Speed Sensor 1977 911 Targa

[targaman55]

My speedo needle was moving erratically and not accurate . I sent the speedo out to have it reconditioned with new circuit board and gear. Reinstalled it, same problem present, not as bad tho, but nonetheless not accurate. Could it be that VSS needs to be replaced? How hard of a job is it to do, and videos/diagrams would be much appreciated. The vehicle has 177k miles on it.
Thanks to everyone for your help.

Targaman55

[rwest]

Since it actually improved slightly with a new board and re-install there could be a bad connection or ground to either the sensor or the speedo.

There is a plastic ring with rod shaped magnets inside the transmission and it is possible that a few magnets have fallen off. There is also a sensor reading the magnets on the side of the transmission near the driveshaft if memory serves on the passenger side.

You could probably test the wave form coming out of the sensor with an oscilloscope to determine if your ring and sensor are operating properly- hopefully someone a bit smarter than me can confirm and maybe elaborate.

Good luck, it’s always best to try and find the problem rather than throwing money at it as parts aren’t cheap.

Also, welcome to the forum!

Rutager

[bdonally]

I just rebuilt the speed sensor magnet disc, and can offer the following:
-the external sensor is easy to change but is likely not the problem. It isn't anything exotic, just a reed switch.
-its easy to test, just disconnect the wires and connect a multimeter set to ohms and turn the right wheel, keeping the left wheel from turning. You should see the multimeter show zero resistance four times per revolution. Make two complete turns to see if all eight magnets are triggering the switch.

-the magnet disc is mounts to the differential housing, so with one wheel turning and one locked the disc turns at half speed. Therefore two turns are needed or you need to turn both wheels at once.
-in my case, half of the magnets had lost magnetism. I saw flaky reading on the switch, and gdetermined the magnet problem after I took out the differential.
- I replaced the original magnets with new ones and all is good.
-the polarity if the magnets is important. They need to be installed with the poles alternating. If the poles don't alternate the resulting magnetic field is strong enough that the switch is held on all the time.

First thing to do is the test with the multimeter and turning a wheel.
You can also remove the switch and use a magnet to determine if it is opening and closing.

Good luck