Introduction to connecting your new LED rear light
A number of bulb adapter and connection options are available in order to make installation of our new LED rear lighting units as simple as possible. This page describes the various connector types available so that you can select the most appropriate one for your own classic motorcycle application.
The four options currently available are the default three-wire connector, the most common BAY15D bulb adapter, the alternative BA15D bulb adapter or a combination of the BA15S bulb adapter plus a separate wire for the brake light. Or maybe you’d like to suggest a new adapter option?
The three-wire connectors are the default option which are included as standard with every LED light board unit. The bulb adapters are offered as optional extras for a small additional charge and are designed to simplify installation by allowing the LED light board to plug into the standard bulb socket.
Note that all of the adapter options are the same for all LED rear light units in both positive and negative earth applications; the LED lighting units are designed to work with either polarity. They also work with both the 6 and 12 volt versions.
Three-wire connectors (default option)
The default option offered free with all of the LED lighting units is the three-wires that you can connect as appropriate to your existing motorcycle lighting electrics.
At one end of the wires is the standard 3-pin connector which clips and locks into place with the special terminal on the rear of the LED lighting board. The other ends of the wires are left unterminated for you to connect as required.
Each of the three wires are approximately 30cm (12″) long which should be more than sufficient to pass through the rear light unit, the number plate holder and rear mudguard in order to reach and connect with your existing wiring. You can of course cut them to a shorter length as required. The wires can then be connected by soldering for a permanent connection, or by using whichever sort of crimp, spade, bullet or choc-block connector you prefer.
This option is ideal if your existing rear lamp’s bulb socket is a little old, worn, rusty or otherwise not making a good connection. The bulb socket can be removed and the LED adapter connected directly to the existing tail and brake light wiring, thereby eliminating any lose connections. The earth can be connected to any convenient point inside the rear light unit, on the mudguard or to the motorcycle’s frame.
However, if you’re existing rear lamp’s bulb socket is in good condition, then one of the following optional bulb adapters means that the LED lighting units can be plugged straight in to the rear light without any modifications to the wiring at all.
BAY15D (1157) offset-pin bulb adapter
The BAY15D bayonet bulb connection is also sometimes known as a 1157, P21/5W or P25-2 type bulb which are all equivalent for the current purposes. This is the most common adapter type for both modern and classic motorcycle applications.
This LED lighting adapter will be the standard choice for the majority of classic motorcycles which already have a dual filament, dual intensity (i.e. tail and stop) bulb fitted. Using this adapter, the new LED light board can simply be plugged in to the existing bulb socket without any modification to the existing wiring.
To identify if you bike currently uses this type of fitment, remove the rear light bulb and examine the number of contacts on the base and the position of the two pins which protrude from the sides. A BAY15D bulb has two contacts on the base for the dual filament (dual brightness) stop and tail lamps. Looking at the metal base of the bulb from the side, the two protruding pins are on directly opposite sides of the base and one is higher up than the other (hence offset pins). This means that the bulb can only be installed one way round and so the brighter filament is always linked to the brake light, and the dimmer one to the tail light.
Otherwise if the pins are at the same height (parallel pins), see the BA15D adapter below.
The BAY15D adapter consists of a metal offset pin bulb base as shown in the above photo with the three wires ready connected for the tail and brake light functions and the motorcycle earth. At the other end is the special 3-pin connector which slots onto the terminals on the rear of the LED might board. The length of the wires are approximately 100mm (4″) which gives you more than enough slack to install the LED, but without having too much surplus wire to get in the way.
BA15D (1142) parallel-pin bulb adapter
The BA15D bayonet bulb connection is also sometimes known as a 1142 type bulb which is equivalent for the current purposes.
This LED lighting adapter is an alternative choice for some motorcycles which have a slightly non-standard dual filament (i.e. tail and stop) bulb fitted. Using this adapter, the new LED light board can simply be plugged in to the existing bulb socket without any modification to the existing wiring.
The BA15D adapter described above should be the normal choice for motorcycles with a dual-brightness (tail and stop) rear light fitted as the offset pins ensure that the correct brightness filament is assigned to the correct light function. However, I have come across several after-market replacement Lucas rear light units which don’t have the offset pins and hence need this type of bulb adapter instead.
To identify if you bike currently uses this type of fitment, remove the rear light bulb and examine the protruding pins on each side of the metal base. For a BA15D type bulb the pins will be at identical heights such that the bulb could actually be installed either way round into the rear light socket. Installing it the wrong way round would give you a very bright tail lamp, but a barely indistinguishable increase in brightness from the low-wattage brake light, so care is needed!
If the pins are different heights (offset pins), see the BAY15D adapter above.
The BA15D adapter consists of a metal parallel pin bulb base as shown in the above photo with the three wires ready connected for the tail and brake light functions and the motorcycle earth. At the other end is the special 3-pin connector which slots onto the terminals on the rear of the LED might board. The length of the wires are approximately 100mm (4″) which gives you more than enough slack to install the LED, but without having too much surplus wire to get in the way.
BA15S (1156) single-filament bulb adapter
The BA15S bayonet bulb connection is also sometimes known as a 1156, P21W or P25-1 type bulb which are all equivalent for current purposes.
The BA15S bulb adapter is different from the other two types described above in that the bulb it is designed to replace only has a single contact at it’s base connected to a single filament.
Brake lights were not standard fitment on many classic motorcycles right up until the late 1950’s and there was usually only a single brightness, single filament tail light for use at night. Most motorbikes will at some point have been converted to a dual-filament tail/stop light arrangement, but if your bike hasn’t, this is the bulb adapter for you!
The adapter connects the new LED light board to the tail lamp power supply and motorcycle earth via the BA15S single-contact bulb adapter. The power connection for the brake light is via a separate unterminated wire which can be connected to the brake light switches in whichever manner is most appropriate for your motorbike. The brake light uses the same earth connection as the existing tail light.
To identify if you bike currently uses this type of fitment, remove the rear light bulb and examine the number of contacts on the base and the position of the two tabs which protrude from the sides. A BA15S bulb has a single contact in the centre of the base for the single bulb filament. Looking at the metal base of the bulb from the side, the two protruding pins are on directly opposite sides of the base and at the same height so that the bulb can be installed either way round.
The BA15S adapter consists of a metal parallel pin single contact bulb base as shown in the above photo with the two wires ready connected for the tail light function and the motorcycle earth. The wire for the brake light supply is separate from the bulb base. At the other end is the special 3-pin connector which slots onto the terminals on the rear of the LED might board. The length of the wires to the bulb base are approximately 100mm (4″) long, whilst the third wire for the brake light supply is longer at around 300mm (12″) to give you maximum flexibility.
Other adapter options?
The four LED light adapters described above will, I think, cover nearly every classic motorcycle installation example I have come across so far. However if you think I have missed one or if you have a specific application in mind, please drop me a message using the contact form at the bottom of this page, or leave a comment below, and I’ll see what I can do!
Inquiry on specific application. I have a single filament LED (2 wire) and want to use it as running AND brake lights. I found this module which converts a single intensity LED to a dual intensity LED (Custom Dynamics Eclipze2 Turn Signal Module). I was planning on connecting the two wire to the LED and the three wire to the existing Harley wiring under the seat. Will this set-up work?
Hi Reed. Sorry, I don’t know anything about that product so can’t really advise. I imagine it uses a resistor to give a dim LED setting for the tail light, then full output for the stop light. Not something I would particularly want to do myself as the tail light may not be very visible. The only way to tell will probably be to try it. Regards, James
where can I purchase the BA15S (1156) single-filament bulb adapter from?
I don’t need any plugs just loose wires although if it comes with a plug it isn’t a problem’
Thanks for your time.
Thanks for a very informative website and all your hard work. I’m pretty sure I require the BA15D adapter but I’m not sure if the LED board will fit my rear lamp which is a MILLER type 37ET. These rear lamps are not as chunky as the Lucas lamps and have less space internally. (I can send images if required). I’m sure you will have been asked this before as many 50s and 60s machines were fitted with 6V Miller electrics (mine is Velocette). Look forward to your reply,