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Thread: How to test the starter relay (also, is this even the right relay?)

  1. #16
    Quote Originally Posted by Guenther View Post
    Your tests show that the starter switch and its connection to the relay is good. Your test 15/31b should have closed the contacts in the relay.

    Leaves the blue wires from the ignition light and D+. Disconnect the blue wire that goes to D+ on the diode board and put GND on the spade where D+ was connected to. Try the starter button now.

    That was it! When grounding the blue wire connected to the diode board the starter worked. So it was the diode board all along which also explains my charging issues. As soon as I get the chance I'll test the diode board.

    1973 R75/5

  2. #17
    Registered User Guenther's Avatar
    Join Date
    Jan 2007
    Colorado Springs, CO
    Very very unlikely the diode board. I think the brush-rotor-brush-GND path is not connected to ground. And this (GND) is what should be coming out of D+ when the motor is not running.

    Could be the brushes' connection to the rotor or, the rotor itself. The resistance of the rotor should be in the 3-4 Ohm. Could be that the wires from the brushes are loose, or one/both brushes are hanging up in the air. But that's the area to look at.

    2017 F700GS

  3. #18
    B Reams brook.reams's Avatar
    Join Date
    Jan 2007
    Arvada, Colorado
    Quote Originally Posted by jakobw View Post
    That was it! When grounding the blue wire connected to the diode board the starter worked. So it was the diode board all along which also explains my charging issues. As soon as I get the chance I'll test the diode board.

    Hi Jakobw,

    If you want the details of how the /5 starter relay with it's transistor work to prevent engaging the starter motor when the engine is running, I documented it here:

    --> Starter Relay

    You will learn that the D+ terminal on the diode board (blue wire on the back of the board) is part of the ground path for a variable resistor that is used to apply the correct voltage to the transistor base to turn it on. If there is no path to ground via the diode board D+ terminal, then the transistor prevents the stater relay coils from getting current so the relay doesn't close.

    The path to ground for the D+ terminal of the diode board is via the voltage regulator D+ terminal and DF terminal. It's the DF terminal that connects to the alternator brushes which provide the ground. SO, anything along the path from the stater relay D+ terminal though both alternator brushes that has failed will prevent you from using the starter motor.

    I mention this because brushes not touching the alternator slip rings could be the cause, the voltage regulator could be the cause, and any wire in that path could be the cause.

    Best of success isolating the fault.

    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||1973 R75/5

  4. #19
    If new graphite brushes are needed I recommend getting them from Rick Jones at Motorrad Elektrik because his brushes now have small ring terminals crimped on the braided copper wire (which needs to remain flexible). It used to be that the braided copper was soldered to the generator. The braided copper looks a lot like solder wick and works like it as well. I don't often need to replace the brushes and it seems that a couple of tries were needed to get the right touch. The first time or two I took too long and the braided copper sucked up solder and was no longer flexible. One could also obtain small ring terminals and crimp them to old style brushes.

    The brushes with the ring terminals are shown below -

    If you are on a road trip and it seems that the brushes are so worn that they are not making good contact with the rotor one could attempt a field expedient repair. As I recall this tip came from Oak (one of the now deceased airhead gurus). Remove negative lead from battery. Remove front cover so that generator is exposed. If it seems like the brush is so short that the spring can't adequately force the brush against the rotor, obtain some old fashioned paper matches. Remove match and match head. Fold paper match in half. Gently lift spring from the top of the brush. Place folded paper match atop the graphite brush. Replace spring atop the paper match. Do for both brushes. [Note: this is likely much easier said than done for the back brush!!]. Replace cover and re-connect battery. This temporary fix should allow one to get back home so that a more permanent fix can be made.

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