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:pat: i have bought a 1968 r model ex concrete truck i cant get it runnin yet but somebody told me this truck could be positive ground i have worked around equipment all my life never saw positive grounding unless maybe 24 volt? does any 1 have any things to look for before i burn something up? could this truck have positive ground??
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Your 1968 Mack should be postiive ground unless someone converted it. You can look at the batteries and see if they are grounded on postiive side to confirm, or if no batteries are installed you can check the terminal size on the ground and confirm using the size of the terminals to check. If you decide to change the polarity there are several items you need to revise- 1. any radios will not convert with out and inverter. 2. the Generator or Alternator will need to be reset (reploarized) for negative ground, 3. the terminals will need to be switched so the battery cables are set to accept a negative ground. I am sure if you can check on google- you can find a detailed explanation on how to swtich polarity and more details.

Good Luck,


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thank u i have bought 4 new batteries and goin to try to get her goin this weekend should the batteries be hot on negative side?

Install them the same way as the ones youre taking out.

If there weren't any batteries in it look at the battery cable ends and see which looks like it would fit the smaller post, that would be negative, and the larger one would be positive.

A more definite way to tell would be to disconnect the ground cables from the starter and using a long jumper wire connect a continuity tester from the starter end of the cable to the battery end of the cables.

Whichever cable shows continuity with this method is the ground.

1968 models were positive ground when built, but as stated before someone could have changed it to negative ground since then.

From what I've seen out there it appears Mack changed to original equipment negative ground about 1976 or 77

"If You Can't Shift It Smoothly, You Shouldn't Be Driving It"

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