In mid-1955 General Motors introduced their long awaited new trucks, often referred to as the Second Series. The first half of the year 1955 (the first series)) GM continued to market the 1954 body style. They remained with the 6 volt system.
Though Chevrolet trucks made a complete change-over from a 6 to 12 volt electrical system, GMC did it different. Only the newly introduced V-8 (actually borrowed from Pontiac) was given the 12 volt system. It was the expected thing for GMC to do as the adopted Pontiac V-8 was equipped with a 12 volt flywheel, starter and generator.
GMC’s almost bullet proof 270 six cylinder was another story. They continued through the end of 1955 with their proven 6 volt positive ground electrical system that they had provided for over 40 years! After all, the main two electrical accessories were a radio and heater, so a 6 volt system continued to be adequate.
Just another area that divided the two marquis, GMC and Chevrolet, into different trucks even though they shared their cabs, beds, transmissions, wheels, suspensions, and most differentials.