Electrical issue, trailer /truck wiring

I am having a problem with the "unwanted" application of the trailer brake on my 22 ft. car trailer. The trailer brake appears to be working properly when applied. The problem occurs when the left turn signal is turned on. The truck/tow vehicle is a '99 Ford F350.

When the left signal is turned on, the left trailer brake is activated. Not enough to lock up the wheel, but only enough to cause a drag and some mild overheating. No problem at any other time.

As I understand it, there is a "left turn tow relay", and a "right turn tow relay" on the tow vehicle, somewhere between the signal switch and the connector box at the rear of the tow vehicle. My mechanic has done some electrical testing,and believes the left side is getting a backfeed of about 6v and causing the trailer brake to activate slightly. I have found a new relay ($13), but cannot locate the relay on the tow vehicle. Wiring diagrams do not show the location, or acknowledge existence of it.

Can anyone tell me the location of the relay(s)? The dealer says that Rangers, F150 and larger, Explorer, and most Ford SUVs have the relays, but they have no idea of location, nor have had the problem to come into their shops. I am assuming that "Ranger"s applies to the larger, earlier series trucks, rather than the compact trucks.

Any help is appreciated.

Thanks, David.

David,

Wires are touching that should not ...

It could be in the trailer or in the truck.

To isolate it to either - hook up another trailer without using an adaptor to the truck

(by that I mean if you have a 6 way or seven way plug - do not use an adaptor to reduce it down)

If the problem persists with another trailer - the issue is with the truck.

If not - it is with the trailer.

From there - you have to start tracing wiring .....

Jim

I would look first at the plug in at the rear of the truck especially if you have a,hot wire running to your trailer. I had this same problem several years ago and found that this stuff that grows on your battery cables had grown from the hot wire in the connector to the turn signal wire. I cleaned that stuff off and solved the problem