From the S&S manual - The vehicle speed sensor is routed first to the speedometer and then the speedometer creates a signal that
is sent to the ECU.
So if the speedo is working OK it could be the problem is in the wire(s) from the speedo to the ECU. I would suggest unplugging the speedo and ECU and then doing a continuity test on the connecting wires.
Tracing the wiring on chassis and engine looms I don't think the quoted routing from the S & S manual has been implemented by MMC Chris.
Just one of the three speed sensor wires, the yellow/pink is split in the chassis loom to feed TSM and instruments as well as being sent to the ECU. The other two yellow/black and yellow/red, route to engine loom and nowhere else in chassis loom.
As suggested I believe these latter two are common reference signal and maybe return to benefit just the ECU. This is born out by the fact that each split via bomb tails within the engine loom to provide common connections as follows:
YR14 ECU
YR15 speed sensor
YR16 MAP sensor
YR17 throttle position sensor
YB27 ECU
YB28 speed sensor
YB29 l/h lambda sensor
YB30 r/h lambda sensor
YB31 engine temp sensor
YB32 MAP sensor
YB33 throttle position sensor
YB34 air intake temp sensor
Given above I conclude that the speed sensor main signal is fed raw directly to TSM and speedo and probably referenced to ground since such is readily available. The speed signal does not appear to be routed through speedometer therefore since it's sent directly to ECU as well via the YK bomb tail.
The other two bomb tails (YR and YB) on engine loom may be worthy of investigation however. Also all sensor connections.
Failing that maybe another ECU could be tried. Indeed given the consistent elements of fault I'm beginning to doubt an intermittent connection.
Hopefully this explanation may help you in fault tracing of your own Laurens.
