MGB Speedometer
What Speed Are You Doing?
When the speedometer on your MGB stops working you have a number of possible faults. The cause of the problem may be with the instrument itself, the flexible drive cable that runs under the tunnel to the gearbox, or even the pinion drive inside the gearbox. The following article descibes the process of locating the fault and making a repair.
Do not assume the fault is always with the cable. I replaced two broken drive cables before it became apparent that the speedo itself had partly siezed and the extra strain on the cables was causing them to break .
Start off by removing the speedometer. It is much easier to remove the speedo than the cable. I found that it was helpful with my BGT, a 71 model, to remove the black plastic cowl from around the indicator stalk to allow easier withdrawal of the instrument. Be careful not to damage the long trip meter resetting control (which sticks out below the speedometer casing) as you withdraw the speedo from the dash.
Instrument failure is not a common fault on the MGB, but from time to time, the Speedo will stop working. This gives rise to a number of problems, the mileage will not be correct and the speed you are travelling will not be shown.
The inability to record mileage covered is not too much of a problem over a very short period of time but if left unsolved will cause the car to show the wrong mileage, possibly invalidating insurance and in extreme cases could be regarded as clocking the car. Also if you are in the habit of using the mileage the car has covered as a guide to fuel levels rather than relying on the fuel gauge it will be very important to get the Speedo working again quickly.
More importantly if the Speedo does not work you will be unaware of the precise speed at which you are moving. While you may have a rough idea of the speed you are travelling, if you do not want a speeding ticket you must resolve the broken Speedo problem.
| You can undo the cable without removing the speedo
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Tracking Down the Problem
Begin by checking the Speedo head. The first and easiest test to carry out is to disconnect the Speedo cable from the Speedo head. This is simply done by unscrewing the large knurled nut at the back of the Speedo and pulling the cable towards the front of the car. With an assistant in the passenger seat holding the Speedo cable, roll the car slowly forwards a few yards and see if the inner cable turns smoothly. If it does not turn check that the cable is fully pushed home into the outer casing. Having checked that all is securely pushed home and if the cable turns smoothly then the problem lies in the Speedo head itself. The only way forward here is to purchase a new or reconditioned unit. Before ordering you will need to note down all the small letters and numbers that are printed on the bottom of the Speedo face because your supplier will need these to ensure that you receive the correct Speedo. The wrong unit wil give an inaccurate reading, more of a problem than no reading at all. |
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 Speedo drive exits gearbox housing
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Checking the Cable
If the cable either does not turn smoothly or does not turn at all, then the solution is slightly more complex and difficult than just changing the Speedo head. You will need to jack up and securely support the car on axle stands or service ramps. Slide underneath the car from the driver’s side (there is more room from this side without the exhaust in the way) and locate the Speedo cable. It runs from the Speedo head forwards and down through a hole near to where your clutch foot rests. It then passes rearwards down the car until is turns left and into the gearbox just before the overdrive. As with the Speedo, undo the knurled nut and check the cable. Check that it has not broken and has a small length stuck in the Speedo drive or that has rounded the edges so that it just turns round in the drive. If the cable has just broken you have two choices. The first is to remove the inner cable and just slide a replacement in. This may seem logical but to be one hundred percent sure I would suggest that the inner and outer is replaced. This way, if there is a fault that you cannot see in the outer cable it will not effect the new inner. It involves more work to re-route the inner and outer together but will save time in the long run. |
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 Clean with care
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Speedo Drive
If the Speedo head is not faulty and the cable is in good condition then the last item on the checklist is the Speedo drive in the gearbox. With the cable removed take a 7/16th ring spanner and release the bolt that holds the forked Speedo drive retainer in position where the cable exits the gearbox. Be careful not to loose the bolt, spring washer and retainer over the garage floor. Having removed the retainer you can now pull the drive out of the gearbox, sometimes you may need to use a pair of long nose pliers to help it on the way. Once out inspect the gears for wear and possibly teeth missing. If in any doubt about the condition then replace. Make sure that you note the colour of the gear as you will need to provide this to re-order, as there are a number of different variations. Replacement is as straightforward as the removal making sure that all the cables and drives are pushed fully home and the drive retainer tightened. It can be difficult to thread the Speedo cable back up through the hole into the car. It is easier if you use a small length of insulation tape to fix the knurled nut to the top of the cable, as this allows you to push the whole unit up into the car. All that remains is to lower the car back on to its wheels and go up the road to check everything works. You should now have a Speedo that both records the mileage and registers the speed you are travelling. As a footnote, if you change the wheel or tyre size on your car it will be necessary to check that the Speedo still gives an accurate reading. If not you must have the Speedo recalibrated, which most MG specialists will be able to advise on. |
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| Removing the Speedometer
78 MGB on 1. Disconect battery |
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