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Author Topic:   Trailer-to-Vehicle Connectors: Round v Flat
PeteB88 posted 09-29-2010 11:52 AM ET (US)   Profile for PeteB88   Send Email to PeteB88  
I recently bought a used COX trailer for my 13 which has the round plug instead of the common flat plug. The round plug is connected to a heavy duty cable that is coiled and quite impressive.

Do I need to rewire the trailer w/ flat connector or can I use some kind of adaptor?


contender posted 09-29-2010 12:22 PM ET (US)     Profile for contender  Send Email to contender     
You can purchase an adaptor $5-$10, Any trailer place should have them, maybe even walmart
jimh posted 09-29-2010 12:31 PM ET (US)     Profile for jimh  Send Email to jimh     
There really is no connector for a trailer which is known specifically as "the round plug." There are several varieties of round connectors which are in use on trailer-to-vehicle connections.
jimh posted 09-29-2010 01:42 PM ET (US)     Profile for jimh  Send Email to jimh     
In the REFERENCE section there is a good article on trailer-to-vehicle connectors. See

Trailer–Vehicle Connector Wiring
by James W. Hebert

Most pre-made adapters that I am familiar with are usually in the form of converting a vehicle with a 7-pole connector to connect to a trailer with a 4-pole or 5-pole connector. I do not recall see too many adapters designed to work in the other direction, that is, to adapt a trailer with a 7-pole connector to a vehicle with a 4-pole connector.

On my own trailer I have installed a 7-pole round plug, which will mate with the (more or less) de facto standard 7-pole round RV-style connector you will find pre-wired on most domestic trucks or larger sport-utility vehicles.

There is no reason why you could not install both a 7-pole round RV-style connector (which you may have already with what you described as "the round connector") and a 4-pole flat connector on the trailer. You can wire them in parallel. In that way you could use the trailer with vehicles which had either the 7-pole round connector or the 4-pole flat connector.

PeteB88 posted 09-29-2010 03:46 PM ET (US)     Profile for PeteB88  Send Email to PeteB88     
Best description is round plug or connector and it is 7-pole. Great ideal about installing two. I need to be careful because I drive 4 Runner which requires a converter thing to make trailer lights work. I generally test first w/ 12V battery charger.


Nauti Tauk posted 09-29-2010 04:15 PM ET (US)     Profile for Nauti Tauk  Send Email to Nauti Tauk     
For those interested, you can charge the onboard battery(s) in your boat using two of the leads that are present in the 7 pin plug Jim referred to above. I have a trolling motor jack in the console of our 2003 170 Montauk which accepts the same male end that is wired to the trolling motor. A two conductor wire (10' or so) is wired into the male trailer plug picking up the 12 volt charging circuit that is present within the 7 pin jack on the truck. Although ecxeedingly simple this setup charges my trolling battery like a charm on the hour ride to the boat ramp. Tavel trailers have used this method for years to keep their house batteries topped off while towing. This charging harness cannot be used to charge batteries that are wired in series to supply 24volts however, since it is basically a backfeed and 24volts would probably hurt the alternator of the towing vehicle.....I think. Jim?
jimh posted 09-29-2010 07:56 PM ET (US)     Profile for jimh  Send Email to jimh     
Please start a new thread to change the topic. Thanks.
jimh posted 09-30-2010 09:03 AM ET (US)     Profile for jimh  Send Email to jimh     
If a trailer is wired with two cables and two connectors for attaching to a vehicle, and the cables are wired in parallel, caution must be taken to prevent the unused cable from short circuiting. In the case of the flat-4 connector on the trailer side of the cable, the exposed pins of the connector will become hot with voltage if the trailer is connected to a vehicle using the other connector. A cap or cover should be installed on the unused connector to prevent its from short circuiting, as would likely occur if the exposed pins came in contact with the trailer chassis.

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