Mattel Talking Engine repairs.

HI all.
I was wondering if if could get some help repairing a Talking Victor.
When I turn him on he talks great, clicks into driving mode, but the engine doesn't drive.
I have pulled him apart and tried his engine with a AA battery and it works.
My guess is that there must be a switch / circuit that is not sending the pulse to tell it to drive.
There are no broken wires.
This thread may already exsist, but I can't find it.
I hope someone can help.
Many Thanks in advance
AOF