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MG MGA - Speedo

Apologies for what is probably a very basic question. My speedo needle bounces but more importantly also shows a very much higher mph reading than I am actually doing.
I assume I should first try greasing the cable. If I disconnect from behind the instrumnet can I pull it through from there or do I need to disconnect at both ends?
Any other suggestions?
Many thanks
GrahamMV

Graham
initially just disconnect from the Speedo- The inner cable can the be pulled out grease very sparingly and clean the top 6-8 inches to avoid any oil getting in the speedo.
Paul
P D Camp

Graham, has your speedo always read high? It may have been incorrectly calibrated at some time (or taken from an early MGB). What is the number on the face? (e.g. SN.6161/12). The bouncing may also be caused by a kink in either inner or outer cable (or both). However, a common cause is a "gummed up" mechanism:
http://www.mgaguru.com/mgtech/dash/st106.htm
Try the cable first, but you will probably have to pull the speedo (for cleaning/recalibration) anyway.
Neil McGurk

Check you have the correct speedo for your car from the Smith's number (SN...) on the dial face. See Clausager for the various numbers. Mike
m.j. moore

Haven't had a chance to try to pull out cable yet but the number on speedo is SN 6161/06 1440. I cant get hold of Clausagers(Amazon has it listed for 75!!)
GrahamMV

Graham
According to Clausangers SN6161/06 was for a 1500/1600/twin cam with a 4.3-1 rear axle ratio should be ok for your car. See you wednesday
Paul
P D Camp

Clausager is available in German for about USD11..... In English it seems to have become something of a rarity, even abebooks only has one, and I've never failed to find a book I wanted on that site.
dominic clancy

Hi
I greased the cable, and although there certainly is an improvement (much steadier), the speedo is still significantly overstating my speed by I guess about 35%. I was told the gearbox is original, it definitely feels like it, and the tyres are 165/15
Any ideas greatly appreciated.
GrahamMV

Graham

There are plenty of instrument restorers around who will recalibrate and clean the instrument. I used Speedy Cables: http://www.speedycables.com/

They just require you to take some measurments (takes about half an hour)and they do the rest. See here: http://www.speedycables.com/calibration.html

Steve
Steve Gyles

The speedometer consists of a rotating magnet which causes a magnetic force on an aluminum cup attached to the pointer. My guess is that somehow the magnet is mechanically contacting the cup which would cause it to jerk and also impart too much force causing it to indicate high. Mine had iron filings stuck to the magnet which rubbed on the cup causing a similar problem. Worn bearings could also cause the cup and magnet to touch. I believe there were problems with some cables being too long, pressing on the magnet drive and causing bearing wear which is something you should check.

Barney has several pages about the speedo on his site.
http://mgaguru.com/mgtech/dash/dash_1.htm
This one talks about the cable end clearance problem.
http://mgaguru.com/mgtech/dash/st107.htm
Here is a good page describing the speedo and how to repair it.
http://home.comcast.net/~rhodes/speedo.pdf




Jeff Schultz

Thats very helpful. What is now bothering me is that the serial number on the speedo is 6161/06 . Looking at the MGOC spares list at http://www.mgocspares.co.uk/acatalog/mga_instruments.html it says that 6161/06 is to chassis 100351. My MkII has chassis GHN2/102315 so it looks like I should have S/N 6161/12.
Does anyone have any idea if that would be my problem. I am happy to spend the 68 and replace (unless they have white facias as in the MGOC picture!) if that would sort the problem.
GrahamMV

Graham,
If I read clausager correctly, that chassis number means you have a Mk 2, so you have a 4.1 rear diff and therefore need the later speedo.
Shane
Shanerj

The 4.1 diff should only produce a 5% error in the reading, not 35%. It needs recalibrating, and regardless of which diff you have, the speedycables measurements will produce the correct calibration result.

Speedy Cables did not live up to their name when they did my speedo - three months and a lot of prompting and nagging was required, and they lost my spare speedo as well. It's a pity Keith Swindell had to retire.
dominic clancy

Not sure about the part number, but I think the 1440 turns per mile is the proper for the MKII. However, even if the speedo was for a 4.3-1 your and yours had a 4.1-1 or 3.9-1, that would cause the speedo to read lower than actual speed not higher.
Jeff Schultz

Graham,
If you or a friend is a NAMGAR member a recent issue of MGA! had an article about calibrating speedos that might be useful. If need be, I could scan it and e-mail to you.
Cheers,
Gerry
G T Foster

Gerry
Thanks that would be brilliant. My email is gmv22 AT hotmail DOT co DOT uk (obviously substitute "@" and "."
GrahamMV

Graham, I recently calibrated my speedo after taking it apart by connecting an electric drill in reverse to the opposite end of the speedo cable. (see a recent thread in the archives called 'Another speedometer enquiry' - in fact there is a mass of information in the archives on speedos).

I did a graph (see pic.) of indicated dial speed vs. 'actual' speed and the points lay on a straight line which meant that the speedo was 7% fast up to 100mph. The line went through the origin which means I managed to put the speedo needle back in the right position! Mike


m.j. moore

I am tempted to have a go at recalibration but as my speedo is so far out, is it likely to be a waste of time as beyond recalibration?
And if so (as it looks like a PO has fitted the earlier speedo by mistake), does anyone know if I buy the correct later reconditioned unit, will it fit straight in without the need to recalibrate?
If I dont do something quick, the speeding tickets will make the 68 for a replacement look very cheap!
Thanks
GrahamMV

How accurate is the odometer portion of your speedo?
With that info you can know which way to go.
Sandy
Sandy Sanders

OK, thanks. I will check it in the morning. Do I need to check both odometers(trip and cumulative)? What will it mean if that is OK?
GrahamMV

Trip it back to zero and do a ten mile run with a known car or roadside markers if available. Then you can figure out where and what the problem is.
Sandy
Sandy Sanders

Graham
Without knowing your exact ratios buying a new speedo may not instantly correct the problem ie do you know which rear axle you have ? My Mk 2 had a 4.3 rear end and was replaced with the correct 4.1 by the previous owner !!. Speedy cables recalibrated and rebuilt mine after the 5 speed conversion and it was returned in 2-3 weeks fully rebuilt and looked like new. and works perfectly- cost around 60. Look up speedy cables www.speedycables.com and their site has full instructions re information they require- and the costs involved. Use your rev counter in the mean time -in 4th the Mk2 with 4.1 gives 18.2 mph per 1000 revs therefore 2000revs is around 35-36 mph The 4.3 is around 17 MPH in 4th
Paul
P D Camp

Graham
Sorry my earlier mail assumed you had a Mk 1 !!! the speedo is wrong for your car (assuming you had a 4.1 rear end) however speedy cables should be able to recalibrate correctly for your car.
Incidentally my car is GHD2 102399 built July 1961 some 65 cars after yours See you tomorrow
Paul
P D Camp

Sandy, Just come in from a 5 mile trip and both odometers look to be correct. Interestingly, I went past a "speed calming" sign that measured me at 34mph when the speedo showed 48mph.

Paul, I fancy having a go at recalibrating myself. As long as I dont break it (famous last words) there is nothing lost. See you tomorrow.


Graham V

This is getting interesting! If both your mileometer readings are correct then I would suggest that there is nothing wrong with the speedo gearing. So if the needle is reading far too fast then, as there is no mechanical connection between the pointer and the cable, then it can only be that either the magnetic field in the vicinity of the aluminium disc is too great or the restraining action of the hair spring is too weak. My money would be on the hair spring being broken but each half not completely free from each other maybe unravelled by a turn.
You may need a magnifying glass to check this and if it is damaged then it's a job for an expert as the choice of spring would be crucial for the calibration. If it's not the spring then it's the magnet so bear in mind Jeff Schultz's explanation above. Whatever it is please let us know. Mike
m.j. moore

Mike,
I was planning to take out the speedo and manually move the needle a bit. Sorry that's putting it a bit crudely, what I should say is follow the instruction in Anthony Rhodes manual at http://home.comcast.net/~rhodes/speedo.pdf.
Do you think its beyond such a fix?
Graham
GrahamMV

I would have thought if you did that i.e remove the needle when at 48 mph and replace it at 34 mph the low speed accuracy will be poor. In fact it probably would only start registering at 14 mph (?) I might be wrong in saying this however. When you remove the insides make sure you dont grip the aluminium disc as it is not too robust. mike
m.j. moore

I hear what you say. My logic is that if it is easy to open it up & try that (I have no idea how easy it is), then as long as I don't do any serious damage, wouldn't it be worth a go before sending it away for calibration?
GrahamMV

Have you checked it at other speeds? Does your needle sit at zero when stopped? Unless it is a constant 14mph off and the needle sits at 14mph when stopped, just moving the needle will only make it right at 34mph and it will be wrong at all other speeds.

For it to be that far off, it probably needs repair and not just calibration. It will probably have either bad bearings allowing the magnet to get close to or touch the cup, or else a problem with the hairspring like Mike said.

Also, it may not be that easy to open. The rubber gasket is often fused to the glass and the bezel.
Jeff Schultz

This thread was discussed between 01/05/2009 and 06/05/2009

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