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'72 R75/5 Front Brake Light Wiring

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David Wallace
Posts: 25
Eminent Member
Topic starter

Up front disclaimer - I've never been very good at diagnosing electrical issues.  Symptoms:

< brake light comes on when activated by the rear brake lever switch.

< brake light does not come on when (just) the front brake lever is pulled.

< I disconnected the two wires (green with red stripe and green with black stripe) from the front brake lever switch so I could check the function of the switch by itself.  Checking for continuity with an ohm meter shows the switch itself is working fine: open when the lever is in at-rest position, closed circuit when lever is pulled.

< the strange thing is that the two wires, whether connected to the switch terminals or not, show a closed circuit with the ohm meter, yet this condition is not causing the brake light to come on.

Any ideas what could be wrong, where to check?


This topic was modified 3 months ago by Richard Whatley
Posted : 06/13/2022 15:27
David Elkow
Posts: 191
Estimable Member

For the best help, please tell us your year and model. 

with the wires off the switch and key OFF, a jumper wire from the battery+ to the green/black wire (I believe) should light the brake light. That will check the back half of the circuit. 

with the wires off the switch and key ON, the green/red wire (I believe) should test 12v with a voltmeter (or test light) to GROUND. That will check the front half of the circuit. 

try to narrow it down. 

Posted : 06/13/2022 17:50
Richard Whatley
Posts: 2255
We need your year and model to be more precise.
Posted by: @david-wallace

Up front disclaimer - I've never been very good at diagnosing electrical issues.

That's OK. Help is here. 

► We will assume that the horn still works. The horn and front BL are on the same circuit and fuse. A working horn proves the governing fuse is 'good'.

► We will assume that all testing is done with the ignition switch in the ON position. Test this by activating the rear BL switch.

► We will assume that your bike will not crank while in-gear, unless the clutch lever is pulled in.


Testing Procedure

1. Start by connecting the 2 front brake lamp switch wires together using a short length of small copper wire. (That's what the switch is doing internally.) If the brake lamp comes ON, then the switch is the issue. Replace the switch. Treat the 2 male pins with anti-oxidation compound before re-connecting any harness connector, including those on the switches.
2. If the brake lamp does not illuminate, then check for the presence of electrical power, usually on the Green/Blk wire. Do this by connecting your mechanic's test lamp to an unpainted cyl head fin and then touching the metal part of the Green/Blk connector.

Test Lamp

3. If there is no power on Green/Blk, then (on a handlebar master cyl model) follow the 2-wire cable down under the fuel tank. Right under the front of the fuel tank will be a tiny 2-pin connector. Make sure that is plugged in.
4. Disconnect the 2-pin connector, and using a short length of small copper wire, connect the 2 contacts in the female harness connector together. The rear BL should illuminate. Treat the 2 male connector pins with an anti-oxidation compound before reconnecting. (Don't confuse [or inter-connect] the FB connector with the identical 2-pin connector from the clutch lever switch.)
5. If there is no BL illumination, then use the Test Lamp to test for electrical power on Green/Blk.

6. Testing the connection inside the main harness between the BL switch and the BL bulb is done by using a 4ft length of insulated copper wire to apply electrical power from the battery Positive (+) terminal to the Green/Red connector.


► However, Airhead harnesses rarely if ever have issues, unless the harness has been cut and re-spliced (as in a shoddy wreck rebuild). Since the BL bulb works with the Rear BL switch, we know all connections at the bulb are good. I believe your issue lies at the BL switch or the contacts connecting the wires to the switch. So you will most proabably solve this dilemma at Step 1 or Step 2.

► Application of an electrical anti-oxidation compound, such as No-Ox-Id, on ALL male connectors and male contacts you find, see, come close to, touch or otherwise come within an arm's length of during this work is STRONGLY advised. This includes male electrical contacts on electrical switches, relays, harness connectors, etc, etc. This also includes the multi-pin box connector for the rear of the harness, that is next to the battery where the harness jumps onto the rear fender.


Hope this helps.

This post was modified 3 months ago 2 times by Richard Whatley

[color=blue]Don't hide 'em, Ride 'em !!

Posted : 06/14/2022 07:02
David Wallace
Posts: 25
Eminent Member
Topic starter

David, Richard    -

Thanks for your suggestions.  Bike is a '72 R75/5.   With your input I've concluded it must be the switch because touching the two wires together does indeed make the BL come on (don't know why I didn't try that earlier, duh).   Though I was a bit mystified because the ohmmeter does seem to indicate a connection in the switch when the plunger is extended, but when I connect up the wires to the switch and pull the lever - no BL.  Another simple test I performed was to remove the switch from the brake lever (to rule out that the switch mounting was somehow an issue) and connected the wires and manually activated the switch plunger - no BL.  So why the heck I got a closed contact indication on the switch with the ohmmeter will forever remain a mystery.  Oh well.  I believe problem is solved.  Thanks again.

Posted : 06/14/2022 12:01


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