It is in Chapter 11 that we find out that the Ultra-Glide has a problem. Kenny wakes up to hear the same phrase repeated again and again on the Ultra-Glide:
I knew what woke me up this time. The Ultra-Glide was stuck. As we drove, the record was saying, "... and don't forget who's tak ... and don't forget who's tak ... and don't forget who's tak ... and don't forget who's tak ..."
However, what is clear is that the Ultra-Glide getting stuck has a hypnotic effect on the rest of the Watsons, and Kenny quickly falls back to sleep before he can tell his Dad. When he does wake up and manages to tell his father, his Dad replies that "Something isn't working right" with the Ultra-Glide, but in a sense, he is quite pleased because the way it repeated the one phrase over and over meant that he was able to drive all the way to Alabama and save money without having to stop frequently, because the rest of the family slept to the hypnotising repetition of the stuck record.
Thus we are not told what precisely was wrong with the Ultra-Glide, but we are told that it actually helped the Watsons get to Birmingham quicker than they had anticipated.