Troubleshooting the Speedo

I only have 3 weeks until the engineering inspection is done on the car so I need to wrap up some loose ends. One of those is the lack of speed on the speedo. I tried to calibrate it both manually and automatically on my last 2 runs but it refuses to detect a pulse. 

The speedo uses what I thought was a Hall Effect sensor, but now know is an Inductive sensor on the diff housing. The inductive pole (magnets) portion is mounted in a bracket close to the end of the CV joint bolts, and it is supposed to measure each time a bolt passes it as a pulse. You then work out how many pulses you need per kilometre and you are set.

I assumed the problem was the air gap between the sensor and the bolts, as I had not paid a lot of attention to setting the gap in my haste to finish the car (will do it later!). The gap is supposed to be between 0.3mm and 0.8mm (hard to measure I know). The problem now is to get to the sensor to reset it I needed to do a bit of dismantling once the car was up on stands. So off came:

• Right rear wheel
• Scuttle mount plates
• Bottom scuttle screws
• Interior side panels
• Seat headrests
• Roll cage 
• Boot liner

Everything is interconnected hence the amount of unrelated stuff that needed to come out. I exposed the sensor, repositioned it down to a very small gap, fired up the speedo and spun the wheels to check the pulse. Nothing!

So now I pulled the sensor out and checked the resistance of the inductive component when pulsed and it is working, so now I needed to test the wiring. I reached under the scuttle and disconnected the speedo connector. With the multi meter I belled out each of the wires from the sensor to the speedo. The blue/yellow wire is showing not connected?!?!. Bugger! Remembering the rear harness connects to the body loom in the fuse box, I figured I should check the connections in there as well. So more bits came out:

• Bonnet
• Pod air filter
• Fuse box lid
• Fuse rail mount

This exposed the rear harness connector and look what I found ------>. 

The wire join for the speedo sensor had come apart and out of the connector!

Easy enough fix. Rebind the wires, add a new terminal and reinsert it into the connector. Before I rejoined each step I belled out the connection to the sensor and the speedo to make sure it was OK, and then did the same end to end connection test. Once I had the end to end connections working, I put the sensor back on and tested it again with the speedo fired up. This time I got a pulse count on the speedo! 

Then it was just a matter of positioning the sensor, final testing and then putting everything back together!