Testing 7-Way Socket on 2015 VW Touareg for Active Trailer Lighting & Power Circuits

Updated 05/10/2017 | Published 05/09/2017

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Hook Probe Circuit Tester

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Adapter 4-Pole to 7-Pole and 4-Pole

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Question:

I understand that unless a trailer is hooked up to my vehicle there will be no voltage on the 7 pin connector. What determines this as i am trying to test the vehicle side connection.

asked by: Gilles A

Expert Reply:

A 7-way trailer socket like # 37185 on a vehicle includes a 12V auxiliary trailer power circuit that is intended to maintain a charged battery installed on the trailer while you drive. This power feed cannot charge a depleted battery but it can maintain one that is already well-charged. If you refer to the linked photo of a 7-way vehicle socket you will note this circuit is carried on the pin at the 1-o-clock position as you view the socket.

Some vehicle electrical systems will have this 12V circuit active at all times, regardless of whether or not a trailer is connected or if the key is in the on or off position. Other vehicles' ignition keys will control whether this power feed is active or not. Applying a circuit tester like # PTW2993 to the ground and 12V pins will tell you when this circuit is active on your 2015 Volkswagen Touareg.

It is sometimes necessary to install fuses and/or relays in the vehicle power distribution box to activate these circuits. Some "tow packages" will include the fuses and relays but others may not; it depends on the vehicle make, model, version and whether the trailer socket was factory-installed, dealer-installed or aftermarket. Circuits for trailer brake, turn and running lights may also need fuses/relays installed to activate them. Usually a power distribution box will have a legend on the lid that indicates each fuse/relay position and required fuse value. This is easy enough to check by opening that power box and checking for fuses installed in any receptacles labeled trailer/tow.

The other functions on a 7-way, the various lighting signals, will be present on the 7-way whenever those functions are activated on the vehicle. Again, this requires that any essential fuses and relays are installed.

expert reply by: Adam R