speedo connection to sender

[craigh]

a mate is using a sensor on his car that senses the driveshaft bolts on the rear of the car,it has no instructions with it and only has the markings vdo on the side
it also has 2 terminals
how can you tell which it is? magnetic,inductive etc....
its being coupled up to a smiths speedo,can anyone recommend which cables to connect to,not sure if it needs a supply cable and a return,even the instrument instructions are not that clear

[SimonP]

Only 2 terminals.... does it have an earth tag on its mounting, or is it metal cased?

Try these tests:
Check resistance between the two terminals, also between each terminal and case if metal or has earth tag. Also cross the leads over and test again.

If open circuit between terminals, the unit may be a reed sensor. Try bringing a magnet near the sensor. If you then get continuity it will prove that it is a reed sensor and you need **magnets** on the propshaft.

If you get a resistance which is the same with the leads both ways round, the unit is probably an inductive sensor.
Try bringing a piece of steel past the nose of the sensor. If the resistance changes, try the same test with the meter on mV dc.
This time the reading should be positive as you bring the steel near, then negative as you draw it away (or vice versa).
Also check on mV ac - the voltage generated will be proportional to the speed you bring the object past the sensor.

If it has an earth connection it will likely be a Hall sensor, which will require a supply. Some use a raw ignition-switched feed, others need around 8 Volts which is supplied by a regulator internal to the Smiths speedo.

Does this help?

Simon.

[craigh]

cheers simon,will check it out, it is metal cased no earth tag on it,would have thought if it was magnetic they would have supplied him with some small magnets to bond onto the driveshaft bolts?

[SimonP]

If the "piece of steel" test does not work, try again with a magnet - results will be similar to those mentioned.

So, if it's a reed sensor or hall sensor you use white/black for input.

If it's a magnetic proxiimity (inductive) sensor, use the red/blue for input.

Remember you will get more reliable readings using a screened cable - but if you have to use what's in the loom, use the second wire as the return for the sensor, earth this back at the speedo.

Refer to http://www.caigauge.com/speedofit.htm for instructions

If it's a Hall sensor though, connecting it up incorrectly could damage it, you'd need to find more info on it.

Simon.

[Grease Monkey]

On a completely different, but related, note, i'm hijacking this post.

I've got an XJ6 diff, with the funky 'cooling finned' rear plate, and on this plate is a speedo sensor. (Knackered, as the monkey (no relation) in the breakers must have pulled it off with a JCB)

The point is, has anyone used this Jag sensor, which i presume is a pulsed/magnetic type device, to drive a speedo ? (I'm guessing a modern speedo with jumpers/dipswitches so it can be re-calibrated to the sensor)

[SimonP]

Test as above to find out what it is - my guess is reed - and the CAI Smiths unit as used in the standard GD kit will work no problem with it.

However you'll need to work out your own calibration number which isn't too difficult. Just have the dip-switch chart with you when you go for SVA.

Simon.