Prop-shaft Speed Sensor

[Joel]

Hi,

 

I'm using a reed switch with magnets on the propshaft for my speedometer. The speedo stopped working on the way to SVA - the propshaft had been making contact with the speed sensor, snapping it in half and knocking all the magnets off (25 quid for a new one - ar*e).

 

I just ran the car with the tunnel side off, and I'm suprised just how much the propshaft moves. Problem is the sensor only works up to a maximum of about 10mm from the magnets, and I can't see how I can position it so the same problem won't happen again.

 

Has anyone else used a similar system?

Where did you fit the speed sensor?

Would it work on the mating-plate where the prop bolts to the rear diff?

[Guest SteveL]

Hi Joel

 

Is the movement a sideways/up/down wiggle? If so, you could set up the sensor so that the body of the sensor is parallel to the propshaft, rather than pointing towards the propshaft, and the magnets aren't stuck onto the body of the propshaft, but on the face of the mating plate between the bolt heads.

 

This way, the sensor doesn't have to be close to the main body of the propshaft, so the propshaft movement will not affect it.

 

I also see no reason why the mating plate wouldn't be another place to mount the magnets, although the movement which caused the problem in the first place may cause a similar problem?

 

Just my 2p worth

 

Steve

[Guest Ant]

In the 2B the diff is rigidly bolted to the chassis, whereas the engine is on rubber mounts. Mounting it all at the diff end should mean there's less movement. Better still mount if on the diff input flange as it won't move at all.

 

Not sure if that's possible - I've not got one.

 

Failing that, then how about a driveshaft pickup, mounting the magnets on the can where the driveshaft leaves the diff, and mount the sensor on a bracket from the chassis? this is my plan for a bicycle speedo pickup (if I ever get round to fitting it)

 

Ant

[Joel]

Thanks for the suggestions chaps.

 

New problem - I rigged up the sensor to read from the prop/diff mating plate, as that doesn't move at all, but now the speedometer only registers up to about 35mph, then cuts out.

 

Could it be that the magnets are passing the sensor too quickly over 35mph, what with the mating plate outside diameter being much bigger than the propshaft?

 

I've double checked the wiring to the speedo, the calibration and the sensor - all are correct and set up properly.

 

The only other thing I haven't tried is to use 6 magnets instead of the 4 I'm currently using. Other than that it's got me stumped!?

[Peter Bell]

Hi Joel could you fit the sensor on the drive shaft instead and recalibrate the speedo? at least there you will have reduced the rotational speed by whatever your diff ratio is. I have a bike one with the magnet on the inner CV joint.

Peter.

[Guest paul mercer]

exactly what i've done. i've made a bracket that holds my sensor next to the hub on the diff half shaft. This is much slower than the prop.

[Guest MJTyke]

Alternatively, try europa, the do a speedo drive adaptor which runs from the existing speedo cable but produces four electrical pulses per rotation. Passed SVA 1st time.