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MG Midget and Sprite Technical - How can I tell if my tacho is dead?

Mk II midget. The car has been converted to negative earth at some point. The tachometer is stuck at 3500. It rarely moves whether engine is running or not. I thought I'd have a fiddle with it this evening. On the reverse there is a spade terminal connected to a green wire and a white wire which loops around a post and off to the ignition switch. The tacho face is marked positive earth. I have taken it out and removed the glass and mechanism. The needle did not budge but freely moves if pushed. The springs look intact.

It appears to have been converted to negative earth as I found the green wire connected to the spade terminal and the resistor to the post as per the very useful information in this post:

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=97&subjectar=97&thread=2008020319580925754

So it all appears to be connected up correctly. Now I'm stuck. How can I test if it is working?
Stuart K

Sound very much like it's dead....

They are cheap enough and easy to find.

MG Live at Silverstone over the weekend of 10th / 12th July will have lots of stalls selling such things.

http://www.mglive.com/


Mark.
M T Boldry

Thanks Mark. Already have my tickets!

After cleaning it up and resetting the needle to zero I refitted it. It did respond to the engine revving on start up but then sticks again at 3500 and doesn't budge after that so I think the wiring is correct but the unit is malfunctioning. Is there a spring to return the needle to zero?
Stuart K

Stuart
It sounds as if you have a foreign body, i.e. swarf, iron filing or similar in the gap in the magnetic circuit where the moving coil moves. It is possible to remove it with a needle, BUT you need a very steady hand.....a bright light and a watchmakers eyeglass (lupe for the USA brigade).

Do you want further instruction.
Regards Arthur.
AG Martin

OK thanks, I'll take a look. I have 2 of the required tools you mention. Not sure about the steady hand <g>
Stuart K

Stuart, sorry about the delay but I only go on the net 2 or 3 times a week. Back to your Tacho problem.....
I've been looking at a Smiths tacho and I think it best if we talk.. If you are an MGOC member you'll find my telephone number in the Club magazine April 2009 or earlier under regional review for 0709, Tewkesbury Musketeers. I've retired so call any time.
But to carry on: does the tacho work normally up to 3500 and return as the engine slows?
I note that you have removed the "works"
Yes it does have two hair springs to control the pointer, they also supply the current to the moving coil. When 'stuck' does it return to zero when you turn off the ignition?
I'll keep an eye open on this thread unless I get a phone call.
Regards Arthur
AG Martin

Hi Arthur,

It does not return to zero when the ignition is turned off nor does it drop with the engine revs once it has got up to 3500. When the ignition is off if I tap the glass the needle moves down a bit. The needle did not appear trapped by the glass when I disassembled.

I've given up on it and plan to replace it with a later model. I read somewhere that the RVC is less prone to trouble.

Stuart K

Stuart, Hi . One more try. Your reply would indicate that the pointer, moving coil and springs are catching on something . I would suspect the wires , black and red connecting the printed circuit to the movement. Anyway with the bright light and your eyeglass look to see if anything touches something it should'n't when you move, gently by finger, the pointer up to and beyond the 3500 position.
If you decide the easy option i.e. change the tachometer, there are plenty tachos about and you can use units from the MGB, just don't get the 80 mm version from the r/b cars. Get one to suit your car polarity and you can use a current or voltage triggered unit and easily rewire the car .
Regards Arthur
AG Martin

Thanks Arthur. I may take it out and have another look. The needle appeared to move freely when I checked. Should it be at zero when out of the car or will the needle just rest where ever it is left? When I took it out to inspect it the needle moved freely but remained in what ever position I left it.
Stuart K

No, it should return to zero. It sometimes helps to hook up the rev counter out of the dash and give it an encouraging tap to bring it to life. Worth one more try.
f pollock

Stuart- Hi
If, a big IF, you can get the tacho out of the car and out of its case without the pointer returning to zero then you can look to see if anything is interfering with the moving coil/pointer assembly with the aid of the bright light and a watchmakers eyeglass.
I would suspect that the pointer spindle is bent and the pointer is touching the scale. Are the two screws holding the scale in place?
If the tacho pointer stays wherever you position it when out of the car and nothing obvious can be found on inspection then you may have damaged/corroded pivots on the pointer and I would hesitate to suggest that you try cleaning them with, say Methanol or Propanol, as the delivery system may need extreme improvising. Hint; try and avoid touching the hair springs!!
Good luck Arthur
AG Martin

This thread was discussed between 01/07/2009 and 10/07/2009

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