Electric speedometer

[ccooper174]

I am coming to the end of a 30 year old restoration of a 1962 Mk 2. I have changed the DG automatic gearbox to a five speed Gatrag gearbox and I am very pleased with the result, but there is one problem that I have not been able to solve. The Getrag gearbox did not have a mechanical drive for the speedo cable. My son Paul purchased a digital electrical speedometer and adapted / grafted the new unit onto the back of the original speedometer head.
We wired in the unit and have purchased and wired in a sensor which is activated by a four post ring fitted to the output shaft on the gearbox. We then programmed the unit as instructions all seemed fine and when the ignition is turned on the needle on the adapted speedometer head flicks over and back to zero as it should. The problem is at the moment that the sensor does not make the needle advance when the vehicle is in motion. Please could I have some direction as to the next steps to take to rectify this problem. At the moment we are not having much help from the supplier of the digital speedometer.
Many thanks in anticipation of your help.

[Doherto]

Ccooper, i have no experiance of the unit you have but have had a similar problem with an ecectronic speedo on a kit car i built a while ago. The sensor i positioned under the car to sence the rotation of the prop shaft and it ok when tested. However when the car was put back on the ground the position of the sensor must have changed due to suspension. In the end it was trial and error to get the sensor very close to the rotating, nuts on the prop shat in my case, most frustrating but it was all about proximity in the end. This may or may not help but !!!

[BrianG]

The problem with this kind of installation is that there is no easy way to test the functionality of the whole system unless you can run the car on a hoist with the rear wheels free to rotate and get at things to make adjustments. To identify where the fault lays there are two main areas which can be the cause; there really is no short cut way of fault finding other than by testing each area in turn and perhaps making some measurements.

Firstly, although electronic unit makes the electric speedo move at power on, this does not in itself prove that it is fully functional. Assuming that the electrical speedo head came with the electronic unit, or is otherwise as-specified by the manufacturer, the only way to rule out a fault here is to inject a test signal instead of the sensor input signal and see if the speedo moves correctly. The manufacturer should be able to help here with a specification, although you may have great difficulty in providing the required signal at home without the necessary equipment.

The second possibility is that the gearbox sensor is defective and is not producing a signal, or the signal is present but not large enough to trigger the electronic unit. The second possibility is usually caused by incorrect spacing of the fixed pickup coil from the moving poles (which could be magnets, bolt heads, gear teeth, etc. as required by the particular setup). The size of the signal could be measured if you have access to suitable equipment (a decent digital multimeter that can read small a.c. voltages and frequency), and this is perhaps easier to perform than providing the test signal to the electronic unit. Again, the manufacturer should be able to tell you what this signal should be for a given gearbox shaft speed.
Good luck!
P.S. Feel free to send me a PM if you need further help.

[ccooper174]

Hi Doherto,
I have no problem with the siting of the sensor as it is set against a four post ring fixed to the out put shaft
on the gearbox and the sensor bracket is mounted on the gearbox. So there is no independent movement to
allow for. Thank you for your input.
Colin.
Hi Brian,
I think that the problem may be with the sensor and cable, in that I am not sure that the sensor that we have fitted has a magnet incorporated within the unit. Without either a magnet in the unit or on the four post revolving ring fitted to the gearbox output shaft I doubt whether it will work. I have ordered another sensor with a magnet incorporated within, so with this fitted I am hoping that the problem will be solved.
thank you for your help.
Colin