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MG Midget and Sprite Technical - Tachometer Dissasembley!!

I need some help,one of the studs has broken off my tacho that hold it in the dash, Ive taken off the chrome trim and removed the glass with no problem, Ive slightly loosened the 4 screws on the back which has loosened the internals(which i need to remove to fit a long machine screw, held in place with a nut to replace the sheared off stud)just inside the tacho in front of the dial is a metal ring which must come out to enable the innards to be removed,i cant for the life of me see how to get this out without causing irrepairable damage,someone must have done this before? Any tips greatly appreciated. Martin. Its a 1967 MK111 midget(1275cc)
m a frame

It *should* just lift out, but it may be stuck around the edge with the remains of a rubber seal.

Try carefully prising it up from the outside edge with a small screwdriver, or you could try running a blade around the edge.
Dave O'Neill2

Dave,Have tried cleaning edge with a blade and gently prying but its stuck fast,its a bit rusty in there!!
m a frame

I suppose you could try squeezing the housing to distort it slightly 'out of round' and see if it pops.
Dave O'Neill2

As Dave says, the ring is not actually fixed to the housing by anything, and should just lift out. As it doesn't, then it can only be corrosion or it has become stuck with the remnants of the rubber seal. Knowing this, you can be quite forceful with a knife blade or small screwdriver to get it out without fear of damaging it.
Guy

Funny you should post this. I was just researching the same thing with a view to renovating my tacho with new cap's and transistors.

In an article I just read, it says DO NOT remove the 2 recessed screws at the back..

Read this.

"The Smiths tach must be disassembled in order to calibrate or repair it. Carefully rotate the chrome bezel until the tabs on the bezel line up with the slots on the case. This can be a very difficult job if the seals have aged badly and stuck. Whatever you do, DON'T pry up the tabs on the bezel or youll ruin it. If you need to, you can CAREFULLY pry a tiny bit around the bezel in an attempt to break it loose. Don't tap the edge as the meter is very fragile.
Once you get the chrome bezel off the tach, the face glass and inside bezel must come out, if they didn't come out with the chrome bezel. Carefully pry the inner bezel from the case. It is not necessary to separate the glass from either bezel if it is stuck to one of them. Be careful prying on anything, especially if the glass is still in place, as it is very easy to damage it or the bezels. I have not been able to locate a source for the seals, so I just try to be very careful, and reuse what I can with whats left of the seals. I always use a lint free cloth and glass cleaner to clean the glass while the tach is apart.
Once the meter face is exposed, be very careful not to mar or get finger prints on the face or break the needle. The next step is to remove the tach innards from the case.
There are four screws on the back of the case, two of which are recessed in holes in the case and two of which are not. The two in the recessed holes hold the innards of the tach together so don't take them out. Put the tach case face down on the bench. While pinching the U bracket stud with one finger and the power spade lug with another to hold up the tach innards, remove both of the non-recessed screws. The tach innards are now being held in the case by your two fingers. Pick up the case and cup your other hand under the face of the case. Carefully let the stud and spade lug slide out of your fingers and catch the face of the tach by the edges in the cup of your hand. You can then pull the case off of the tach innards and turn it over."
Lawrence Slater

Great advise, Ill have another go in the morning, Ill tighten the 2 screws i shouldnt have loosened first!!!Martin.
m a frame

Managed to get the inner bezel out, it was just rust holding it in!!!now realised connections on back of tacho are different!!my car is 67 but neg. earth, the new tacho is marked neg. earth but has a male bullett connecter and a male lucar connecter whereas my old one has a white wire looped round a block!! and i think a green wire going to a male lucar connecter, is the new tacho going to fit ok and how do i wire it up? electrics arent my forte!!!!!!!! Martin.
m a frame

Does the model number - on the tacho face - start RVC or RVI?
Dave O'Neill2

Your original one was current sensing. The loop induces the signal to the meter. The front face with have RVI on it.

From your description, the new one sounds like a voltage sensing tacho. This will have RVC on the front face.

The green wire in each case, supplies the power to the Tacho.

If the new one is Voltage sensing, then it's very easy to wire up.

Just leave the white wire alone, this goes to the +'ve side of the coil.

Run a new seperate wire from the -ve side(points side)of the coil, and connect this to the tacho signal connector.

PS I poached this diagram, but can't remember the link to where I got it.


Lawrence Slater

Lawrence, Many thanks for the reply,so the green wire which is a female lucar goes on the male lucar on tacho and the new wire i run from the points side of the coil would connect to the male bullett on the new tacho, and the white cable leave alone. Better not do it tomorrow!! Ill wait till Boxing Day. Have a good xmas everyone. Martin.
m a frame

Hi, Wired up new tacho as Lawrence said and it didnt work,pulled off the green feed from back of tacho and ran another from the + side of coil and it works fine, albeit slightly fast or high (about 200rpm Id say!!!)will it be ok wired like this, strange thing is i had 12.5 volts on the green at the tacho with ignition on so cant understand why it didnt work like that? Anyone any ideas? Martin.
m a frame

Have you got an RVC or an RVI?

The cable from the points side of the coil, will go between 12 and 0 volts, as the points open and close.

The green should come from the fuse box, which is made live when the ignition is switched on.

Running a feed from the +ve side of the coil, amounts to ABOUT the same thing, except that it is UNFUSED. Not a great idea.

Lawrence Slater

Thanks Lawrence, its an rvc, Ill amend the wiring this w/e. Martin.
m a frame

This thread was discussed between 23/12/2011 and 30/12/2011

MG Midget and Sprite Technical index

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