The 7 pin N type plug and socket is still the most common connector for towing. This supplies power to the road lighting of your trailer or caravan. This has now been replaced by 13 pin Euro plugs on all new caravans.
Below is a diagram for the original plug and socket showing the functions of each pin.
| Pin No. |
Colour |
Function |
| 1 |
Yellow |
Left Indicator |
| 2 |
Blue |
Fog Light |
| 3 |
White |
Earth |
| 4 |
Green |
Right Indicator |
| 5 |
Brown |
Right tail / Side light |
| 6 |
Red |
Brake / Stop lights |
| 7 |
Black |
Left tail / Side light |
Here are two wiring diagrams for the 7 pin ‘N’ type trailer electrical plug.
The first diagram is a simple set up of two brake lights, two indicators and two side lights. Click on the image below to enlarge it.
The second diagram shows two brake lights, two indicators, two side lights and a fog light. Click on the image below to enlarge it.
Other wiring diagrams on this site are listed below:
7 pin ‘S’ type plug and socket wiring diagram.
13 pin Euro plug wiring diagram.
7 way universal bypass relay wiring diagram.
Split charge relay wiring diagram.
To purchase components for trailer wiring please visit – www.towingandtrailers.co.uk
TEC2M Auto Switch Combi Relay
This relay automatically senses the condition of the towing vehicles alternator and diverts some of the current charging it’s battery to the caravan’s battery or fridge as well. This happens automatically when sensors trigger a split charge relay at a set voltage. This does not require a separate lead from the ignition switch as the switch in the relay is automatic.
It is important to use cables with sufficient stranding to prevent voltage drop as this can stop the relay from functioning. Cable that has a minimum stranding of 35/0.3 (2.5 sq mm) should be suitable for the 12v feed from the battery to the relay. Cable with a minimum stranding of 16/0.2 (0.5 sq mm) should be enough for the earth.
This relay has been designed to handle loads of up to 15 amps.
Below is a wiring diagram for the TEC2M, please click to enlarge it.
To purchase trailer parts and towing accessories please visit towingandtrailers.co.uk
Cable cover plates form a protective cover for the exposed area of brake cable that connects to the expander in the brake. The cable cover is made up of two plates that slot together and are held in place by the brake cable itself. The first of these two plates is permanently fixed to the back plate of the trailers brakes.
Cable cover plates on trailer brakes are a common area for maintenance as they are usually the first to show signs of corrosion. This is because the underside of your trailer is far more likely to suffer the effects of corrosion due to the water and salt picked up from the road.
The example below shows a poorly maintained back plate from a brake unit. You can see that the back plate is badly corroded, and the fixed half of the cable cover plate has snapped.
If you are sure that the corrosion to the brake unit is only superficial and there is no severe damage, it is possible to clean it up with a wire brush and replace the damaged cable cover plate.
As you can see in the picture below, the hub and brake assembly have to be removed in order to perform these repairs.
In order to take off the old cover plate you must remove the weld fixing it to the back plate using a grinder. Take care not to damage any other part of the brake while doing this.
After removing the weld, the old cover plate should not be too hard to separate from the brake back plate.
Use a hand file to clean up the area you have just been grinding.
Slot both parts of the new cable cover together and position them in the back plate. Next you will have to weld the top cover plate in place.
Remember that only the top cover plate needs welding to the brake back plate. The bottom cover plate is held in place by the brake cable and can be quickly released to allow access to the expander.
The weld only needs to be about an inch in length, just enough to hold the cover plate in place.
In the example below you can see the top half of the new cover plate firmly fixed to the brake back plate.
Providing that the expander inside the brake is in working order, you can use a wire brush to clean up the rest of the brake and re-assemble it. Connect the end of the cable to the expander, re-fit the bottom cable cover plate, and slide the metal ‘cup’ of the cable over the ends of both cover plates to hold them together.
Related articles:
Identifying trailer brake cables
Brake cable replacement / fitting guide
Replacing trailer brake shoes
To purchase trailer parts, including cables and cover plates, visit towingandtrailers.co.uk:
Here is a guide to fitting / replacing a triple lock head on a braked coupling.
First of all you will have to remove your old coupling head.
If you have a standard coupling head you should be able to remove it by undoing the two bolts that fasten it to the drawbar. If you are replacing a triple lock head, things are going to get a little trickier.
Triple lock heads have a small security device that seals one of the bolts securing it to the drawbar. You can only remove this is by cutting through it with a grinder, exposing the bolt on the inside.
The example below shows where to cut through.
You must also turn the head over and cut through the device securing the nut.
Cut the tie wraps from around the rubber bellows and slide them out of the way. Now the bolts have been exposed you can remove them with a ratchet or power tool. Remember though, you will need to use thin walled sockets to fit into the ground off security devices. Spanners and impact sockets will not fit.
Remove the old head from the drawbar.
Slide the new head onto the drawbar and line up the holes where the bolts go. Begin to fit the bolts that secure the head to the drawbar, remembering to fit the new devices that will later seal the bolts.
You will notice that the hole on the side of the head is obscured by something inside the drawbar. This is a damper that connects to the back of the head. It must be levered into position so that a bolt can pass through the eye on the end of it.
Lever the damper with a screwdriver until you can see that the eye of the damper and the holes in the head line up. When they line up, push the bolt through them. This is a little tricky as you will have to hold the damper in place with one hand, while inserting the bolt with the other.
When both bolts are in place tighten them up. Remember that you will need thin walled sockets to fit inside the security device.
Now you can seal the security devices by hammering these small metal disks into them.
The metal disks have tapered edges to make them easier to hammer in.
The metal seal for the device protecting the nut has a hole in the centre of it, this allows it to fit over the end of the bolt. To hammer this into place you will need to use a large socket so that you can hammer the metal disk without hitting the bolt. See the example below.
Turn the head back so it is facing the correct way up and re-fit the rubber bellows. Stretch the bellows back over the coupling head so that it covers the unprotected bolt. Use tie wraps at each end to keep it in place and make a good seal that will keep water out.
To purchase trailer parts, including replacement couplings and coupling heads, please visit towingandtrailers.co.uk:
Many modern cars have Multiplexed or Can bus wiring systems. Wiring systems such as Can bus were developed in order to be a simpler and more cost effective alternative to the traditional wiring harness.
This advancement in wiring has also led to the development of various fault finding and diagnostic systems. One fault finding system sends small test pulses to all of the lights in the system to identify faulty bulbs. If a faulty bulb is detected the diagnostic system will display a symbol on the dashboard, alerting you to the fault.
This system can cause problems when trying to fit a tow bar, LED lights, or even when towing a trailer that has been fitted with LED trailer lights. Unlike regular bulbs, LED lights are so sensitive they will pick up the small test pulses through Can bus and flash constantly. As well as constant flashing, LED lights do not have sufficient ‘Load’ enough to trigger the diagnostic system and dashboard warning symbol.
This can be avoided by fitting an expensive manufacturer approved plug-in wiring kit when your tow bar is installed. This should resolve any problems with the diagnostic or fault finding systems, but it won’t be cheap.
Fitting this Universal Bypass Relay to your car between the wiring system and trailer plug socket bypasses all of the fault finding and diagnostic systems created by Can bus. This basically leaves you with a standard 12v power supply. This allows LED lights to function properly, without flashing, at a the fraction of the cost of a dedicated plug-in kit.
This relay isolates the 12v power supply and bypasses any fault finding or diagnostic systems after the relay. A fuse should be fitted to the relay’s power supply so in the event of a fault or short circuit your car remains unaffected. Please be aware that no system on your car will alert you to faults with your trailer’s lights while using this relay.
On the left hand side of the relay are 8 colored cables, a 12v power terminal, and another terminal marked TT. The 12v power terminal requires a dedicated feed (with fuse) directly from the cars battery, while the terminal marked TT is for an optional ‘tell tale’ dashboard light.
The 8 colored cables have the following functions:
Yellow – Left Indicator
Blue – Fog Light
White- Earth
Green – Right Indicator
Brown – Right Tail / Side Light
Red – Brake / Stop Lights
Black – Left Tail / Side Light
Gray – Reverse Light
On the right hand side are 7 terminals marked 1, 2, 4, 5, 6, 7 and ‘R’. There is no terminal 3 as this is an earth wire which must be secured to the vehicles bodywork.
These correspond to the pins in the trailer plug socket and are wired as shown below.
Click the image to enlarge it.
To purchase trailer parts and accessories, including tow bar wiring components, please visit towingandtrailers.co.uk :
Making sure your trailer is fitted with the correct tyres is very important. Here is a short guide to tyre speed ratings.
Tyre speed ratings are writen on the wall of a tyre. They can be identified by alphabetical letters ‘J’ through to ‘V’, with ‘J’ being the slowest.
| Tyre Letter Rating |
Max Speed (mph) |
Max Speed (km/h) |
| J |
60 |
97 |
| K |
69 |
111 |
| L |
75 |
120 |
| M |
81 |
130 |
| N |
87 |
140 |
| P |
93 |
150 |
| Q |
100 |
161 |
| R |
106 |
170 |
| S |
113 |
182 |
| T |
118 |
190 |
| U |
124 |
200 |
After 200km/h (124mph) the letter ratings are for performance cars.
| Tyre Letter Rating |
Max Speed (mph) |
Max Speed (km/h) |
| H |
130 |
209 |
| V |
150 |
241 |
After 240km/h (150mph) tyres are marked with ZR, and may also have a W or Y. The W means the tyre is restricted to 270km/h (168 mph), and the Y means the tyre is restricted to 300km/h (186 mph).
Related articles:
Wheel Measurements
Working Out Your PCD
Trailer Tyre Load Index
Trailer Wheel Offset
If you are looking to buy trailer wheels and tyres, choose from a large selection at 
Here is a step by step guide to replacing brake shoes.
Lift the trailer off of the ground and onto stands so that all of the wheels can turn freely. Make sure the handbrake is locked in the ‘off’ position.
Now you can begin by removing the wheels and the hub caps.
Slacken off the brake adjuster until it becomes loose, do not remove it. On some brakes this adjuster is a bolt on the back plate of the brake, others have a ratchet type adjuster inside the brake drum. To reach this you will have to use a screwdriver to turn the ratchet through a small hole in the back plate of the brake. This may be concealed behind a plastic stud.
Remove the axle nut. This could be a simple locknut, or it could be a castle nut secured by a split pin.
Next remove the brake drum by turning it left and right while pulling away from the back plate. Make sure you do not disturb the bearings or brake shoes at this stage.
It is also important to note that brake dust is harmful if inhaled, so be careful not to inhale any while removing the drum. If you have a buildup of brake dust, the best way to remove it is to wipe it away with a cloth.
Now the drum is free from the trailer, check its condition. The drum should be replaced if there are deep score marks around the inside.
The next step is to remove the brake cables. For a detailed guide on brake cable replacement please see here.
Using screwdriver as a lever, carefully lift the sliding end of the shoe away from the expander. Once the top of the brake shoes are out of the expander you should be able to remove the expander and top brake shoe retaining spring.
The whole brake shoe assembly should now be lifted away from the back plate. Be careful not to lose the two small adjuster wedges that may drop out during this process.
Now check all of the brakes components including springs and the adjuster. Clean the entire mechanism, making sure everything is free to move and undamaged. Any damaged or rusty parts should be replaced. Avoid any kind of lubricant or penetrating oil as this could prevent your brakes from working.
The expander is a very important part of the brake unit and it has to remain free of rust in order for it to work properly. You can check that the expander is working correctly by simply holding it by the hinge. It should swing open freely. If it does not, rust will be preventing it and it should be replaced immediately.
Re-fit the lower spring to the new brake shoes and re-position the new brake shoe assembly back onto the back plate. Remember to re-attach the assembly to the adjuster wedges, or cam block if you have brakes with a ratchet style adjuster.
Return the top retaining spring and re-fit the expander between the two brake shoes. You may need to carefully use a lever like when it was removed in order to do this.
You can now re-attach the brake cables and cover plate. If your cables are showing any signs of wear or corrosion they should be replaced for new ones at this time.
Re-fit the brake drum and bearings and secure them in place with the axle nut. For tips on how to do this, see this guide on replacing trailer wheel bearings here.
Replace the hub cap and wheels before moving on to adjusting the brakes.
Adjusting brakes
Two important things to remember when adjusting trailer brakes are as follows:
1. The trailers drawbar must remain fully extended. Adjusting the brakes while the drawbar is even slightly compressed will result in the brakes becoming incorrectly adjusted when the drawbar becomes extended during use.
2. When adjusting your brakes only rotate the trailer wheels in the forward direction. This is because most trailer brakes will lock when turned in reverse. This is because of an auto reverse mechanism.
To adjust trailer brakes you have to rotate the wheels in the forward direction. Now tighten the adjuster (either a nut or ratchet adjuster) until you can just feel the brakes start to bind. When you can feel this resistance carefully turn the adjuster back until the wheel can just rotate freely.
Now adjust the nut on the brake rod until it makes contact with the compensator. Be sure not to over tighten this as it will slowly apply the brakes, causing them to bind and overheat.
Turning each wheel backwards with the handbrake engaged will cause the brakes to lock after a short time. If they do not lock, there is too much slack in the cables and they will need further adjustment. Well adjusted brakes should leave the brake compensator at 90˚ to the brake rod, and parallel to the brake cable anchor point. As shown below.
Finally, check all components have been correctly installed and all wheel nuts are tight. Remember that new brake shoes take a while to bed in and brake adjustments should be re-checked after a short journey.
For a wide selection of trailer parts, including trailer brakes and suspension, visit our website:
Here is a guide that will help you install trailer wheel bearings.
Trailer wheel bearings can become damaged or worn out over time just like any other, so it is important that you remember to maintain them. Wear and tear through regular use is a gradual process, but you can cause unnecessary damage through two easily avoidable mistakes.
The first mistake many people make is overloading the trailer. If you exceed your trailers maximum weight you are putting more stress on the wheels and axles than they are designed to take. The wheels and axles themselves are less likely to break, but smaller internal parts such as wheel bearings are at great risk of becoming badly damaged.
The second mistake people often make is loading the trailer unevenly. This is a bad idea in a few different ways.
Firstly, this can cause major stability issues while towing. Towing an unstable trailer can be dangerous to everybody around it, if the police see a badly loaded trailer in tow they can, and probably will stop you.
Secondly, uneven pressure from a poorly loaded trailer can cause different rates of wear for each bearing. For example, a trailer loaded heavily to the right will cause the right hand bearing to work much harder than that on the left because it is under more pressure. If this is a regular occurrence, it will effectively shorten that bearings lifespan.
The first step when fitting new bearings is to check the condition of the shaft on the suspension unit. This should be clean, free of rust, and have no splashes of paint from when the suspension unit was painted. A quick rub with sandpaper will clean this up.
At this point it is important to identify which bearing is the front and which is the rear. Some rear bearings have a built in seal, others have separate ones that will need tapping into place with a rubber mallet.
Your new bearings now need to be thoroughly greased and prepared for fitting. For best results grease new bearings by hand.
Slide the rear bearing onto the shaft making sure it is firmly pushed all the way back. Remember that the rear seal must face outwards from the hub (towards the suspension unit).
Grease the inside of the hub and insert the front bearing with the tapered edge facing in.
Slide the hub onto the shaft. Remember that the wheel nuts face out.
Grease the front bearing again now it is in the hub, and place a large washer on top of it.
Tighten up the castle nut until it just ‘nips’ tight. Turn the castle nut back a fraction until you can clearly see the small hole that the split-pin passes through.
Push the split-pin through this hole and open it out with a screwdriver and small hammer.
Now the whole unit is pinned together, all that remains is to fit the hub-cap that covers the bearings to keep dirt and water out.
For a wide selection of trailer parts, including wheel bearings visit our website: 
