Speedo stopped working

[Peter-j8c3u]

Speedo stopped working, unexpectedly started working again and stopped again all on the same day. I cleaned the speed sensor and checked the gap but it still wasn’t working, there was no connector by the diff it was a solid soldered connection. Bought a replacement sensor and fitted it, didn’t work so I swapped the wires polarity but it still didn’t work. The sensor is adjusted by screwing it in until it touched the ring and backing it out slightly.
Ive got the car on jack stands and I’ve been running her in gear to see if I’d fixed it, I noticed that if I tapped on the speedo glass the needle would occasionally move round to 20-30mph so I removed the binnacle & dash,checked the connectors but the pins were bright with no sign of a problem and I removed the speedo from the dash hoping to inspect the inside for a loose connection etc, unfortunately as per my other post I can’t get the back cover off the speedo and don’t want to force it.
Does anyone know how to get the cover off or can think of something else to check?

[ukkid35]

You could check that about 5V AC is reaching the speedo

I used a USB oscilloscope, but a multimeter should be fine

[Peter-j8c3u]

I’ll try that….someone in a recent post attached wiring diagrams….which is nice.
You don’t know how to get the back off the speedo do you?

Edited by Peter-j8c3u on Tuesday 7th March 09:28

[Peter-j8c3u]

I got nothing testing across the yellow and black wires that attach to the speedo so I went back to the plug, should I be testing green and black?.
I’m getting a reading that rises as the speed does to 3v ac, I was doing about 3k rpm in 3rd. I was expecting a solid 5v.
Is it better to have the sensor further away?, ive got it just backed off from touching at the moment.
I noticed the wiring seems to loop around the chassis a bit back there before going off in two directions, have you any idea where it goes?, I’ll have a look tomorrow and see if there’s a break in any of the wiring before it disappears into the body.
I’m tentatively pleased now but praying I haven’t got to pull the interior apart.

[Peter-j8c3u]

Panic over, speedo is working now. I’m not sure whether I wasn’t going fast enough on the jackstands or I had a dodgy connector but I got rid and soldered the joint. When I got her going a bit faster it sprung into life.