Forman, Adams and Hunter, Circuit Judges.
JAMES HUNTER III, Circuit Judge.
In the instant case the jury awarded appellee Delmar Dennis damages of $13,471.00 in his diversity products liability action against appellant Ford Motor Company. Dennis' recovery from Ford was based on the standard of strict liability imposed upon a manufacturer or seller by the Restatement (Second) of Torts § 402A, which was adopted as law in Pennsylvania in Webb v. Zern, 422 Pa. 424, 220 A.2d 853 (1966).*fn1
On this appeal from the judgment entered pursuant to the jury's verdict, Ford contends that the district court, 332 F. Supp. 901, erred in denying its motions for judgment N.O.V., for a new trial and for a remittitur.
The facts in this case are adduced from the district court opinion. On September 28, 1967, Dennis purchased a new Ford truck-tractor from an authorized and franchised Ford dealer in Uniontown, Pennsylvania. Upon delivery the next day, Dennis' brother mounted a fifth wheel upon the tractor for the purpose of attaching a trailer to it.*fn1a He then drove the tractor on a short test run, and it was also tested by Dennis' regular driver. No difficulty was experienced in the operation of the tractor during either of these test runs.
On September 30, 1967, the tractor, pulling a semi-trailer loaded with sawdust on its first regular run, veered off the highway to the right while going downhill, demolishing the entire unit. The driver testified that he felt the right front wheel give way, causing him to lose steering control. The jury's $13,471.00 verdict in favor of Dennis included, by special interrogatory, the sum of $3,865.00 for the fair rental cost of replacement equipment.
Section 402A(1) of the Restatement (Second) of Torts provides:
"(1) One who sells any product in a defective condition unreasonably dangerous to the user or consumer or to his property is subject to liability for physical harm thereby caused to the ultimate user or consumer, or to his property, if
"(a) the seller is engaged in the business of selling such a product, and
"(b) it is expected to and does reach the user or consumer without substantial change in the condition in which it is sold."
In denying Ford's motions for judgment N.O.V. or a new trial, the district court relied on MacDougall v. Ford Motor Co., 214 Pa.Super. 384, 391, 257 A.2d 676, 680 (1969), which held that "the occurrence of a malfunction of machinery in the absence of abnormal use and reasonable secondary causes is evidence of a 'defective condition' within the meaning of § 402A . . . ." See Greco v. Bucciconi Engineering Co., 407 F.2d 87 (3d Cir. 1969).
Ford's primary argument in this appeal is that if a user like Dennis makes a change in a product which could have an effect on its operation, he cannot proceed under § 402A and the rationale of MacDougall unless he shows that the change was not substantial and did not contribute to the happening of the accident. Specifically, Ford alleges that for Dennis to recover on the basis of § 402A under the circumstances of this case, he must affirmatively prove that the admitted addition of the fifth wheel was not a substantial change in the tractor and did not contribute to the accident. Judge Knox charged the jury on this point as follows:
"Second, you may find the Ford Motor Company liable to the plaintiff for damages if you conclude that the Ford tractor did not function properly even though the specific defect causing the malfunction has not been established. That point is affirmed with the modification: ...