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Thread: How to install left horn/dimmer switch on 1972 BMW R75/5

  1. #1
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    How to install left horn/dimmer switch on 1972 BMW R75/5

    In an effort to give something back to the forum, I will offer a quick tutorial of my left switch install on the 72' R75/5. I will preface this that I am not a mechanic or electrician so please perform this at your own risk. I am sure there is a more professional way to perform this replacement but it worked and I am happy with the results. I was able to find an original switch from an eBay seller that was new old stock.

    Step 1: Disconnect the negative battery cable

    Step 2: Remove the old switch at the handlebars. There is a single small screw at the top and the switch will pop out. As you can see mine was in bad shape since it was missing the thumb button.
    IMG_0968.JPG

    Step 3: Since I had no use for the old switch or wiring, I simplified the process by just cutting it before the input into the headlight (be sure you have disconnected the negative battery terminal).
    IMG_0969.JPG

    Step 4: Feed the cable at through the switch and toward the headlight and attach switch to handlebar mount with screw.
    IMG_0977.JPG

    Step 5: To make it easier to feed the wires through the small rubber grommet on the rear of the headlight, I taped the end with a little electric tape. I also added a little delectric grease to the wire housing which helped to slide in the small grommet much easier.
    IMG_0975.JPG
    IMG_0978.JPG

    Step 6: It is not easy to access anything inside the headlight assembly unless you have really tiny hands but I was able to work my way through this by unplugging the three bulbs from the speedometer. Make sure you take a picture or mark the order.
    IMG_0972.JPG
    I pulled the old wire harness through the hole but kept the ends connected so I could keep up with what connections went where. There are three wires that connect to the top relay board red/green, yellow/white and solid black.
    IMG_0974.JPG
    It was simple to unscrew each connection (not all the way, just enough to release the old wire) then plug in the new wire and tighten. There is a clip on the right side of the housing that the holds the harness so be sure to slide it in after connecting the three wires.

    Step 7: Finally, there are two wires that connect to the headlamp bulb for the dimmer switch. They are yellow and white. These already had a butt connector and I decided to cut the two old wires and use another butt connector for each wire. I used ones that heat shrink so used a heat gun to secure them after crimping into the connector.
    IMG_0982.JPG

    Step 8: Time to test everything. Reconnect the negative cable and insert the key. I was pleased everything was working (including the horn) and I didn't damage any speedometer bulbs in the process.
    IMG_0984.JPG

  2. #2
    B Reams brook.reams's Avatar
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    Quote Originally Posted by yourpalcal View Post
    In an effort to give something back to the forum, I will offer a quick tutorial of my left switch install on the 72' R75/5. I will preface this that I am not a mechanic or electrician so please perform this at your own risk. I am sure there is a more professional way to perform this replacement but it worked and I am happy with the results. I was able to find an original switch from an eBay seller that was new old stock.

    SNIP ...
    Thanks for posting this. It should help anyone who has to replace the switch.

    Best.
    Brook Reams.
    Brook Reams - Arvada, CO
    Endeavor to Persevere
    Various Two-wheeled Vices, All BMW || Website: Airhead Rebuild Projects
    2004 R1150-RS||2002 F650-GS||1983 R100RS||1977 R100RS||1975 R75/6||1973 R75/5

  3. #3
    Day Dreaming ... happy wanderer's Avatar
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    @YourPalCal
    In step number 6 in the second photo I noticed a YEL/WHI wire next to a GRN/RED wire that appear to have melted insulation just before they go into the black cover sheathing together. They appear to have melted together a bit and this usually means that one of the wires has carried too much current or had a short circuit that melted the insulation.

    I'd pull them apart and repair the insulation to prevent further mishaps.
    MJM - BeeCeeBeemers Motorcycle Club Vancouver B.C.
    '81 R80G/S, '82 R100RS, '00 R1100RT

  4. #4
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    Quote Originally Posted by happy wanderer View Post
    @YourPalCal
    In step number 6 in the second photo I noticed a YEL/WHI wire next to a GRN/RED wire that appear to have melted insulation just before they go into the black cover sheathing together. They appear to have melted together a bit and this usually means that one of the wires has carried too much current or had a short circuit that melted the insulation.

    I'd pull them apart and repair the insulation to prevent further mishaps.
    That was actually a pic of the old wiring prior to removing. I appreciate the notation nevertheless.

  5. #5
    Quote Originally Posted by yourpalcal View Post
    That was actually a pic of the old wiring prior to removing. I appreciate the notation nevertheless.
    Your Headlight wiring needs some repair that is causing the heating on your headlight to switch wiring. Those ground wires are barely attached, which can cause heating problems and the wires you sliced to your switch to the headlight look really bad.
    Just and observation.
    Gator

  6. #6
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    Quote Originally Posted by gator View Post
    Your Headlight wiring needs some repair that is causing the heating on your headlight to switch wiring. Those ground wires are barely attached, which can cause heating problems and the wires you sliced to your switch to the headlight look really bad.
    Just and observation.
    Gator
    Thanks, I will check it out.

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