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Tractor Talk Discussion Forum

OT: 1979 Kenworth positive ground ??

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Helper

01-08-2004 06:07:22




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Got a call the other day from my son who lives out of town and is working on his friends 1979 truck, I think it is a Kenworth. Anyways, he was installing a hookup for a phone in the truck dash and he found out that the truck has a positive ground electrical system. The truck has 4 six-volt batteries (I assume two pairs each hooked up in parallel then the pairs joined in series). He says there is no doubt that it is a positive ground system. Was this the factory setup? Somehow I doubt it. Is it possible for an alternator to end up being reversed polarity?

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Fred P

01-09-2004 09:27:52




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 Re: OT: 1979 Kenworth positive ground ?? in reply to Helper, 01-08-2004 06:07:22  
My 1968 Mack is 12V + ground. It had four 6V batteries, wired to make 12V. The starter is 12V. I changed it to two big 12Vs. I was going to put a CB in it and then said no way.



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Bob

01-08-2004 08:21:32




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 Re: OT: 1979 Kenworth positive ground ?? in reply to Helper, 01-08-2004 06:07:22  
The truck probably has an electrical system with a series-parallel switch. It connects the batteries in series to provide 24 Volts for starting, and connects 2 pairs of them in parallel for charging with a 12 Volt alternator. There are other goofy schemes, too, with 24 Volt isolated ground alternators which charge one pair of batteries with 12 Volts (+) when referenced to chassis ground, and charge the other pair of batteries with 12 Volts (-), referenced to chassis ground. With one of these systems, it should be possible to locate a source of (+)12 Volts referenced to chassis ground to operate the accessory.

One thing for certain, alternators are "purpose built" for positive or negative ground operation, and cannot change polarity on their own, or by being polarized in the opposite direction, unlike old-fashioned generators.

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rod

01-08-2004 09:08:49




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 Re: Re: OT: 1979 Kenworth positive ground ?? in reply to Bob, 01-08-2004 08:21:32  
we had an older freightliner that was 24 volt starting and was pos ground from factory



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