0-12V square wave to -6 +6 sine
When it ”dies” means the speedometer drops dead to 0.
The instrument reads a sine in stock form from a VR-transmitter in the stock gearbox. Depending on the speed the amplitude varies between tenths of volts and around 5V at top speed. 9920 pulses/cycles per kilometer. 
The instrument cluster also has a circuit that converts this sine into a 0-5V square wave and passes this on to the now non existing stock ECU for speed calculations. The big unknown here is whether the instrument itself works frontside of the sine input or backside of the 0-5V square wave. Perhaps it is the built in circuit that needs a more sine like wave to function correctly.