The speed transducer box is under the hood(s) and attached to the bulkhead, you should be able to locate it as the speedo cable is in two sections one feeding into the box and the other feeding out of the box. Maybe a first step is to check that the signal cable is still attached. I will need to refresh my grey cell with regard to the expected output voltage. Essentially there should be an electrical pulse coming out of which the frequency will be related to road speed.

Reference is made here https://www.gomog.com/allmorgan/StepperMotor.htm#TRANSDUCER - see 3. THE ROAD SPEED SENSOR (aka transducer)

One way to check is to put an oscilloscope on the output of the speed transducer, or a DVM may suffice at a low speed

0-2.0V and 10.5-13.0V. Signals when road
speed >3mph.The signal should go up and
down between 0-2.0 Volts and 10.5-13.0
Volts, changing every 4-5 metres. The
signal is 8000 pulses per mile.

See https://www.gomog.com/allmorgan/HOTWIRE/RV8diagnostics2.pdf

Another is to put a code reader onto the ecu, Rovergauge will display the road speed signal - Rovergauge is free software, you just purchase a cable from your PC to plug into the connector in the EFI loom, located somewhere close to the rear of the underside of the glove box.
http://www.remap-14cux.uk/gadgets.html
https://github.com/colinbourassa/rovergauge/releases/tag/0.10.7

---------------------------
Mike Smith

2007 +4