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MG Midget and Sprite Technical - Odometer.. or thermometer?

Sorry to raise this one again but....

My odometer is playing up. On Friday is was very cold and the odometer would measure for a mile or so and then stop. Fiddling with the reset managed to get out another mile or so. At the end of a 25 mile run it had recorded about 2 miles.

Saturday was a bit warmer and it registered 11 miles out of 25 or so. Oddly it seemed to be turning when ever I looked.

Today I did 25 miles in a relatively warm garage assisted by an electric drill on the gearbox end of the cable, and a stop watch. It didn't miss a beat!

On all occasions the speedo itself has kept registering and not indicated any sort of fault which might have upset the odometer. It does seem to be affected by the cold. I am reluctant to take it apart because I am not sure I will see anything obvious and may well upset the calibration or do some other nasty to it.

Has anyone come across this sort of temperature related reliability issue before? I wonder whether the lubricant or crud perhaps, thickens in the cold sfficiently to halt the mechanism.

(I can't see how anyone can "clock" a car with a drill. Top speed on the drill was 50 mph so the 25 miles took getting on for half an hour.)
G Williams (Graeme)

IIRC have a look around the web people have taken speedos (gauges?) apart and fixed them, might even be in the Archives here
Nigel Atkins

Nigel - there are several posts. I found a couple for a recent thread about resetting the odometer to match the old, replaced one.
I have tried to get a spare one apart and concluded that I need to remove the needle and face inorder to "wangle" the trip reset lever thro the case.
As I have no spares (the spare speedo is for a different turns/mile) and it isn't completely broken I'm a bit reluctant to have a go as I'm not sure I shall see anything obvious.
If it were dead then there would be nothing to loose, but it "sort of.." works and "if it ain't quite broke, don't mend it as doing so may really B**gg*r it"
G Williams (Graeme)

Graeme there must be loads of unused spare/scrap speedos about see if anyone has one for you to pick up, you can match it to yours by the number on the face

ETA: I've just remembered I've had a face out of an old speedo sitting on the book shelf for 5 years - the number on it is SN - 6142/06A, just to give you an idea of the sort of number not that yours will necessarily be this
Nigel Atkins

Graeme,

After realising a broke speedo cable and replacing; my speedo worked very intermittently. On replacing the instrument all is working fine. If you are getting a readout of any sort (mph or miles registered) I would say the drive cable is working OK and the instrument is duff. I have now replaced mine and all is well.

Re taking it appart: I have read this up and its quite delicate. You are correct about the drill and how long it would take. To reset it would seem that on most instruments the face has to come off and therefore the needle (how this fits and therefore how it goes back depends on the manufacturer) and then you need to turn the digits while the one way locators for each digit are held open (usually a small plastic afair between the numbers apparently). This way the instrument can be set to any value.

I hope to have time to take old speedo appart by the summer so I can see how it really works.

If anyone has more specific info I would, as Graeme would, much appreciate it.
Dave Squire

Dave: I replaced mine after I discovered the po had bought and installed a nice new speedo but failed to realise he had fitted a 4.2 instead of 3.9 diff. Decided that the diff was sufferable considering the cost of replacement and opted to change speedo. Good old Ebay!
The replacement worked fine and is a spot on match to a gps speed readout. It's just that over the winter the odometer is intermittent.
If it is ok during Spring, Summer and Autumn then I will leave it as that is when it is more important that it works ok.
I've read all about taking it apart but as I said above, if the rachet operating arm is worn, I don't have the parts anyway.
G Williams (Graeme)

Gotcha.
Dave Squire

The odometer works off a worm gear on the speedo shaft such that as it turns it moves a small pawl lever back and forth against the teeth of a circular ratchet gear on the end of the odometer shaft. If the odometer is working intermittently it is likely that this pawl is worn and not catching on the steps of the ratchet every time.

If the problem is heat related then I imagine that the problem is to do with expansion of the pawl in some way moving it away from the (possibly worn) ratchet. You wouldn't think this would happen with plastic or nylon components but some of the mechanisms I have looked at use a small metal lever which just might react in this way. More likely is that it has accumulated muck in the light grease used to lubricate the mechanism and this is just sticking when it is cold.

To get innards out of the casing does need the bezel and glass to come off first. In theory this is done by twisting the bezel until the tags on the rear edge line up with slots in the casing. But this can be very hard to do as the rubber sealing rings stick to the glass and prevent it twisting. So then you have to carefully bend the tags out until the bezel comes off. You don't need to remove the face to get the mechanism out. And you could only remove the face by pulling the needle off which is very likely to damage the ultra-delicate hair spring - then you have bu&&ered the speedo!

If you attempt this I would remove bevel and glass, lift out the complete mechanism and then use an artists' paint brush and meths. to carefully clean around the gear mechanisms. Take the opportunity of cleaning out the inside of the casing and maybe spraying it with a reflective silver paint which will significantly improve the instrument lighting levels! Clean the inside of the glass with an optical wipe or similar camera lens cleaner and reassemble. Be extremely cautious with any attempt to clean the dial face. It is very easy to smudge the white lettering or get visible fingerprints on it! Better probably to leave it alone or at most "dust" it with a soft dry artists' brush. A nice delicate job most appropriately done on the living room coffee table of an evening.
Guy Weller

IIRC, the inside of the speedo I took apart was sprayed on the inside with one side white going into blue the other side - which explained why one side of the speedo was more illuminated with the instruments lit
Nigel Atkins

Correct Nigel. They used a very light blue graded through to white, but it is difficult to replicate. I redid mine with a "chrome" silver paint which was more reflective (thoughtful perhaps?) and certainly improved the illumination levels.
Guy Weller

On my spare unit I could not get the innards out as the reset arm stopped it. Even with the little extension removed I tried all sorts of rotations, twists, pushes and it wouldn't come free. A thread I read recently suggested removing the needle and face improves clearances, which I accept, but it does introduce the risk of inadvertant damage.

I think the idea of grease bunging things up and perhaps being more viscous when cold makes sense and would explain the problem. Today is was warm-ish, and 20 mile round trip the odometer was fine, not a beat missed!
G Williams (Graeme)

Graeme, they are a bit like those Chinese puzzles. You think the innards just won't come out; you try turning the thing every which-way and it won't come out. And then, for no apparent reason, the thing just separates! BTDT!
Guy Weller

..... I think the reset rod is longer than standard on the spare. THe PO/rebuilder decided to put the speedo on the wrong side (no handbook!) when of course the cut-out for the reset in the bottom edge of the dash is on the left side. So he bent/modifed the reset!

When I replaced the speedo I decided the struggle to swap the rev counter and speedo over and untaping lumps of wiring harness wasn't worth the effort. Other jobs were more pressing. I think he did it that way because there was a very convenient hole in the bulkhead which lined up with the back of the incorreclty positioned speedo and thro which the cable drive could be routed. That I have sorted!
(And I cut a new recess in the bottom edge of the dash).
G Williams (Graeme)

This thread was discussed between 18/02/2013 and 19/02/2013

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