The overload shorted out the time circuits, and destroyed the flying circuits.

4 seconds sound clip from the Back to the Future Part III movie soundboard.

You can hear this line at 00:07:38.758 in the DVD version of the movie.

Quote context

[...]

- You sent me a letter.

- Dear Marty, if my calculations are correct, you will receive this letter immediately after you saw the DeLorean struck by lightning.

- First, let me assure you that I'm alive and well.

- I've been living happily these past eight months in the year 1885.

- The lightning bolt that hit the DeLorean caused a gigawatt overload which scrambled the time circuits, activated the Flux Capacitor, and sent me back to 1885.

- The overload shorted out the time circuits, and destroyed the flying circuits.

- Unfortunately, the car will never fly again. It actually flew?

- Yeah, we had a hover conversion done in the early 21st century.

- Incredible!

- I set myself up as a blacksmith as a front while I attempted to repair the damage to the time circuits. Unfortunately, this proved impossible, because suitable replacement parts will not be invented until 1947.

- However, I've gotten quite adept at shoeing horses and fixing wagons.

[...]