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Modification of K-bike Turn Signals to Dual-Filament Bulbs

by Jeff Dunkle

This procedure is my adaptation of one posted some months ago by Mick McKinnon which has worked, he’s recently reminded me, successfully, for many miles on his K/LT.

My bike’s an ’86 K75T with that little cowling around the headlight. So I can’t attest to the complete accuracy of this method for other K models. But judging from Mick’s use on an LT…it should work. On my bike, all four turn signal assemblies are identical, both externally and internally. I’ve included a hideous amount of detail to try to help someone who’s not tackled a project like this before. Once the shopping is done, the job should only take an hour or so. Two at most. The second time through on a friend’s bike should only take about 45 minutes.

Good luck!

Basic Strategy:

We’re going to tap current from the parking lamp in the headlight assembly and from the tail/license plate lamp in the rear assembly to power the running lamp filament in the double filament bulb we’re going to install in each turn signal assembly. The existing wiring will power the turn signal lamp. Dual filament 1157 are the standard bulb for car turn signal assemblies. These bulbs have a high resistance, low current filament for the running light, and a lower resistance, higher current filament for the turn signals.

The existing turn signal lead in your bike will be connected to the turn signal filament in the new bulb via a soldered wire and blade connector. The running light filament will be powered through the modified socket contact by our added power tap in the front and back.

The existing signal lamp sockets will be modified to allow use of the double filament bulbs.

Suggested Tools:

  • medium duty soldering pencil – 25 watts is fine, rosin core solder
  • wire strippers/crimpers
  • tin snips (or similar tool)
  • Phillips screw driver


Note: The following materials are the quantities needed to modify all four lights and make two spare bulb assemblies. I suggest you get extras of each to allow for practice and mistakes in such things as mis-crimping a connector, etc. All of these materials should be obtainable at a good auto parts store or hardware store with electrical supplies.

  • about 6 feet of #18 (OR LARGER) wire
  • 6 each 1157 bulbs (the dual filament turn signal bulbs
  • 2 each male blade connector doublers
  • 6 small male blade connectors
  • 2 each female blade connectors large enough to fit over one of the doubler male blades
  • 4 each small female blade connectors
  • 6 each small male blade connectors that fit the females in the line above

Modifications to the 1157 dual filament bulbs:

  1. File the locking pin off each bulb of the six bulbs that is furthest from the base. Now put two of the bulbs aside as a test fit bulb to help you modify the sockets. Do the soldering described to the other four bulbs.
  2. Cut four lengths of wire a bit over two inches long. Strip about 1/4″ inch of insulation from each end.
  3. Hold each bulb base up, with the remaining locking pin pointed away from you. Solder a length of wire from step 2 to the left contact, with the wire pointing toward you. This positions the wire to have minimum interference later. This is is the turn signal filament connection.

Front turn signal modifications:

Additional wiring installation

  1. Remove turn signal lenses, lamps and disconnect socket bases from wires and remove them. Note which wires come from which connectors. The brown wire on BMWs is always a ground wire. The other wire is the power lead for the turn signal. This wire will eventually be connected to the soldered wire on the dual filament bulb. We will connect a different, to be installed ,wire on the base connector that this wire came from.
  2. Remove enough body work to access the back of the headlight connector.
  3. Disconnect the female blade connector from the “parking” lamp in the headlight assembly. Install one of the male blade doublers onto the existing male contact for the parking lamp. Reinstall the original female connector onto one of the doubled blades. If the existing female connector doesn’t fit the new male blade, install a new, larger female connector onto the bike wire.
  4. Take two lengths of #18 (or larger) wire about 16-18″ long. Strip about 1/3″ inch insulation from one end of each length. Twist the stripped ends together.
  5. Crimp a female blade connector to these twisted wires. Install this blade connector to the other male blade on the doubler on the parking light.
  6. Reinstall the headlight assembly taking care to reposition the adjustment knob bracket properly. Bolt the assembly down to the frame. Snake one of the newly installed wires through each turn signal stalk and reinstall the turn signal brackets. Leave enough of the remaining fairing off for the moment to permit headlight removal again, if necessary.
  7. Cut the excess off the wires just pulled through the turn signal stalks, leaving about 4 to 6 inches of excess so that the wire can be easily inserted again should the fairing and turn signals ever be removed. Strip the end and crimp on a female blade connector.

Socket modification

This step will modify the existing single filament base so that the existing contact will touch the running light filament button on the base of the dual filament bulb.

  1. Temporarily twist one of the dual filament bulbs (with the upper lock pin filed off but without a wire soldered to it) into the socket. Note that one side of the socket interior has a metal side next to the bulb base. Insert the lamp so that the remaining locking pin engages that metal side of the socket. Twist till locked.
  2. Note that the base contact covers both contact buttons on the bottom of the bulb. Scribe a scratch on the contact so that a cut there will permit the metal to touch only the nearest bulb contact button. Remove the bulb and carefully cut the contact where marked. A little bending and filing may be in order to reposition the contact properly. Test fit the bulb to be sure of adequate contact on only the near button.
  3. Repeat for the other socket.

Socket/Bulb Installation

  1. Insert a modified bulb with a wire soldered to the one button, into a modified socket. Connect the brown wire to the ground lug of the socket (THE SAME ONE IT CAME OFF OF…THE ONE THAT CONTACTS THE SIDE OF THE BULB BASE).
  2. Connect your newly installed wire to the blade that contacts the one bulb base button.
  3. Connect the original colored lead to the blade connector soldered onto the bulb. If the original female connector doesn’t fit, cut it off and install a female blade connector sized to match the male on that lead. Connect these wires.
  4. Repeat the above steps for the other side.
  5. Carefully allow the sockets/bulb assemblies to dangle without touching any wires together. Test the lights. If they work correctly, proceed. If things don’t work properly, first test the lights with to be sure they’re not burned out. Then trace wiring to check all connections. Be sure that the double blade connector in the back of the head light isn’t bent against the body work, etc. Be sure all crimped on connectors are properly installed.
  6. When things are working properly, snap the light/base assemblies into the lenses, and reinstall the lens assemblies.

Congratulations! You’re over half done….the rear goes faster.

Rear Turn Signals:

This is essentially a repeat of the above sequence. Here we’re going to tap current from the lower light which is the tail light/license plate lamp.

  1. Remove the turn signal lenses, remove the existing bulbs, disconnect and remove the socket bases.
  2. Remove the tail light lens. Access to this is via thumb screws inside the rear compartment on my bike, at least.
  3. Disconnect the female blade connector going to the lower lamp. Install a male blade doubler on this connector.
  4. Reconnect the existing female blade connector to one of the doubled male connectors.
  5. Fashion a twin lead wire with approximately the same length leads as done for the front. Take two pieces of wire about 16 inches long, strip one end of each, twist together, and crimp on a female blade connector.
  6. Connect this wire assembly to the other doubled male connector.
  7. Snake one of these installed wires through each turn signal base.
  8. Reinstall (TEMPORARILY) the tail light assembly. Check to be sure that the doubler connector doesn’t impact other wires or structure when things go back together.
  9. Pull the new wires through each turn signal assembly. Cut off excess wire, leaving about a 4″ inch excess (TO FACILITATE RE-SNAKING IF THE TAIL LIGHT IS EVER DISASSEMBLED AGAIN).
  10. Strip 1/4″ inch of insulation and crimp a female blade connector to each wire.

Base Modification:

Modify each rear turn signal light base exactly as was done for the front.

Light Modification:

You should have two more bulbs with leads already installed. Make two more as earlier directed, if necessary.

Install a bulb in each socket and reconnect the socket base wires.

  1. Brown wire to ground lug.
  2. Installed wire to the base contact.
  3. Original turn signal connector to the soldered on lead. Again, install a replacement female blade connector if the one on the wire is too small for the crimped on male on the soldered wire.
  4. Allowing the sockets to carefully dangle without touching anything, test the newly installed lights.
  5. If they work, snap the sockets into the lens assemblies and re-install the lenses to the stalks.

If things don’t work, first test the bulbs for damaged filaments. Then trace all connections to be sure they’re proper and that crimped on connectors are properly installed.

Reinstall all remaining body work.

Now, fabricate at least two spare 1157 bulbs to cary along. File off the upper lock pin and solder on a 2″ lead with a crimped on male blade connector as discussed above.

Congratulations. Done!

Please contact me with any corrections you note in your work or any questions about this installation.

Ride Safe,
Jeff Dunkle

  | Jeff Dunkle                        '81 BMW R100/7 "The Black"   |
  | Boof #17, K-whiner #41             '86 BMW K75T "Shadow"        |
  |              Assorted Other Bikes       |
  | Four Winds BMW Riders                   (minus one)             |
  | Pittsburgh, PA                                                  |
  |                                                                 |
  | "We must learn and make practical in our everyday lives the law |
  | of Good Relationship."  Brooke Medicine Eagle                   |

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