For the majority who don't own an expensive Fluke meter with it's Hz button. Set dial to read 12 volts DC. Revolve wheel slowly, and it should read zero or 12V, alternating as the wheel revolves. If that works OK, set meter dial to read up to 12 volts AC, and spin the wheel at a low speed, with a drill. It should read around 7V. If that reading is not steady, and your speedo needle is jumping around, the you may have found the fault. How I do it. Raise the drive wheels off the ground. Attach a wire to the terminal at the rear of the speedo head, connect that wire to one meter lead, and connect the other to a good ground point ( battery negative ). Start engine and engage 5th gear. You should then read a constant 7 ( ish ) volts AC. Yes, you will need to remove the speedo then reconnect it to do it my way. If that shows no fault, then remove the speedo head, and change the electrolytic capacitors on its circuit board. They dry out over time and are the weakest part of the circuit. They have a +ve and -ve leg, so note which way around they are connected before removing them. Avoid removing that sensor unless you really need to. Dropping stuff down that hole into the gearbox might be the source of your next job. Have fun.
For the majority who don't own an expensive Fluke meter with it's Hz button. Set dial to read 12 volts DC. Revolve wheel slowly, and it should read zero or 12V, alternating as the wheel revolves. If that works OK, set meter dial to read up to 12 volts AC, and spin the wheel at a low speed, with a drill. It should read around 7V. If that reading is not steady, and your speedo needle is jumping around, the you may have found the fault.
How I do it. Raise the drive wheels off the ground. Attach a wire to the terminal at the rear of the speedo head, connect that wire to one meter lead, and connect the other to a good ground point ( battery negative ). Start engine and engage 5th gear. You should then read a constant 7 ( ish ) volts AC. Yes, you will need to remove the speedo then reconnect it to do it my way. If that shows no fault, then remove the speedo head, and change the electrolytic capacitors on its circuit board. They dry out over time and are the weakest part of the circuit. They have a +ve and -ve leg, so note which way around they are connected before removing them. Avoid removing that sensor unless you really need to. Dropping stuff down that hole into the gearbox might be the source of your next job. Have fun.