of low voltage problems. Other instruments and clock were normal.
After stopping for gas and resetting the tripmeter to zero I rode
home, noting that only 0.4 miles were recorded on a journey that
normally registers 1.2 miles. Being in traffic, I didn't note the
speedo reading, but it was a less-than 25 mph trip.
Back home I ran the bike on the centerstand, same story. At about
40 mph the speedo would momentarily drop to 30-35 mph and come back
up. Watched the battery voltage with a meter, at least 13.6 volts
at the battery, LED voltmeter showed no hint of low voltage
transients under close observation. Ran it long enough to make
the fan cycle on, no obvious change.
After re-seating the speed sensor connection the tests were repeated,
same result. Next, I tried spinning the sensor with an electric drill
while the ignition was on but the engine not running. The speedo ran
up to 120 mph with absolutely no hesitation.
For the moment I'm tempted to think it's a faulty ground. Unfortunately,
that amounts to a long and winding road with a poor view of the scenery.
If anybody's been down this road before and knows a shortcut, please
post!

After much head scratching I resorted to browsing the forums at
vfrdiscussion.com and ran across somebody who wore out the coupling
between the speed sensor and sprocket nut. Since my bike has ~85k
miles on it, I looked again at the coupling and noted worn spots in
the middle of the flats, suggesting it had slipped. After sliding the
coupler a couple mm away from the sensor and reassembling, the speedo
is now steady past 50 mph indicated when running on the centerstand.
When the weather cools off (105F expected today) I'll do a road test.

Thanks to any who had the patience to read this silly saga.....

bob prohaska

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