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1928/29 Plymouth Q - 4Dr


I recently purchased a 1928/29 Plymouth Q Series 4 Dr and need to replace the battery. I now know these are 6 volt battery vehicles, however, a friend advised me to be sure this isn’t a positive ground battery system . . . I guess meaning that the battery connection is reversed from what I’m used to in our current vehicles. Unfortunately, I did not pay attention to the battery connections when I removed the current battery (assuming it was same as my other vehicles). Is this a positive ground battery vehicle, does the new battery need to be connected opposite (positive to car frame and negative to engine/starter), will it damage vehicle and/or battery if connection incorrectly, etc?

Thanks Much

D - Dodger Fan in California

You might post this on an antique car forum, there you’ll find lots of folks that might know this. I recommend the AACA site:

Are you handy with a digital multimeter? If you know which cable is the positive one you can check it with the “Ohm” setting. If you touch one lead to the frame ground and the other to the “Positive” cable the reading will be 000. That would make it a positive ground system. You can also do this with the “Negative” cable. If the reading is 000 then it’s a negative ground system. This will only work if you know which cable is which.

go to rod knockers web site, they will tell you more than you ever want to know.

Google “polarize auto generator” I expect that you will have done no damage. The generator will not work with incorrect polarity. It is likely that you can run either positive ground or negative ground with no advantage to either. With no polarity sensitive components in an older car unless you have installed a radio, polarity will not matter for the ignition, lights and any heater fan motor or accessory electric wiper motor.

Polarizing means that the generator field electromagnets have been temporarily energized with a current pulse in the correct direction to ensure that the residual magnetism in the field electromagnet cores is correct. An auto generator is self-exciting and needs residual magnetism for that.

My guess is that your Plymouth is positive ground. Most Chrysler products were 6 volt positive ground through 1955. I purchased a 1948 Dodge where the battery had been installed backwards. The ammeter showed discharge when the car was running, and charge when the engine was off and the headlights turned on. It did start and run just fine. I disconnected the battery and completely discharged it then brought the battery up on a small charger. I repolarized the generator. It seemed to me that I put a screwdriver between the Batt terminal and the field terminal on the generaor, but it has been so long ago that I don’t remember exactly how I did this. One polarized a generator one way on GM cars that had an externally grounded field coil in the generator and the Chrysler products another way that had an internally grounded field coil.

A 1928 Plymouth sounds like a lot of fun. I don’t think that the gears in the transmission were synchonized, so you will be doing a lot of double clutching.