Cujo.

The Cambers' once-friendly St. Bernard turns into a killer after being bitten by a rabid bat. Donna Trenton's husband is in New York trying to contain a disastrous ad campaign. Feeling abandoned by her workaholic husband, who is frequently out of town, Donna Trenton embarks on an affair with a local handyman. Left to fend for herself, she takes her ailing Pinto to Joe Cambers' garage for repairs only to be trapped with her son Tad in the sweltering car by the monstrous dog. (source)