Very occasionally mine reads (IIRC) 000.0.

It generally goes away when I switch off the power and on again. Once I had to wipe the pulley at the back end of the spindle where the thing wot triggers the sensor is, and maybe I wiped the sensor thing too.

Most likely messing about testing the motor and fitting a new switch has dislodged a wire. I have a WM280 which may be similar inside.

First, as suggested by others, check and clean the sensor (silver object with red led and black wire) mounted on the change-gear end of the spindle:

Your sensor may look different, but the wire is a giveaway. Note where the wire enters the headstock in case cleaning grease / swarf, doesn't fix the problem. The sensor is actuated by a small block magnet stuck to the splndle, at least one, maybe 2. It's unlikely but make sure the magnet hasn't fallen off.

Behind the front panel you should find the tachometer power supply, on mine it's part of the mains filter, green board mounted on the floor of the headstock.

As the display is working, the problem is unlikely to be due to a power supply problem. Note however the large black cable bundled together and plugged into the rear of the tachometer/display mounted on the aluminium front panel (bottom right of photo): this is the sensor input. Make sure it's plugged in properly.

Another view of the tachometer board, on mine the red/black pair are low voltage DC power, and the green/white pair are the sensor input.

The wiring of my lathe isn't arranged as a tight loom fixed firmly inside the headstock, rather the various parts are wired point to point and tidied up by gathering with a few nylon cable ties. This simple arrangement makes the lathe vulnerable to dislodged connections due to pulling on wires elsewhere on the machine, for example when your switch was investigated. In the worst case, a board could be damaged by pulling a plug hard in the wrong direction, but considerable force would be needed.   It's all simple enough to replace if need be.

Hope that helps,

Dave

Edited By SillyOldDuffer on 17/05/2019 12:54:38

I must admit, the magnets and pick-up do seem a bit vulnerable to contamination. It doesn't look to be the smartest design. Has anybody thought about some sort of cover, maybe using the pick-up bracket, or cable clip screws to secure it. Clearly not possible to get full coverage, and make it easy to remove, but as a famous supermarket says "every little helps".

Well in answer to my question. My sensor was duff. It was locked on all the time and now I have a new sensor I can see that the light comes on when inline with a magnet. When the magnet moves past the sensor the light goes off. This can be seen by moving the spindle by hand. And the proof is that my rpm is back.

I still do not know what type of pickup it is but have since learned that hall effect sensors need power for the internal electronics as well as the LED. There is a good chance that it is in fact hall effect but now that it is working again it is of less interest.

The sensor and display are the same items as fitted to the WM290 lathe. Hopefully this information may be of use to someone else looking at the same problem.