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Jumpy Speedo


deano266

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After a lengthy rewire and a new alternator I have finally got everything back together.

 

However when driving along anything above 50mph and the needle jumps around as if it was Tigger.

 

Does anyone know how to solve this

 

Cheers

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Normally a symptom of a 'sticky' inner cable. Or maybe you have put it in with too acute bends. So make sure it flows properly with no sharp bends . And maybe drip some oil inside ?

 

Later: Ignore this, didn't realise it was electronic.

Edited by alanrichey
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Electronic speedo I assume? Most likely interference then - voltage spikes on the supply line, or interference on the sensor line, are being read as impulses from the sensor and are causing incorrect readings.

Is the wire to the sensor shielded?

Do you have an electric cooling fan fitted? Could be essentially running as a generator with enough forward speed, maybe that is creating some motor noise on a circuit near to the speedo sensor?

Otherwise it's finding what the noisy circuit is, what's creating the noise, and either shielding it or filtering it. Easy option is to just filter the speedo. Without a portable oscillosope it's not really easy to find out where the noise is coming from...!

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Hopefully it came with cable to run to the sensor, and they should have supplied shielded cable - so a braid around the outside of the inner wires. That braid should be earthed at one end only - typically the plug end (ie. at the gauge). The braid should run pretty much the whole length of the cable, bar maybe the last few inches at the sensor end.

 

If it only happens above 50mph is it speed related, or is it *engine* speed related (does it happen at slower speeds if you hold a lower gear)? Just wondering if revs are contributing - ie. alternator or ignition noise. The latter is normally the worst offender. Keep the gauge supply wires, and the speedo sensor wire, away from anything to do with the coil ;)

 

edit: here's an idea, if you're comfortable with it. Get up to the problem speed then, when it's safe to do so, dip the clutch (ideal would be to turn the engine off but that would turn of the dial as well I guess!), does the problem go away? If not, maybe it's just a bad/rattling connection somewhere...

Edited by brumster
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OR the pulse is affected by a varying distance between pick-up & bolt head/magnet. Found with Florin I had to shave the points off the diff bolt heads if they were closest to the sensor, got a reliable speedo once the sensor was constantly just a mm away . Is the sensor mounted on the diff so it moves in unison with bolt/magnet?

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Might be my mistake, but I was assuming from the original post that the problem has occurred after a re-wire and new alternator, ie. nothing else has changed, and the speedo was working fine before?

 

Is that the case, deano?

Or has some aspect of the pickup, sensor, or speedo changed (or is new) in all of this?

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It happened before the rewire but thought it could be a symptom of the old superspec nest which was an absolute nightmare.

 

It is better than it was as under 'slow' acceleration it's not so bad I can reach 50 before wobble.

 

I'm going to see if the sensor has moved tonight but when I do dip the clutch the speed does sometimes come back and is steady.

 

@DanE do they make one for the mt75

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It will definitely be the cable. If you haven't got one make sure you get a 90degree right angle adapter as the space between tunnel and gearbox will be too tight and will have pinched and bent the cable.

In this case eBay will be your friend or a good motorfactors will be able to source a cable for you

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