I have a 1986 300e with the antilock brake light that comes on at about 25 to 30 mph. It then stays on until the car is turned off. The OVP relay and fuse looks ok and the voltage while the car is running is at 13.3 volts. I know the ABS is not functioning since I can now skid and lockup the wheels for the 1st time in 20 years. After looking at many other posts my best guess is that it is either one of the two front wheel sensors or the rear differential sensor. Without having a computer to plug in to the car and tell me which one is bad, how can I diagnosis the problem? One post said to use a ohm meter but gave no details on how or where to plug in the leads. Is there a better way to diagnosis the problem? Please be specific. Any pictures would also be appreciated. (And yes this is my 1st post)

Thank you