This is the original wiring diagram printed by GM for dealer mechanics. You can follow the wiring in your truck from bumper-to-bumper. It will help you to understand connector configurations, and locate & identify circuits, relays, and grounds. Although the schematic is in black and white, the color of each wire is noted. You will not find this diagram in your shop manual. The schematic measures 8.5" x 11" when folded up, and opens up to 11" by 38". The diagram pictured is an example of this type when folded, but is not the actual diagram for your vehicle. Buy now for the best electrical repair info - the factory wiring diagram.
1978 Truck Wiring Diagrams MD16591
This item is original.
11.00 x 8.50 x 0.10 inches
Covers medium duty Chevrolet and GMC conventional truck models M6, M60, M6000, C5, C50, C5000, C6, C60, & C6000 with gas engines. For diesel engine see my other items. To verify that this is the correct diagram for your truck, you can check your VIN code. The 2nd digit should be a D for the medium duty conventional cab, the 3rd digit should be an E or an S to indicate the gasoline engine, and the 6th digit should be an 8 for the 1978 model year.
1978 Chevrolet 40-60 Medium Duty
1978 GMC Series 4500-6500
1978 Chevrolet 40-60 Medium Duty 1978 GMC Series 4500-6500