OK - Working on the basis that the geniuses of the site have been bolstered by an overdose of chocolate (it would be bad parenting to let the kids eat that much, etc., etc.) and therefore can solve this in a tic.

Telemetrics speedo using a magnetic reed switch as the pickup. 5 magnets on the input flange to the rear diff, gap between the sensor and the magnets c. 0.5mm and all lined up dead straight. Have tested the switch and it is definitely working as the magnet passes underneath (but only when the magnet is absolutely bang under the sensor).

Speedo picks up to about 15mph and then as the propshaft gets faster it just drops to zero. Wiring is an ignition feed to one side of the sensor and then returns to the white/black wire on the speedo - Everything else on the speedo works just fine (as far as I can tell, odometer, lights etc.)

I think it's a knackered sensor but thought I would seek second opinion(s) first before ordering new bits / ripping things apart

p.s. Magnets all the same way and diff isn't wobbling!!!

[Edited on 8/4/2007 by NeilP]

1) How far away from the prop is the sensor? Normally needs to be quite close

2) How flexible is your mounting bracket? Needs to be really stiff or the vibration will cause issues...

I have eaten so much choc I feel sick

Sensor is right on the prop and magnets (prob. less than 0.5mm gap between them). Vibration could be an issue - I'll need to have the cover off and get someone to drive whilst I tinker...

Any other thoughts, you lot (other than not wanting to think about choc!!!)?

If it is a reed switch it would not cope with a high frequency, as it is a mechanical design. Is it really a reed switch?

Darren

[Edited on 8/4/07 by turbodisplay]

try junking the reed switch and using an old ABS sensor, nothing to wear out in those.

Regards Mark

Reed switches are good for up to about 150hz for the really fast ones.

I think you might be reaching limit of how fast it can switch...

Really u need a hall effect sensor but you need one that gives the right ouput, some give sine wave outputs and others square wave.

rswww.com do a digital output sender for about £28, would b cheaper elsewhere but theyve got all the data sheets too

Geoff

I would guess that the reed switches are running out of puff. How about removing alternate magnets and reprogramming the speedo to suit?