speed sensor

[mike.bike]

how do i test wheel speed sensor on my 915 trans.

[dap930]

Remove the access cover on the transmission tunnel for access to the shift coupler. Find and disconnect the wire terminal coupler for the wires going to the speed sensor mounted to the transmission. Note the the speed sensor is a reed switch that has been embedded in the sensor housing (hockey puck). Connect a VOM meter (for continuity) to the two leads going to the speed sensor. Jack up the back of the car to allow spinning of the wheels. Rotate one or both tires and observe the VOM. There should be 8 pulses per wheel revolution. There is a disk inside the transmission at the differential that has 8 magnets epoxied to the plate. As the wheel (disk) turns, the magnets pass by the sensor, magnetic flux from the magnets act on the reed switch to close its contacts which should show as a closed circuit on the VOM. Less that 8 pulses per revolution indicates that one or more magnets on the disk have come loose.

[mike.bike]

Have done the test you wrote, it has 000.5 on odm and stays on that even when turning wheel. Is it the sensor that is finished?

[Jonny042]

I thought the sensor was a hall effect sensor? In which case measuring voltage pulses would be a better test. A DVM is not always the best tool for that. I have an old school analog voltmeter that I keep for such things, sometimes it just can't be beat.

[dap930]

I tested my sensor as stated and I got 8 pulses showing continuity when each magnet was present while rotating the wheel. The sensor switch provides a ground to the speedometer internal circuit, likely a 12VDC pullup circuit to provide a square wave signal to drive the speedometer detection circuit. If you look at a wiring diagram for the speedometer there are two wires (both BRN/Red) connected to the speedometer from the sensor. One is an input and the other goes to ground. So the sensor provides a intermittent ground to the speedometer input.
When I setup my MS3 EFI, I piggy backed the speed sensor pulsed ground signal as an input for VSS (vehicle speed sensor) to the MS3 ECU. The ECU input requires that the pulse signal go to ground in order for the input to calculate vehicle speed. I use this function when data logging to see speed verses other functions.

Mike, you sensor appears to be shorted. Check to make sure the wires going to the sensor have not been shorted somewhere, but most likely it will need to be replaced.

['78 SC]

dap930 has it right, the sensor is a reed switch triggered by magnets in the transmission. A simple continuity test shows if the magnets are closing the reed switch.



The marks show where there was continuity when rotating the tire.

[ClickClickBoom]

The sensor is a reed switch. Very simple. Mine died at 120,000mi. Replaced with OEM, works like new. Not cheap...

[Jonny042]

You learn something new every day! Thanks, guys.