the ABS error log will give you a solution ... only the PSD offers a DTC code, and only when the ABS/PSD has actually detected a malfunction.
DTC WSM Test Error
21 D 39 236 6 ABS speed sensor front left
22 D 39 237 7 ABS speed sensor front right
23 D 39 238 8 ABS speed sensor rear right
24 D 39 238 9 ABS speed sensor rear left
When the 928 has an operational RDK , you can use the RDK ecu to monitor the ABS sensor signal in real time. This will only work in low speeds up to 30 km/h. More to this on my website.
here's some more detailed connection info for the non-PSD ABS ecu:
Rear Right= pin 24 to 26
Real Left= pin 8 to 9
Front Right= pin 11 to 21
Front Right = pin 4 to 6
Resistance at a new sensor =775 Ohm. It should provide approximately 900mV, a sensor that only offers 400mV when turning the wheel might be broken.
A different reading can also be caused by bad contacts in the connectors, not a
bad sensor. Check the connectors at the wheel hub.
Both signals should have a very high impedance to chassis ground. If resistance to chassis is low, the sensor could have cracked due to internal corrosion. That will trigger buying a new sensor.
At MY 1986 the number of teeth on the sensor has changed from 90 to 45, so that there is ness chance on dirt getting caught up in the teeth.
Careful removing the screws that hold the sensor in place. They are often frozen due to steel-aluminum corrosion.