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MG MGB Technical - Speedometer Question

The sppedometer n my '71 mgb roadster has stopped working. At first I thought it was the cable, and naturally bought a new one. I decided not to deal with the angle connection on the transmission, so AI ordered an OD cable to get the extra length. As I was beginning to disconnect the old cable from the speedometer, I thought I should see if the inner cable was turning as I drove the car. Maybe the problem wasn't the cable, but the speedometer itself.

As it turns out, the inner cable is turning fine as the car moves along. I tried to hold the inner cable to stop it from turning, but couldn't. It seems to turn strong and constant. I did read that sometimes the end that fits into the speedometer becomes rounded, and then begins to slip. How rounded does it have to become before that happens, and how can you tell?

Also, as I drive with the cable connected, the speedometer does not work at all for 99% of the time. But every once in a while the needle will jump, or I will hear something that sounds like a muffled grinding sound from inside the dash.

Does anyone have any ides as to where the problem lies - cable or speedometer? Any chance of me repairing the speedometer myself? Thanks. Curt
Curt

Sounds like your cable is fine. Does the odometer work? If it does, the cup and magnet assembly in the speedo is bad. Best bet is to take/send it to a speedometer shop.
John H

John, No, the odometer does not work either. Curt

Curt

Curt. In that case, the speedo is bad. There are two instrument repair shops normally recommended. Nossiger (sp?) located in New York and Moma located in New Mexico. Both have good reputations, but the one in New York would be closer.

Les
Les Bengtson

I concur with Les's advice, just stepped in to say that the rounding Curt mentions is highly unlikley, I would have thought, in a correctly installed cable, the load on it being negligible ... with a good speedo.
Paul Hunt 2

I have a basic question regarding speedometers. Having never looked inside one, I have no idea how they work, or how they are repaired. What makes them so expensive to repair? Can someone with mechanical experience repair them? Are there any good websites or archives that provide information? What do the numbers on the face mean? My 1971 B has S/N 5227/06 1280 on it. Is that the Serial number for the speedometer for cars from 1968 to 1971? Others on eBAY have different numbers, depending on what year's MGB they fit. Any information will be appreciated. Thanks.

Curt
Curt

Curt, the 1280 number indicates revolutions of the speedometer cable per mile registered. This number changes based on gearing in the transmission and rear axel, so the speedometer is built to match the design of the cars drive train.
The speedometer part is baisicaly a magnet rotating in a cup that is connected to the needle. A spring, much like the mainspring of a watch supplies resistance to the movement of the needle. Changing the tension of the sping changes the calibration of the speedometer. The odometers in most of our cars is a gear train that turns the individual number sections of the odometer One numbered section turns the next everytime it makes ten revolutions. It is operated by a paul that is driven off a gear rotated by the speedo cable and will advance a gear one tooth at a time. While it is possible to fix your own it is somewhat like being a watchmaker, hence the cost.
John H

Not so much a serial number (which increments by one for each unit produced) but a reference number which indicates the spec it is constructed to - e.g. turns per mile, face markings etc. There were dozens over the years (I've found 50 so far!) according to market, model and year. I've assembled a list which can be seen at http://www.mgb-stuff.org.uk/wn_miscframe.htm and click on 'Speedos'.

Whilst I list a number of 5227 variants 06 is a new one on me, so maybe it is 51! If your car needs a 1280tpm (as a 71 USA model did) speedo then any 1280 unit will give the same speed and mileage readings, but the face markings and/or internal features will differ if it has a different reference number.

As to whether you can repair it yourself it all depends what is wrong. Sometimes it just needs some hardened lubricant replacing with some fresh stuff, but if a part has broken it must be replaced. Repairers have new parts available, and they calibrate it afterwards. The speedo is a fairly precision piece of kit, the reference to a watch is apt.
Paul Hunt 2

Thanks, both John and Paul.

Paul, you mentioned replacing hardened lubricant with fresh. How would I go about determining whether that is the problem? I have no idea how to get into a speedometer, and don't want to make matters worse trying to do so without some knowledge of how it comes apart and goes back together. Do you know, or is there somewhere I can find out? Thanks.

Curt
Curt

Hi, Curt - there is an excellent paper done by Anthony Rhodes on how to work on the Smith and Jaeger Speedometers. You can download the PDF file at:

http://mywebpages.comcast.net/rhodes/speedo.html

or view a page by page version of it at:

http://www.tigersunited.com/techtips/Rhoades-Speedo/rt-Rhoades-Speedo1.asp

Tells how to get inside it, and what you'll find, and has some excellent pictures. Not hard, just "fiddley", in Britspeak - - Best of luck - - Alec
Alec Darnall

If neither are working, then possibly the speedo head has siezed and you are 'grinding the gears' at the gearbox end. With no drag from the speedo the inner may well be turning, with some apparent force unless you have fingers like a Mole wrench, but it may be slipping with the speedo connected. Put a suitably shaped rod in the square drive hole in the back of the speedo and you should be able to turn it easily. If it doesn't turn easily it may be a siezed bearing, in which case if you can get some movement of the drive shaft a little light oil while you work it back and fore may be all that is required. If it turns, you will have to open it up.

The biggest problem I had with my 73 UK speedo (other years and markets may vary) was with the trip reset winder. Anthony mentions a 'tiny screw', but I either it was too tiny for me to see or there wasn't one, or a roll-pin or anything else I could see a way of disconnecting. This made it quite tricky to get the innards out of the case, but I succeeded, and *almost* all the way in, but right at the last minute the speedo needle pinged off, which was what I was trying to avoid. Fortunately it didn't break or bend the spindle, or lose the needle, but I ended up having to 'recalibrate' it anyway. I jacked up one rear wheel, calculated what revs I should be doing at 30mph in 4th, used the internal and an external rev counter to set that speed, then slipped the needle back onto the spindle.

Once you get the innards out you can put a suitable rod in the square drive hole and twiddle it to see what is happening inside. Later on you can use a variable speed drill. It is a matter of looking at things to see what is happening. With neither the speedo nor the odo working it should be quite easy to spot, although you could have two separate problems - one affecting the speedo and another the odo. The speedo should be easy to spot, Basically one disc is on the end of the input shaft, in close proximity to another disc which the needle is on. The first disc is free to rotate with the input shaft, there is a magnetic attraction between the two discs, but the second only moves against return spring pressure.

The drive ratchet and pawl for the odo should also be easy to see, less so if it is something inside the odo assembly. You will notice that the ratchet and pawl always drives the odo in the same direction regardless of which direction you turn the input shaft (note to Stephen King ...).
Paul Hunt 2

Well, here is where we stand. I have the speedometer out, the bezel and lense off, and I started to take the insides out by removing the two screws. However, it was difficult to do with the right angle drive in place, so I removed that. Is it normal to have a right angle drive on the speedometer? I know it's on the transmission, but I have not seen any reference to one on the speedometer. As you may guess, a light bulb went on when I saw it, because the needle did not move when the dirve was in place. Hovever, when I removed it, and used a narrow flat tip screwdriver to turn the speedometer drive, the needle moved in what appeared to be a normal way.

Does that sound like the right angle drive could be the problem, similar to the problems it gives at the connection of the cable on the transmission? My next question is can I forget the right angle drive and just connect the cable directly to the speedometer? Everything seems to fit correctly, and, since I was sure the original problem was in the drive at the transmission end, I have a new OD cable.

Hoping this makes sense, I would appreciate any and all input. Thanks.

Curt
Curt

Curt, The speedometers on 68-71 left hand drive cars used angle drives on the speedometer with the cable exiting through the right firewall. Later cars didn't have the angle drive on the speedometer but the cable exited through the left firewall. Yes, the angle drive could be the problem. It's easy to check by spinning the drive. You can leave it off, but you may have some problems routing the cable to the right firewall, cables need to have gradual bends if you want a steady speedometer needle.

Clifton
Clifton Gordon

The right-angle drives are very expensive. They were fitted to all cars at the gearbox end, and I didn't realise this for some time as neither of mine have them. If your gearbox end drive is sound you might be able to use this at the speedo end, and use a longer cable to get a reasonable radius at the gearbox end as mine have. I say 'might' because I thought they were the same part but I see from the Parts Catalogue that they have different numbers.
Paul Hunt 2

Curt, my 69 MGC with OD has had both rightangle drives removed with no problems. The cable is routed into the car thru a vertical hole that is located in the pasenger footwell, forward from the heater outlet. Just be gentle when you install the speedo, the cable should curve from the back of the speedo to the firewall.
John H

I want to thank all of you for your vaulable input and information. As things stand now, the speedometer appears to be working with the cable connected to it without the right angle drive. It also appears the right angle drive on the transmission is working properly. The concensus seems to be that the speedometer should work with the drive removed from it, if the cable is routed from the transmission with all gradual bends. I have a new OD cable, and I will gingerly attempt to attach it to the transmission right angle drive, then rout it up to the speedometer as suggested and connect it straight into it. And then we shall see what we shall see.

It will also give me a chance to visually see if there is a physical difference between the two right angle drives. It may be the way the drive is attached to the speedometer that makes the difference.

I will let you all know the outcome. Thanks again. Curt
Curt

This thread was discussed between 09/01/2006 and 16/01/2006

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