I am having issues with my stack st3801 speedo. My current set up is a 3 wire Hall effect sensor mounted on a bracket on the diff, this picks up a ferrous bolt head glued to the outside diameter of the inner cv joint. Basically to set the stack speedo up you have to put it into Calibration mode and drive 2 miles for it to count the number of pulses. I cannot drive the car as I am pre iva so I have jacked the rear up and set on axle stands and calculated the amount of pulses in 2 miles and run the speedo in calibration mode ( which shows the number of counted pulses)

My issue is that the speedo counts pulses when the wheels are spinning at a constant speed however if I dip the clutch to change gear occasionally the counted pulses will spike, or I have had it when I've stopped the wheels but the pulses keep counting, the sensor gap is correct and remaining constant, I have changed the amount of bolt head pick ups with no change, pick up point is per the instruction manual, when not in calibration mode the speedo tends to just wander all over the place. The speedo and sensor have been sent back and checked by stack with no fault found. There is also an LED on the sensor which illuminates when the target passes it, from having my head under and turning the wheel the pick up only flashes when it's supposed to so there is nothing else triggering it.

It's something to do with the way I have things set up or wired ( which I have triple checked) what else can I do?

An arcing connection or excessive voltage ripple can drive electronic instruments wild and even without an itermittent connection car is a very noisey electrical environment. I would start by checking the alternator wiring particularly the main connections on the alternator. If you don't find anything you can do a rough and ready check for excess ripple from the alternator by measuring the AC ripple voltage produced by the alternator using a standard multimeter set to AC volts measuring directly at the alternator between the B+ termminal (heavy gauge wire to battery+) and the alternator body. In general 0.5v AC alternator ripple is quoted as acceptable but up to around 0.36v AC is more typical of a good alternator.

Beyond that if you only have basic tools you could try fitting capacictors as suppressors, wrapping the feed wire to the speedo round a ferrite core and re-routing the wiring away from sources of interference such as the igntion wires.

See

http://locostbuilders.co.uk/forum/5/viewthread.php?tid=202405#pid1697140

[Edited on 13/9/16 by britishtrident]

Ripple test on the alternator at idle showed 0.08Vac. However this started around 0.10 and dropped down as the engine revs dropped as it was a cold start