one side of robot not running - and need general electronics troubleshooting advice

[alb4h]

Hi,
We currently have a simple drive train running, a single motor connected to a black jag on each side, and we ran into a problem that we're having difficulty diagnosing. The robot ran fine when up on blocks. We put it on the floor and only one side ran. We swapped the PWM cables on the digital sidecar and the other side ran but the side that originally ran stopped running. It's as if pin 1 is outputting a signal, but not pin 2. We swapped out the DS and for a short time (first few seconds) both sides ran fine. Then the same thing happened. I just realized we haven't changed out the ribbon cable, which I will do first thing when we get to the shop. All the lights on both DS's were strong, and the robot signal light was as expected.
Other ideas?

I am a new electronics mentor, but I don't have an electronics background. I do have a virtual mentor who has been EXTREMELY helpful but she is not always available right when we have a problem, so I'm putting this out to CD. I need to understand the general process to go through when diagnosing electronics issues on the robot. I'm an engineer by training, but have not practiced engineering for 20 years. I can follow technical advice, but I've never used an oscilloscope. Thinking I probably need to figure that out.

Can someone give me a general run down on how they approach electronics troubleshooting?

Also, how do I check whether a ribbon cable is good, beyond swapping it out and seeing that a different one behaves as expected?


Thanks very much for your time,
Ann