ABS warning light for truck.

99 freightliner. FLD 120. Wabco system. Truck abs warning light stays on in dash.
I found the abs computer behind the passenger seat under the bunk. I have pushed the test button and the dash light blinks twice repeatedly.
I had a mechanic look at it. They determined the ECU was receiving only 2 of the 3 required ignition power sources.
He said the pin was in the terminal connection closest to the center of cab. He said the center row of pins the 3rd and 4th pins, one should be a ground and the other a power.
They didn't have any more time to look at my truck.
Thanks for any help.

 

I'm not sure which one to power. I don't want to fry the computer.

 

The tech said the laptop determined it was an ignition power not powering up when ignition switch is turned on.
Don't the tires and drums need to be removed to replace sensors.

 

Other than the Washington DOT wrote me a warning on an inspection. I kinda have to fix it.
I wasn't worried about it for 6 years. Even had an inspection and truck was cvsa stickered.
I wish all inspections were the same.
No problems with my brakes working.
They don't lock up on hard braking. Well the hand full of times I did.

 

To close this thread. The problem was a (previous) lazy tech decided to test power in wires by piercing the insulation with his test probe.
This happened before I owned the truck.
Several wires corroded internally. They had to be replaced.
This tech said 15 breaks in about a foot of harness.