road speed sensor

[red griff 500]

Hi chaps, Griff interloper here! I was hoping that someone could tell me something about the road speed sensor on a 500 fitted with SD1 box.

Problem symptoms are..speedo will sometimes fall straight back to 0, and stay there for between 5 seconds and 1 hour whereupon it'll instantly fly back up to the correct reading. Also, the engine won't tickover correctly whilst this behaviour's going on (although the tickover problems do not occur immediately after the speedo problems). Also (and this may be a dead herring) the radio doesn't switch on during these antics (again it's not immediate).

Question - is the output of the sensor of the Schmitt variety ie it either definitely works or not, there's no in between? What is the setting procedure for the sensor?

Thanks for any help.

Steve

[shpub]

The sensor is in the gearbox up near the top. No setting up needed. It uses Heisenberg's Uncertaininty principle - a sensor observed is a sensor disturbed...
Putting a meter on it may or may not make it work/fail.

Really need a scope to see the waveform and the sensor has to be connected to the speedo. It should generate pulses. I would check for continuity on the wiring first and bad earths.

Big speedo section in't bible.

Steve
www.tvrbooks.co.uk

[spend]

On my rover box, the gear box sensor is only for the ECU. The Speedo sensor is off a toothed wheel on the nose of the GKN diff (actually above toothed wheel where the propshaft UJ connects to the diff). If you have this type it could be wiring under tunnel cover or to the sensor (mine was held in with sealant) or even the sensor itself (clean & gap I guess).

As usual YMMV, Dave

[red griff 500]

hmmm, Heisenbergs uncertainty principle huh! that must've been covered in the lecture I missed all those years ago - I knew I'd regret skiving off. One of these days I will invest in a scope!

Thanks for yer help fellas.