92 Chev Rear ABS wiring

[chuck's]

I'm working on a 92 C3500 with rear wheel ABS.I believe that the controller is not functioning.I would like to know if the controller supplies 12volts to the isolation and dump valve.I checked the white wire pin of the four prong connector and only got 1.56 volts.I believe that this wire should have 12 volts and the green and black are the ground side that the controller will use to actuate either the isolation or dump solenoids.I don't have any fault codes to indicate any electrical problems.The main reason for checking the rear brakes is that the front brakes lock up very easily on slippery surfaces and appears that the rear are not doing much.I did test the brakes on dry pavement and the front did lock up,but I did not get any sense of an ABS application of the rear.The rear wheel cylinders,drums and shoes are going to be changed.The vehicle is a mechanic's work truck and I am wondering if the extra weight on the rear would be a factor.If anyone knows of wiring schematics for the controller or has ideas about the rear brakes,your input would be greatly appreciated.
chuck's