Boat Trailer Lights




Lights usually are a constant headache if used on trailers that are submerged in saltwater.  You freshwater guys don't know what you are missing.  There will be all kinds of lights, some of the "standard" automotive lights will even be advertised as "waterproof".   In actuality very few are.  Some marine dealers will have simple automotive trailer lights on their shelves, and let the buyer beware attitude.


First Things First :  The light bulbs used here are a dual filament, meaning that one bulb will function for both the dimmer tail light on one filament and the brighter stop light on the other.   If you don't get the proper lights functioning, CHECK THE BULBS first.  They may not be burned out, but simply corroded.  One common somewhat cure is to clean the electrical nubbins on the rear with fine steel wool, then cover them with Vaseline, to keep off the corrosion.  More on the bulbs later in this article.


Other than the bulbs, the most common cause of light failure is a bad connection SOMEWHERE in the system.  This is usually in the form of corrosion at a connection and usually at a grounded wire location.


 Shown below is an e-mail plea from a reader.   First off this could be more than one thing wrong.  I would first look at the bulb and the contacts in the bulb socket or clean the small lead contacts on the bulb.  Then I would check any connections for good continuity starting from the back and going forward.  There may even be a chaffed wire that is cut into somewhere under the trailer.   I have since revised this article for more clarity. 


I have one taillight on my trailer that works only when you brake but otherwise doesnt work.  any advise would be appreciated

Also how many times do you still see someone unplug the trailer light pigtail from the vehicle just before they dump the trailer in the water?  This may have been the order of the day when the light units were open to the water and they figured that when the bulbs were submerged, that when you hit the brakes to slide the boat off, that the cold water would break the now hot bulb.   This has been long abandoned with the advent of more sealed type light units especially LED sealed units.


The Light Units Themselves : Most trailer light units will be wired for 2 wires coming out of the light unit and will need to have at least one mounting screw that is grounded inside, which will be grounded to the trailer frame, either under the light unit mounting bolt or by a separate wire all the way forward to the plug in.


This spot will need to be cleaned so that good contact can be made.  It is also recommended that some chassis grease be smeared at this point to help reduce rusting and future non-conductivity of the connection.  You can tell which one by looking inside the unit, finding the metal part that the bulb locks into.  This one will then more than likely have the attaching bolt go thru it to the outside.    


As a better alternate, the white ground wire could be wired together from each the RH & LH light units and clear back to the connector at the hitch, then grounded at the vehicle thru the plug.


The wires will need to be connected to the wires coming from the plug-in connection unit.  At the point you connect them at the light unit, it is recommended that you get some heat shrinkable tubing.  Slide a 1 1/2" section over the wire, and away enough so that the soldering does not effect it.  After the solder job is finished and cool, slide the tubing over the new joint.  Now take a match and heat the tubing up just enough that it shrinks and makes a tight seal on both sides of the joint.


You could use wire nuts or crimp on connectors, but these are not waterproof.  It also has been this persons observations that if you happen to be traveling over an extended rough road, (the Alcan Hiway for instance) that just the weight of the wire nut connections will vibrate enough to where it will break the wire at the connection if the wire is not secured to the axle, frame or part of the trailer very close to the connection.


You may have to make a choice of light units that match your trailer needs.  This can mean that you need a low unit if your trailer is so designed.


All light units are mounted so that the license plate holder is mounted on the LH side of the trailer looking forward.  All replacement light units will come with about 10" of color coded wires protruding for you to attach to the main trailer wiring.  The RH unit will have a brown and green wire, while the LH unit will have a brown and yellow wire protruding.


Universal Trailer Wiring Color Code : The normal trailer wire colors when used with the 4 wires will be white, brown, green, & yellow.
      White  --  Gr  =  Ground
      Brown --  TM  = Tail lights / park lights
      Green --   RT = Right Stop and Right hand directional lights
      Yellow --  LT =  Left Stop and Left hand directional lights


The directional light bulbs also function as stop lights, so in testing if the directional light comes on so will that brake light when stopping if everything is wired right.

Other colors may be red, blue or black,  which will normally be used for auxiliary (side clearance lights on larger trailers) and electric brakes.  However the red and blue are 2 that you normally do not see on boat trailers.


Ground wire :  Be sure that your wiring is grounded to the frame or bumper on the towing vehicle and then ran into the ground lug on the receptacle.   Now be sure that this ground wire is again bolted to the trailer so it is also well grounded.  Do not rely on the ball coupler to make good enough ground.  If you start to have light problems, check either of these ground connections for good contact FIRST.


If you have an old trailer and have light problems, pull the lens cover, turn on the directional for that side.  Recently I had a old Chevrolet pickup bed trailer that was giving me fits.  Tracing the wires back from the vehicle, I had power all the way to the bulb, by using a a clean metal location for my ground clamp.  The wiring looked good with power going thru it.   Look closely in a dim or shaded area for even a glimmer of the bulb glowing.  I did and yes there was a glimmer of the filament having power.  It may just be a bad ground.  This particular light unit had the receptacle grounded to the light base which was then screwed to the metal bed.  I just ran a jumper ground wire to each of the metal light bases, then attached the wire crimp on end with a sheet metal screw.


Universal plug in Wiring Code :  Check the wiring diagram of your receptacle unit.  Years ago (like maybe 40-50) there was no universal wiring and I have even seen some of the old plugs with marking for red, black, white and ground.  Using this they usually ran a blue or green as ground, rather confusing.  The standard color code as recommended by the US Society of Automotive Engineers  and the Canadian Standards Association is as listed in the chart below.  Also some automobile manufacturers stray a bit off this standard code.   So when you are trying to trace down a problem, do not go by color alone, but locate wires by their function, as someone may have changed the wiring for some reason. 


7 Pole trailer connectors


6 Pole trailer connectors


4 Pole connectors





Ground (GD) return to towing vehicle White   White   White
Left-Hand directional & LH stop Yellow   Yellow   Yellow
Right-Hand directional & RH stop Green   Green   Green
Tail lamp / license plate illumination & clearance Brown   tie tail lamp load on with marker circuit   Brown
Marker, identification & clearance for large vehicles Black   Brown    
Stop lamp / electric brakes / back-up or anti-lock devise Red   Red    
Auxiliary circuits Blue   Black or dark blue    


The auxiliary prong can be used for an auxiliary battery in a RV or a backup light.   The stop prong is for a unit that uses individual stop lights on the trailer instead of a combo directional / stop which is commonly seen.


If the receptacle unit you buy does not offer a wiring code, check with a different brand as they are usually the same.  It also is a good idea when you are finished, to mark on the diagram or make up a wiring plug diagram with the wire colors to which prong that you do use, & then keep it with the towing vehicle or boat.  You may find the auxiliary terminal may be uses as a backup light connector.


Shown here is the wiring diagram & reference marking for the 6 prong as viewed from the rear of the towing vehicle Shown here is the wiring diagram for the towing vehicle receptacle as viewed from the rear as taken off a Ford pre-wired towing package vehicle
TM = Tail Marker (Tail Light)  Brown
GD = Ground,                         White
LT = Left Tail (Directional)    Yellow
A = Auxillery                           Blue
RT = Right Tail (Directional)   Green
S = Stop Light/Electric Brakes  Red

The LT & RT can also function as stop lights


Plug in Receptacles :   Most common receptacles will be either a 4, 6 or 7 wire unit.  Currently the simplest ones are a flat plastic 4 wire male/female receptacle using 3/16" round prongs, with color coded quad wires in and out.  These will have 3 wires insulated plus 1 bare coming from the vehicle.  The trailer end will be reversed  using 1 insulated and 3 bare.  You may see one of the trailer end with 4 prongs going in but 5 wires coming out.  If you look farther, you will probably see 2 sets of Siamese wires, a yellow/brown and another with green/black with yellow stripe (or some combination thereof).  What these are would be a wiring kit, where each Siamese set would go directly to the light for the directional and tail light.  In essence, the brown and black are connected inside the plastic coupler.


Most all boat dealers now are pre-wiring the trailers with these 4 wire plastic connectors.  They are many times very short at the trailer, so short that you will need a short, (18" to 24") section of wire coupler.  This is good in that with the many types of receptacles on the different towing vehicles, you make up one to fit your vehicle, with the 4 wire on the rear and a 6 wire going to your vehicle.


Most newer towing package vehicles are pre-wired from the factory with a standard 7 wire RV plug in receptacle, they are large in comparison to the standard 6 prong.  So for the trailer plug ins, there is a standard 7 wire RV plug using all 7 prongs, then a 7 prong to 6 prong that only has internal connections for 6 prongs, in that it does not use the center prong receptacle, which would normally go to a backup light or an extra battery on the towed unit. There is also the 7 prong RV plug that only has connections for 4 prongs, which is normally used when you connect to the 4 wire plastic pig-tails.  All the above will still plug into the standard 7 prong RV receptacle.


Times have changed as in previous years the 6 prong plug units were made of pot metal castings, now you will see all plastic, which seem to be better in my mind after using them.

Just because the plug in, or receptacle you purchase has places for 6 wires does not mean you need to use all of them when 4 will normally do for standard boat trailer lighting.  In the LH illustration above, when using only 4 wires to the trailer, you would not use the "S" or "A" lugs.  These 6 wire units are more readily available and better built than the 4 wire units.  You will also note that the ground uses a larger plug than all the others, this makes for a more foolproof situation even though they all have a raised section that mates in to a grooved section to ensure alignment.


I have one of each that no matter who's vehicle is used there is one that fits, (if the vehicle is wired right).  I even have an extra 4 wire flat connector (to the trailer) with about 24" of wire attached if someone has no wiring or it is wired to current non coding.  Using this I can splice into the trailer wiring, then plug my pre-wired pigtails and be on the road without all the hassle of trying to figure these old ones out.


The smaller metal ones which are more common, usually will be round, with the female part that fits the vehicle will have a spring loaded door that covers the prongs when the 2 units are disconnected.  They have the 6 prongs and will normally have some internal marking for the prongs showing where which wire goes.  I have settled on the 6 prong plug unit, as this gives me the option of using the same receptacle unit for many different trailer requirements with spare connector prongs if needed.   Also it is readily available from many different manufacturers and is usually interchangeable between manufacturers.  It is even available in Canada.


As mentioned before, many single axle trailers come with the flat plastic 4 wire unit.  It is best in this case, to make up a short, 18" or so wire set that plugs into this plastic unit on the trailer, then on the other end use the 6 prong male plug to your vehicle.  In this manner you can then get another plastic 4 wire unit and wire in a different male plug if there is a chance that your son or neighbor with a different plug may tow your boat.  Wire this other unit to fit theirs ahead of time if they are different.


This is important if you may ever have the need for someone else to tow your trailer.  It is especially important if you have electric brakes.  As IF you have the brakes wired to the lug that would normally have been the lights or a directional, your brakes will come on at times you never intended like turning your lights on.  And this could lead to HOT brakes and or a fire.   My brother borrowed my stock trailer once and his pickup was not wired according to the diagrams,  Everything was fine during the day, but when it got to where he had to turn the lights on coming home in the evening, things started happening.  And the electric brakes were on, got hot enough to lock the trailer up until he figured he had to unplug the light cable from the vehicle.

When using the 4 wire molded connectors and adapting to the 7 wire plug, you will not have access to the other 3 wire connectors.


Wal Mart sells a 7 wire RV plug to a standard 6 wire commercial, plus another 4 wire flat on the other side, a truly universal unit.
4 wire flat to 4 wire flat extension 4 wire to standard 6 wire extension, note the extra blue wire not attached, it is so it can be used it for brakes by wiring around the plug if needed later
Here is the 4 to 4 seen above with a 4 to 7 RV adapter Standard 4 wire flat to 7 wire metal RV plug
6 wire plug to 4 wire flat sold by U Haul Standard 7 wire RV plug & 7 wire cable to be wired directly to the trailer wires.


Troubleshooting :   If you ever have to track down a problem in the wiring, you will be be chasing your tail without a circuit tester.   The 4 wire tester shown on the left in the photo below may have some benefit at times as a preliminary tester.  It can not tell you that the wire or a bulb is OK behind this tester however.  It is designed tell you if you have the wiring to the receptacle on the towing vehicle right.


But the one on the right in the photo below is the old standby.  It has a 12volt bulb in the red handle, an alligator clip for the ground wire and with a sharp probe into which you can make continuity to a hot wire.   In use you have to have the lights on and if it is directional related, that blinker on.  You can clamp the clip onto any GOOD ground, push the sharp probe into a suspected wire until you make contact with the inner wire, (be careful as you MAY WELL POKE YOURSELF).  If you have a connection, the red handle will light up.


If you have all the wires checked out and still no lights, the first thing to look at would be a blown fuse.  Second you may even have to remove the lamp lens cover, test the contact points where the bulb mounts.  Again, clean the bulb contacts if need be.   The stop lights will be the same bulbs/elements as the directionals, but both elements will have power and be on at the same time if that is the case.  


In the LH  photo below the 4 wire plug in tester would be good to test the wiring you have in the pig-tail and from the vehicle.   It has 3 indicator lights, when plugged into the vehicle pigtail, these lights will light up individually when the directionals are turned on and or the tail lights are on.   This unit has markings near the indicator lights to let you know which wire should be on.    Since this tester needs power to light up the small indicator bulbs, it would not work trying to run down the trailer lights themselves but will tell if the towing vehicle is wired properly or the vehicle fuses are functional.


Testers designed to perform different tests. Here is one that did not need the tester


You may find a simple problem and then again it could get real complicated.  When the later is involved it will usually be related to the wiring on the vehicle itself.  Do not believe any kind of wiring code color unless you can prove it yourself.  The previous owner may have been a computer genius, but did not understand that normally a red wire is a hot one and either black or white would be a ground wire.  They may not even understand what a ground wire really is or how it functions on a trailer.  I recently spent 5 hours under the back of a Chevy pickup trying to figure out what was wrong until I finally just cut all of the rat's nest under the rear of the bed, traced down every wire by trial and error with the above probed tester.  It appears that the factory RV unit must have had some short problem (yes on of the prongs was folded back to where it shorted out against the metal housing) and had burned the #10 black ground wire in two.   They abandoned that plug, but left it in place, but jumpering to the new 6 prong plug mounted beside the RV plug, with the wrong wires of course. 


We even ran the battery down in this testing process.  There were yellow wires taped into green ones, red into brown.  I could get the LH directionals to work (part time) but not the RH ones.  The part time was because the splices were just wrapped around the spiced onto wire, then taped (somewhat), but they were coated with dirt & made little connection.  Finally I just bypassed the green RH wires that were not functioning, spiced into the proper RH directional green wire under that side of the bed and ran a new wire to the trailer plug in.  BINGO it worked.


4 Wire Vehicle to 3 Wire Converter for Vehicles With Separate Directional Lights :   For newer vehicles (especially non domestic) with separate and usually amber directional lights, it is a pain to wire in an extra set of trailer lights just for the directionals.  However there is a "little black box" that you wire between your vehicle and the receptacle that converts 4 wires coming in to 3 wires going out.  The 4th wire is the ground wire.  They are not overly expensive, $20 or so and available from about any automotive supply store.  It has been noted that these do not last indefinitely however, and may have to be replaced in 4-5 years.


Bulbs :  The standard combo trailer light bulb has dual bayonet lugs on the base.  These short lugs will be offset, one being slightly closer to the base than the other.  This will ensure that the bulb is positioned so that the dual internal filament of the bulb will always have the stop light filament (which is brighter than the tail light side) taking power from the brake sender unit.   The number of this bulb will usually be  #1147




Automotive :  These lights will usually have the housings made of plastic, with the rear lens cover screwed on.  Many times they will be universal, meaning there is no dedicated RH or LH side.  Most times you will find a LH (license) side with a clear lower lens cover to illuminate the license plate.  You can also find the RH or LH with a small outside lens cover to act as a side signal light.  They usually will have no provisions for keeping water out.  There may not be any gaskets under the lenses & WILL NOT have any sealing grommet around the wires.


Weatherproof :  Many times these are not really much different than the automotive, except they may have a gasket under the lens cover.  But usually do not have any type of a grommet or seal around the wires.  They must mean weatherproof from water spray on a rainy day on the freeway.


Waterproof :  These can be had in a few different styles.   Some can be the same as the weatherproof, but with a seal or grommet around the wires.  Not enough waterproofing in my book however.
Trapped Air :  You will find this version having an open air chamber on the bottom where the lights are.  In essence it is the same as a one ended open can upside down.  In use, the air is trapped in the upper chamber as the trailer is submerged.  The height of this unit is made so that water never reaches the bulbs.  A couple of drawbacks when using this style, is that you need to be sure there are no cracks in the plastic case and that any seals are intact around the wires.   Also it is not the best when using around saltwater as just that close of proximity to the saltwater, as over time the light bulb connections will corrode anyway.  Usually the light bulb is mounted in a slide out unit that comes out from the bottom.   But I have not had any luck trying to find replacement internal parts for these.


Sealed :  These are the true water-proof lights.  They may be made as a sealed unit for either incandescent bulbs or sealed beam type units.  They may be more expensive to start with, but will last many years.


LED :  These appear to be the best out there.   They do cost more, but as with the sealed units they are pretty maintenance free.  Many have bullet type plug-in wiring at the unit.  These connections may need to be looked at occasionally and cleaned, even greased with chassis grease or Vaseline to keep corrosion off.


Things You Can Do :  In a pinch and or on a short budget, you can be selective when purchasing automotive type light units.  Then either silicone seal the lens cover gaskets to the body, or if you have ideas of ever replacing the bulbs, buy some soft neoprene or rubber type sheet gasket material, this stuff comes in different thickness, however about 3/32" would be about right.  It is usually offered in 12" square sizes.  Now make your own gaskets to go under the lens covers.  Also you should apply some silicone caulking around the wiring exit hole.


Some trailers will have side or clearance lights that are wired into the same brown wire as the tail lights, silicone any opening here also.  Do not silicone the bulb receptacle as silicone seems to contain ammonia and will promote corrosion if in close contact metal.


You will see some boaters unplug their trailer light pig-tail from the vehicle when launching.  Some trailers use a 2 wire rubber connector plug at each light.  IF the plug unit can allow water to come in contact with both wires when submerged, the water could short out the wiring and possibly blow a fuse.  If this happens to you, consider unplugging the connector at the vehicle when you launch.


If you have a smaller boat, you can take a wood  2"x 4", clamp it onto the stern of the boat, with automotive lights attached to this, with wires running forward to the vehicle plug in.  In this manner you can detach the light bar when you launch the boat.  The lights will never get wet.  This works great if you have a smaller boat or where the effort of re-wiring a old trailer would be a hassle, especially if you can use this light bar on another trailer or on a towed vehicle as well.


Here as mentioned above is a simple removable light bracket


I have also seen trailers that have the side rub rails with the rear upright is taller than actually needed.  On the top of these uprights are mounted the trailer lights, high and dry.


If you happen to blow a light fuse, say when you turn on the headlights, check the brown wire (tail lights) for a short to ground somewhere in the system.  This happened to me and I found that somehow 6 months before, when tracing out a wiring problem, I had inadvertently pinched the brown wire when retightening a trailer bracket.  I apparently had never driven the vehicle towing the boat at night before.   It took that long before I did and then after hooking the vehicle to another trailer as a control, that worked OK, I then found the brown wire was pinched and grounded at near the light to the boat trailer frame.


One bit of advise, always unplug the connector before you drive the vehicle off.  I have found that on a recent used boat that I bought that one brake light was not coming on.  Everything checked out on the trailer, but still no functioning light.  I was at the point to pulling a new wire from the plug in back to the bad light unit.  But what I finally found was that the wire was broken inside the rubber 4 wire connector on the trailer wiring.  While checking continuity with help of a neighbor, of that wire by working my way forward, if he twisted the rubber connector just right, I could get continuity.  Apparently the previous owner had forgotten to unplug from his vehicle and had stretched that one wire beyond it's limit.  And it was unseen , being all inside being the 4 prong rubber unit at the trailer that was the problem.


Also it may be a good idea to find out what fuse functions for your trailer lights, some vehicles have a separate fuse for trailer directional lights as compared to just adding it to the regular vehicle lighting fuse.  Purchase a couple of spare fuses no matter which style or size just to be prepared.


One use where a sealed light unit can prove helpful is if you need to launch before daylight and do not have a helper to guide you down the ramp.  These lights since being waterproof, will light up bright when you apply the brakes even under water.  If the light is LED, this gives you enough light even under water that you can be aware of just how deep you need to back in order to launch, this can help you immensely to judge how deep your trailer is.

Launching Light for Your Trailer :   There may come a day, either morning or evening when you would wish you had backup lights on your trailer.  Having your buddy walk behind the trailer wheels with a dim flashlight showing the launch and your tires is not ideal.   Occasionally you may be in a situation where you may need to launch before daylight if you have to reach your intended fishing area for the daylight bite at tide change.  


Here you can mount a set of sealed backup lights to the rear lower part of the trailer fender, wire them in with the backup light wire of the towing vehicle.  This way they come on every time you back up.  My thought was not to go for the more expensive but much brighter LED lights, but stay with the regular incandescent type bulb, not only because I do not think it would be necessary for that bright a light when it only is only to identify the ramp's edge a few feet away, but the LEDs may pretty well blind anyone near you or on a double ramp when backing up.


I chose to mount one on each trailer fender step.  These units that I picked are oval shaped so they can fit under the fender's rear or can be bolted onto a galvanized plate that is in this case bolted to the top of the plastic fender step.  They come with a rubber grommet to hold them into a Ell shaped mounting bracket.   I wanted a sealed type type light (these are not sealed beam, but a regular incandescent bulb that is sealed inside a plastic unit) because of them being near or possibility immersed in salt water.  The brand I that suited my needs was made by Peterson Mfg Co.  and purchased them thru Car Quest automotive for a total price of about $65.  The light unit kit itself is #416K where the replacement light itself would be #416.  This included the light unit, rubber grommet and 2 wire pigtail right angle plug, but needed the Ell bracket as a separate item.  Then of course a new 7 wire plug in and matching 7 wire RV cable.


These units consist of polycarbonate lens, sonic welded to housing to form single unit, so supposedly being waterproof.  12-volt lamps are designed with single filament, 2.1 amp, 32 cp bulb.

Most towing type vehicles will be pre-wired for a towing package which will include the standard 7 prong receptacle on the vehicle.  One of these wires will be from the vehicle's backup light to the receptacle.  HOWEVER identify it with a 12 volt test probe and connect a wire from the wiring harness going into the trailer tongue, then extend that wire from there to your backup lights.   NOTE, locate the wires to the terminals by function only as color coding may not be the standard with all manufacturers.


If you wanted to be able to manually turn these lights on, you would need to run a switched fused wire from the battery or splice in to these wires at the tongue of the trailer to install a switch below the winch.


Assembling the light to the grommet and bracket is only accomplished by using soapy water as it is a very snug fit.  The pigtailed wire plug to the light uses bayonet type connectors & comes with dielectric grease already on the female ends.  There are 2 wires, a red & white.  The white is ground. 


On this installation, a sheet of galvanized 1/16" sheet metal was used as a base and was extended rearward enough over the step to accommodate the light unit.  This was then bolted on top of the original step while being sure to utilize the step bracing under the plastic fender. 


A piece of old carpet was cut to match the galvanized metal, 1/4" stainless steel RH bolts, lock washers and nuts were used for attaching both the step plate, the light unit, and the carpet to the trailer.   On top of the carpet at the light  attachment, a piece of galvanized metal 1 1/4" wide was bolted down using the same light bolts.  This was to provide a more secure rear edge of the carpet, which now did not need any other means of attachment.   This carpet provided for better foot traction when climbing in or out of the boat when on the trailer.


The light unit had the wires coming off on one side.  It was inserted into the grommet/bracket so that the wires were closest to the frame of the trailer, thereby not hanging down.  Wiring was ran up inside the trailer side rails using a electrician's snake and connected inside the tongue to the wiring pigtail was terminated.


One bit of advise, when locating the mounting holes, if your trailer has fender guides to facilitate reloading the boat like the one illustrated, be sure you locate the mounting holes in a location where you can get a drill motor in close enough to align up to drill the holes.


Also after about 3 years of being immersed in salt water. the painted steel began to get a bad case of rust.  I solved this by making light the bracket out of stainless steel.


Peterson #416 backup light kit unit mounted on an EZ Loader trailer


These lights also come in real handy when backing the trailer into a dark or not so well lighted storage bay.


Trailer Wiring ??? :   Shown below are examples of questionable trailer wiring.  I would not even call it Redneck wiring

Readily accessible wires OK, but does it work ?


Did he loose a light or does he only make a RH turn ?



Copyright 2002 - 2015  LeeRoy Wisner  All Rights Reserved 

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Originally started  02-18-02, Last Updated 11-17-2015
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