Motors in rear A unit aren't running

I don't own these and have never worked on them.

Whenever I have an issue like this, I apply small amount of DC power straight to the (un-hooked) motor after I check for binding. ( I've never worked with AC motors).

Sometimes it's a broken wire or bad solder joint.

Someone with experience on these may help you better.

First try what Joe suggested and apply dc directly to the motors. The 4 motors are all wired in parallel powered by a single DCDR motor driver board. Lights and couplers are powered by separate wires. 

If the motors work OK on straight DC then you likely just have a broken wire or bent pin in a tether. 

Pete

Sounds like the broken wire or connection in the tether that Pete mentioned, that's certainly the most likely.

Thank you, gentlemen.

Following the direction that you all pointed in, I started with the easiest thing, which was to check the continuity of the tether going through the B unit. I found that 6 of the 8 wires had continuity; those would be for the smoke, lights, and coupler. Two of the wires did not; those would be for the motors in the trailing A unit. I tried just pressing the wires tighter into the connector. That did the trick. Possibly the wires had been pulled sometime when the B unit and the leading A unit came uncoupled.

Again, thank you!