Tach Swings Wildly

[rellquist]

I have an FL style dashboard and tank conversion on my 89 883 Sportster. The speedo has a tach built in. The tach needle jumps around wildly. I put a resistor in line with it but that didn't help. The wires going to the coil and the tach are tight and no corrosion. Can a coil that works fine to run the engine cause this problem or is it just the tach? Anybody have this problem with this style of speedo? The speedo says Harley Davidson on it so I believe this conversion was a Harley kit back in the day.

[decman]

Check the ground on the tach.

[rejeanprimeau]

Check the battery terminals after checking the others gizmos.

[IXL2Relax]

Are you running the original Ignition Module?

Or what aftermarket module are you running?

Single-Fire ignition setup or dual-fire setup?


Pre-2004 Sportster tachs were intended for dual-fire ignition
with the uneven firing.

2004-later, Sportsters went to a single-fire ignition configuration
and the tachs used the digital data bus information.


On the BigTwins, this difference happened in earlier years.
You might need to pin down the Tach model to know what it expects.


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[bustert]

+1 mr. relax
the resistor is not the way to go. IF you are picking up rf interference by the wires running to close to the high tension leads, first try a diff route. you can also try a filter capacitor to shunt the rf to ground.

[Hellbent_4]

My battery cable came loose and did some weird re-booting type things like that.

[rellquist]

Well, the speedo froze at 40MPH so I got a Drag Specialties unit that looks just like the HD one that froze. That cured the tach problem but at 1200 miles on the new speedo, it went up to 60MPH while I was doing 50 and froze there. I got a replacement Drag Specialties and on my first ride I was doing 55 MPH and the speedo started climbing all on its own. It went all the way around to 6 o'clock where it hit the tach needle and stopped. It stayed there even after I stopped the bike. This is a mechanical speedo with a cable. Anybody experienced this? Is the Drag Specialties unit just a POS?

[IXL2Relax]

I don't know what DS speedo/tach you bought, but I'd be checking the wiring & power to the unit. Even though the unit takes a mechanical drive, it may operate electrically (as does the tach)...

I would check voltage levels to the unit. And make sure you have a good ground on it.

On the other hand, maybe you're right about poor quality...

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