Speedo not working until 40 mph

[KeithS]

My speedo has suddenly started playing up, it bounces between 0 and 30 mph when I'm moving at between 0 and about 40 mph, then it seems to work normally when I get above 40 mph. Just adjustment or knackered speedo/sensor?

[shpub]

Could be knackered speedo sensor or the speedo itself. Worth cleaning the sensor and checking for funny wires. Beyond that you need a scope to check the pulse train coming from the sensor. Details in the Bilbe 2.

Steve
www.tvrbooks.co.uk

[apache]

Steve, would this not affect the idle? mine has become a bit erratic by staying high sometimes, if I drag the revs down using roadspeed it does what it should, it also hunts

[KeithS]

Thanks Steve I'll have to get the new bible anyway so I'll check that out. It was working fine until the car went in to the dealer for a check on a slight misfire, after about 5 miles on the homeward journey the speedo started playing up. Is it possible for it to be caused by a loose connection in the loom somewhere? The car has to go back on Saturday for a new fan belt, I'll get them to check it while there I think.

[shpub]

apache said:

Steve, would this not affect the idle? mine has become a bit erratic by staying high sometimes, if I drag the revs down using roadspeed it does what it should, it also hunts

It all depends. If the sensor is going a bit intermittent it can be enough to satsify the ECU but not for the speedo. What you describe sounds like a stepper motor problem but it could be made worse by a duff speedo input. The only real way of testing that sensor is with a scope. It also means that you can see what signal is appearing at the ECU as well.