Turn and Brake light on TT only work on one side

Problem fixed. I replaced the $25 relay for the driver side stop and turn signal but that was not the problem. Found a blown fuse in the motor fuse box and the socket that the bulb sat in was too loose so it was not getting a good connection. Problem solved. Thanks to everyone for your valuable input.

Cecilt wrote:
OP here. Truck is a 2009 F-250. TT is a 2014. I put in a brand new bulb and same result. I swapped the passenger side bulb that worked fine to the driver side and still it would not blink or get brighter when hitting brake. Not the bulb.

Should I remove the housing for the tail light and see if there is a loose connection behind the rear wall of the camper?

On my 2009 F-250 I will occasionally have to wiggle the 7 pin connector to get one side to work. I know you said you cleaned it but this sounds like a connection problem with the trailer connector and 7 pin receptacle.

All the way back to our 1997 mini-van, Ford used separate fuses for the trailer lighting. Good place to check. If it's blown, question becomes "Why?" You may have simply shorted it somehow but could be a pinched wire etc.

Sometimes I use a low-amp (like 10) auto-reset circuit breaker for troubleshooting. The breaker clicks out and back in while I look and feel around for the problem.

X2 on checking truck fuses / relays. Trailer fuses / relays are in the fuse box under the hood. There's a chart in the owners manual. The diagram in the box is very difficult to see.

Seems the F250 has a separate fuse / relay for everything. Mine does.

The truck probably has a separate fuse for the dead lights. My 2007 ford did and the new 2014 does too.

In April the trailer's drivers side lights went out I did everything listed above, then took the time (and a new pair of cheaters) to read the teensy print on the truck fuse box diagram to figure out which of the 300 grain-of-rice sized fuses was blown. Of course no retail parts dealer has the new fuses, so a trip to the dealership added to the fun. Bought me 5 new fuses. Cause you know if you only buy one, the other blows right away. If you buy too many you never need them. I marked the new fuse so I know where to start looking if it ever blows again.

Artum Snowbird wrote:
If you can take the red lens off and look carefully at the bulb. When you have the turn/brake on you should be on the thick filament. What you might see is the thick filament is very faintly lit, and the thin filament is brighter.

Very often what happens is you see a flashing/brake indication through the red cover, but it is actually lighting the thin filament. If you find that to be the case, the problem is the ground connection to that bulb socket.

You can check that by bringing your negative from your battery to the bulb socket, and touching it to the bulb socket side. If the lights work properly then you have found your problem.

On re-edit... I presume you know that the brake/turn signal to the trailer is fused, and that you may have actually blown the individual side fuse that feeds the trailer brake/turn lights, one for each side.

No, bad presumption. Electrical is not my skill set. Would this fuse be in the truck or TT? I assume the truck but I don't assume either :)

Uses the same wire in the TT to truck connector. Try wiggling the connector while someone presses the brake. Might just be a loose connection at trailer plug in.

If you can take the red lens off and look carefully at the bulb. When you have the turn/brake on you should be on the thick filament. What you might see is the thick filament is very faintly lit, and the thin filament is brighter.

Very often what happens is you see a flashing/brake indication through the red cover, but it is actually lighting the thin filament. If you find that to be the case, the problem is the ground connection to that bulb socket.

You can check that by bringing your negative from your battery to the bulb socket, and touching it to the bulb socket side. If the lights work properly then you have found your problem.

On re-edit... I presume you know that the brake/turn signal to the trailer is fused, and that you may have actually blown the individual side fuse that feeds the trailer brake/turn lights, one for each side.

OP here. Truck is a 2009 F-250. TT is a 2014. I put in a brand new bulb and same result. I swapped the passenger side bulb that worked fine to the driver side and still it would not blink or get brighter when hitting brake. Not the bulb.

Should I remove the housing for the tail light and see if there is a loose connection behind the rear wall of the camper?

1st thing I would do is swap bulbs. If that doesn't work, then it is time to start tracing wires with a meter.