speedo drive
 

Anyone know how the drive unit works. From the wiring diag it has an inductive coil, coupled to a switch mechanism. My speedo has stopped. I suspect the problem is here, I get continuity across both the sensor units wires, implying the switch is connected. The diag has it open, but I dont know if that is static or running depiction. I checked on another car and static there is no continuity.
Anyone know how the switch is supposed to work?
I'm talking about the unit fixed to the side of the g/box.
Thanks
Alan

Just under the sensor inside the case is a disc with 8 magnets in it. Running at the same rate as the hubs each revolution is counted 8 times with the sensor and the field causes an induction in the wire to the speedo.
I have had magnets fall out giving false speeds because each magnet is 12.5% so speed on the speedo is reduced.
Bruce

I am thinking that each magnet givesan electrical pulse via toroidal coil in the pick up unit. This pulse opens and shuts a switch/gate? So the gate is pulsing at a certain rate = speed. If this is so, I think my pick up unit may be faulty and the gate stuck at contact point, rather than oscillating.
If I have deduced the mechanism properly.
Just trying to work out which bit is faulty. But on that basis, each magnet missing would affect the speed by reducing the number of pulses. Half the magnets = half the pulses to speedo.
Alan

I would go for failure of the pickup unit, least expensive and more probable.Check the access to the shift mechanism as the wires run right there and couple there
Bruce

The wire coupling in the tunnel is OK - that is where I measured the continuity of the pick up wires from. I checked one of my SC units at same place - in that I get open circuit static, ie infinite resistance. On the faulty speedo, I get zero resisitance across the pick up terminals. Somehow I think the switch/gate is stuck in a position it should not be.
Thanks
Alan

Well, swapped a speedo sensor unit over. Still not working.
Swapped a speedo gauge over. Still not working.
Swap gearbox over??
Hardly seems feasible that all the magnets fell out?
Alan