My speedo has never worked. New cable was installed and all I get is either nothing or wild swings of the needle. Took it out and found this bent up piece wedged in the works. The needle is on the wrong side of the peg and won't stay back at zero where it belongs. Any ideas other than finding a replacement unit?

 

sounds like it needs repair or replacement.

 

The speedometer is simple in concept though very complicated in reality. It consists of two parts that run together. The measurement of distance is basically a gearbox with a lot of gears that step down the rotation of the cable to the rotation of the numbers on the distance displays. The measurement of speed is achieved through induction between a magnet and a metal disc. The rotation of one relative to the other causes a torque. The torque is proportional to speed of rotation and is resisted by a hair spring (like the balance wheel of a clock). The needle deflects in proportion to the torque and gives an indication of speed.

My guess would be that something is/has been blocking the gears causing them to move unsteadily. It may be that piece you found. However, it's likely that piece has some other function in the gears. If the input to the speedo turns smoothly, it may now work at least for measuring speed. If it does turn smoothly, try driving it with an electric drill, starting from very slow.

For a home repair, the difficulty is the complicated gears and the non-availability of spare parts. You probably have to buy another for spares. At that point, why not just buy another. If you do that, make sure it's the same turns per mile as the one you have. TPM is a number on the front face.

 

Thanks for explaining the inner workings of the speedo. It never ceases to amaze me how complex seemingly simple things are on this car. I think the tacho is another example although the instrument itself is just a voltmeter.

 

there are repair shops in the UK that will fix it and calibrate it.
www.speedycables.com is one.
www.speedograph(something) is another but I forgot the full name.

 

The second is Speedograph Richfield. They are both well known In the classic car world. In fact, there are several repairers here and in the US probablr because the ones fitted to motorcycles suffered from exposure to the weather. Nisonger are well established in the US - I think they were Smiths' official agents there. I don't have personal experience of any of these companies.

 

What is the "TPM' number on your unit?- I ask in case someone may be willing to supply one.

 

It's not a bad idea to have a GPS speedo or use Waze on your phone in addition to the OE speedo or at least to calibrate the it. Old Jaguars are possibly unique in that the speedometer might even read slow: a reading of 80 mph could be a true 85. This wasn't much of a problem years ago. The boys in blue were sympathetic to old Jaguars, but nowadays speeding is more strictly regulated and automated.
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This is very true. I put a speedometer app in my phone for my car because I could see by other traffic that the mechanical one was wrong. Its speedo consistently reads 10% faster than the car is going. Tire size has a lot to do with it but that's not the issue in this thread. My tires are as close to correct as was available at the time.

 

Looking for a 1020 version
Thanks

 

awwww, nope I have 1080 and 1152.

 

1020 being the TPM ??

 

The needle is mounted on a very thin shaft and held by a fine hairspring. Behind that is a rotating magnet which is turned by the cable. The faster the cable turns, the faster the magnet rotates, and that deflects the needle further. The hairspring returns the needle to zero, and also provides the correct amount of resistance to counteract the magnetic field, so the needle points to the right speed.

The pivot points of the needle shaft are probably either dirty or rusty, you can take it apart and clean and relubricate ( very sparingly!) and it may read correctly. Before disassembly and with the needle at zero, make an index mark on the rotating disk that turns with the needle and is attached to the spring. You need to know where the spring disk assembly needs to be at zero. The needle itself should simply pull off the shaft, so you can move it to the correct side of the stop peg and reinstall. Make sure the disk assembly is at the zero index mark when you push the needle onto the shaft at zero. The spring should have a bit of tension to hold the needle at zero.

There are several guides on the internet that describe overhauling Smiths speedometers, just go slowly and methodically and take lots of photos of how everything fits together. Then disassemble and clean everything.

You can test the speedometer by using a drill in reverse with a #2 Robertson (square) screwdriver bit, that is the same size as the drive cable.

 

1020 TPM is for a 3.54 axle, generally an automatic or non-overdrive. They were also used on the series 1 XJ6, but (I think) without the ignition lamp. I'd guess the internals from an XJ series 1 speedo could be transplanted.