Inliners International Forums General Discussion Truck Talk 49 GMC 100

I've never owned a truck this old so I'm little unclear on "reverse polarity" on a the 6 volts system. My newly acquired 49 GMC has this set up.

Can anyone enlighten me on proper terminal connections. I have a grounding strap from frame with post connector; a huge power wire from starter with terminal connector; and lastly I have a smaller (1/4") wire with post connector that looks like it goes to the outer casing (ground) of the starter. The truck runs with just the power line from the starter to the negative side of battery and the grounding strap to the positive side.

Pretty awesome truck.......

thanks

Hi Mike . . .

Welcome aboard. If the truck were mine I would invest in a Shop Manual. They are available in reprint and will include a wiring a diagram.

There is nothing particularly tricky about positive ground systems - they just fell out use as the auto world standardized on negative grounding.

In terms of the wiring harness (like everything from this era) if there is a wire it is hot, with grounding coming from the body, frame or engine to complete a circuit. The only connection from the + post on the battery will be to the frame or engine-block.

From an electrician's perspective - ground is just a reference. On a 6-volt negative grounded system hot wires carry +6-volts, on positive grounded system hot wires carry -6-volts.

regards,
stock49

Just because I could never get my head around it I switched every positive ground vehicle I had to positive ground just to make it easier on me. It seemed strange that Chevy was negative ground & GMC was positive ground using the same components. Starters, generators, switches, relays.......When you bought a starter or generator you had to "polarize" it by doing a momentary connection to let it know which system it was in.

I've been running 6v positive ground in my '51 GMC for 32 years. Noooo problems. The biggest problem that I see in 6 volt systems is the use of 12 volt wires, which are roughly 1/2 the gauge they should be for a 6v system. Battery cables should be two ought (2/0) for 6 volt. That's a heavier gauge than 12 volt. Folks complain about slow cranking 6 volt systems but that is always a result of undersized cables.

Back in the dark ages (1960's) I ran a 47 Mercury convertible with an 11:1 compression flathead V8- - - -and a 6 volt positive ground electrical system. That one taught me a lot about maintaining low resistance connections in the starter circuit. BIG conductor, CLEAN connections, and short ground paths are essential to any 6V system. The best thing you can do for your 6V GMC is to run a #0 or bigger welding cable from the battery (+) post to the starter housing or one of the bell housing to engine block bolts. The flat braided ground strap from the battery to the frame can corrode and induce lots of resistance into the cranking circuit. Run smaller conductors from the engine to the frame and body to ground circuits like lights, heater blower, radio, etc.
Jerry