Starting Issue on a Harley Davidson

Today at Rewired C&C, we investigate a starting issue on an old Harley Davidson motorcycle.

A quick look at the motorcycle itself.

The Issue

The motorcycle was suffering from an issue preventing it from properly starting. When you turned on the ignition and pressed the start switch, the starter motor sounded like it was trying to start, but mostly clicked - a symptom typically associated with a flat battery. However, even with a charged battery, this issue persisted.

The owner found a workaround: adding an extra switch to bypass the starter circuit would allow it to start. Knowing that the battery checked out and the starter physically worked, they brought it to us to investigate the starting circuit.

A switch was previously added to the starter, bypassing the starter circuit.

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The Investigation

The starter is controlled by a starter switch on the handlebars. This gains a supply from the ignition switch and sends the start signal down towards the engine once pressed. However, the current draw of a starter motor is far too high to be controlled directly by such a small switch, so a starter relay is used. When the relay receives the low current signal from the start switch, it triggers a high current output to power the starter motor.

The starter switch is situated amongst the handlebar controls.

The starter relay is fitted under the rider's seat. The relay was disconnected from the socket and tested seporatly. For example, when providing a ground and a live across its control coil, we heard an audible click, showing that the coil is properly engaging the contacts inside. Additionally, when the relay activates, the contacts join and allow current to flow with good continuity as measured with the multimeter.

The starter relay is located under the rider's seat, above the main battery.

With the relay verified, the motorcycle side of the connection was tested next. The relay had a good ground connected to the control coil, and was receiving a signal from the start switch too. By directly supplying the output of the relay using a link wire, the starter motor engaged, verifying the connection between the relay and the motor. However, when checking the power supply to the relay, the volt meter revealed a noticeable power drop of about 1 volt, which worsened when the starter tried to turn.

The supply to the relay is provided by the ignition switch. When the relay is activated, this ignition supply is directed towards the starter to power the motor. If this supply is hindered, the appropriate amount of power will not be transferred to the motor.

The ignition switch, along with much of the other circuitry, is located near the fuel tank, under a switch/display console.

A top view look at the ignition switch and console.

Removing this lid reveals the main ignition switch - a large circular switch that directs power to a multitude of subcircuits around the motorcycle. A number of bulbs and resettable fuses are also exposed.

The main ignition switch that supplies many of the motorcycle's circuitry.

The voltage across the battery was approximately 12 volts with the ignition switched off and could be observed at the supply going to the ignition switch. With the ignition switch activated, a similar supply was measured at the switch's output. A small power loss was expected, as bulbs and other components were being powered.

However, the output supply to the starter motor resulted in a higher loss than the other outputs, such as the supply to the starter switch. As this was measured at the switch, this suggested an issue with the ignition switch itself, such as tarnished or burned-out contacts within, causing a resistance and hindering the supply.

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The Repair

The optimal solution would be to either replace or repair the ignition switch. However, these switches are becoming scarce items to source, with a high likelihood of breaking when taken apart. With a budget being a factor in the repair, a cheaper solution was found.

On most motorbikes and cars we work on, the starter solenoid has a direct feed from a battery, sometimes through a fuse, rather than taking a supply from an ignition switch. Though there are exceptions to this rule, especially with some vintage vehicles.

Our findings were that the ignition switch was perfectly capable of supplying the various components of the motorcycle, but could no longer cope with the high demand of the starter motor, likely taking on some damage at some previous point. To resolve this, we removed and insulated the original supply wire from the starter relay's socket, replacing the connection with a direct supply from the battery itself. Though the starting supply is taken from a permanent supply, the start signal remains sourced from the switched ignition output, requiring the bike to still need the key to start.

This repair resulted in two benefits: providing a higher current supply from the battery to power the starter motor, and retaining the original supply wire, which is insulated at the end. This allows the starter to start the engine more easily without putting a strain on the ignition supply. Additionally, by unplugging and insulating the supply wire's terminal, rather than cutting the wire, it makes it easier to restore the bike to a more original condition. If the owner decides to source a replacement switch at a later time, the waterproof insulation on the original supply wire can be removed, and the terminal can be re-placed into the socket.