Strange issue with my MkII Vitesse rev counter

[dave.vitesse]

From experience a worn bearing can cause over-reading as it allows the magnet to become closer to the disc.

Dave

[NonMember]

Also from experience: residue from ill-advised attempts to clean / lubricate with oils or WD40 can also cause over-reading, often by a huge factor. 120mph in 2nd anyone?

[martin62]

Seems like a prudent next step is to try another tacho. There is one local to me on eBay at the moment so that maybe worth a go.

[johny]

yes this article is quite good although its for a speedo:

http://obswww.unige.ch/~wildif/cars/docs/Smith-jaeger_speedo_repair.pdf

Looks like the coil spring can become weaker so allowing the instrument to read proportionally higher across the complete range or possibly a lack of lubrication in the bearing between the magnet and cup might have the same effect....

[martin62]

I definitely believe the spring is the issue. After re-assembling and a little manual testing, I found the needle returns to the bump stop at a different rate if the gauge is upside down. Seems, at least in part the needle is currently returning under gravity not because of the spring.

[johny]

hmmm not easy to adjust the spring. I take it youve not found any dirt? How about getting a drop of thin oil in the bearing between cup and magnet? At least you can test it easily unlike a speedo.....

[Pete Lewis]

the basic  drive in the tacho is identical to the speedo 

if the needle has been banging the end stop its likely its moved , so its worth a try at hold the disc and move the needle back to retension the hair spring,

apart from its pure size i wish i kept our smiths head drive tested , you could remag and re calibrate anything, but it was in a box about 2.5 ft square like all good things  it  went in the skip 

there becomes a problem with hair springs and needles you can get the reading correct at on speed but the hysterisis of the spring 

bu**ers up all other  readings ....can be lots of fun getting it right for the whole range 

keep at it 

Pete

[Mad4classics]

It's not impossible to re-calibrate NEMAG gauges yourself; encouraged after reading Anthony Rhodes document posted above and based on the pointers given in the the very old US War Department document TM9-1829A, I decided to re-calibrate both my speed and tacho; it's a fiddly task, ideal as a winter project but in the end you can get them spot on and have some confidence in what they're saying. Took me several magnetizing demagnetizing cycles to get it right the first time.

You do need a few pieces of kit to do it; most notably a large electro magnet which I had to make based on the pictures in TM9-1829A, a small geared electric motor and an optical tacho or some means of measuring the cable RPM.

David

TM 9-1829A.pdf

[martin62]

Well the old rev counter was beyond what I was capable of repairing, but a replacement unit (from a Spitfire of the same period), is working just fine.