Trailer lighting diagnostics

[superhawk996]

I was prodded into this, but before that I thought I should do a writeup anyway.  Warning: I'm better at doing than writing.

 

Trailer lights not working at all: Using an incandescent test light grounded to the ground terminal of the vehicle connector (white wire) check to make sure the vehicle is doing its thing.  Green is right turn signal/brake light, yellow is left turn/brake, brown is taillight. (exceptions below)  If ok, move on.  If nothing then move the test light's ground clamp to a clean chassis ground.  If it now lights up you have a bad ground on the vehicle, follow the white wire and fix it.  If no change the it's the vehicle's lighting power supplies.  If the vehicle isn't doing its thing stop reading cuz I ain't covering that here.  You could use a volt meter or LED test light instead of an incandescent test light but since they put little load on the system you can get a false "ok" which is why I recommend an incandescent light to put some load on the system.

 

So the vehicle is ok.  None of the trailer lights work or all of the trailer lights are dim, and possibly doing goofy shit like flashing the tail lights when a turn signal is on and even feeding back to the vehicle effecting it's lights: most likely a bad ground.  Easy check is to use a jumper wire between the vehicle and the trailer, any clean metal surfaces you can clamp to.  Beware of screws in plastic, you need to attach to the vehicle and trailer chassis.  If the lights now work you know it's the ground wire of the trailer not conducting.  It's generally screwed to the trailer near the tongue; the white wire.

 

Both taillights dim or not working but marker/clearance lights are ok: sometimes, tho rare, the taillights share a ground wire; that's the likely suspect.

 

Only one light not working or dim: check the bulb, bulb connections, wires to that lamp.  The ground may be a wire screwed to the frame near the lamp (sometimes under it) or the lamp's mounting screws making the ground connection.

 

Lights are bright and normal except that one of the turn signals makes the other lights flash: single filament bulb in a dual filament taillight, defective/broken bulb receptacle, defective bulb, wires crossed/shorted.  This is a pretty rare issue but it happens.

 

If you have a 5 way connector the fifth wire (should be blue) connects the vehicle's reverse lights to a lock-out solenoid at the brake cylinder output used with disc surge brakes.  If the trailer doesn't let you back up it could be a bad solenoid, but most likely a wiring issue.  It can also be used to add reverse lights on the trailer, but beware of the total load to not exceed what the vehicle can supply.

 

If you have a 7 way connector it can get confusing as there are some options.  Blue is for electric brakes and white is ground;  don't think there are any standard deviations for these two.  Depending on how it's wired there could be a red wire for independent brake lights-meaning that the brake and turn signals are different lamps and not shared, or the red could be an auxiliary/charge lead.  The auxiliary lead can also be orange or black.  This wire is used for charging/maintaining a 'break away' battery for the brakes which is activated by a cable that pulls out of a switch if the trailer detaches from the vehicle locking the brakes.  This wire can also be used to charge/maintain an auxiliary use battery in the trailer but it would likely be illegal to share those circuits as the auxiliary use battery could drain the safety battery if they're joined.  The standard of yellow=left turn/brake can instead be the reverse lights.  There are at least 3 different 'standard' 7 way wiring schemes that I remember.  I've also found that with 7 ways people tend to get creative because there are so many options available.

 

There's also a 6 way that is rarely used, too tired to bother.

 

If the trailer is to be submerged in water, especially salt water, all connections need to be sealed.  Not just some tape but totally sealed using something like marine heat shrink connectors.  I've gotten by with dielectric grease and/or CorrosionX for quick fixes, but if it falls off the honey do list it'll become a complete harness replacement later on.

 

If you find a connection that's corroded you need to cut the wire back to clean copper.  Corrosion will travel through a wire an unbelievable distance and if you just cut and re-connect bad wire it's likely to be a temporary fix.  If you're working with a trailer that has old wiring that's cracked or has/needs multiple repairs you're best off replacing the whole harness.  It's usually pretty easy but if the wires are inside of frame tubes and difficult to remove/replace just abandon them and lay out a new harness.  Make sure the harness is well secured with little dangling, but if it has electric brakes make sure there's enough free wire to handle suspension droop.  Do not give in to the temptation to solder connections, if there's any movement/vibration of soldered wires they will likely break.

 

Wiring a vehicle for a tow connector: If you're making your own connection beware of modern electrical/electronic systems. If it's an old ass truck you can generally just splice into the existing wiring but with some modern systems it can lead to trouble.  If there's a plug & play option use it.  If you're slicing in do some homework.  If the vehicle uses canbus or any other 'smart' electrical system splicing in can cause a shit-ton of issues.  If you slice into an older vehicle and experience quick flashing turn signals you need to replace the flasher unit with a HD flasher.  If the hazards use a separate flasher unit you'll probably have to replace it too.

 

Warning #2, and my excuse, most of this was written under the influence of Sailor Jerry.

[googus]

Great write up