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How to Check an Old Motorcycle Battery
- Set your voltmeter to DC volts in a 20v range.
- Lift the motorcycle seat to get to the battery.
- Static: Place the positive and negative leads of the voltmeter to the positive and negative terminals on the battery (positive goes to positive, negative to negative.)
- Read the voltage on your voltmeter with the bike off, if it’s under 11-10v, the battery needs a charge.
- Start load: Start the motorcycle with the voltmeter leads still touching the battery terminals.
- Read the voltage, if it drops below 9.5-9v, the battery will likely need to be replaced.
- Charging system: Start your motorcycle with the voltmeter leads connected to the battery terminal.
- Bring the engine to about 3,000 RPM and read the voltage, it should read around 14v
- Refer to your owner’s manual for specific RPM ranges and expected voltage readings.
How to Check for Voltage Drops on Your Old Motorcycle
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You can check for voltage drops anywhere where voltage is expected to flow. (dim lights, lights won’t work, old switches, components, etc.)
- Place the positive end of your voltmeter (set to DC 20v range) to the positive terminal of your battery.
- Place the negative end of your voltmeter to the place you’d like to check (headlight power, for example)
- A positive voltage reading on your voltmeter indicates a voltage drop at the electrical component you’re checking.
- Drop could be caused by loose connections, dirty/corroded wires and connectors, etc. See below for cleaning details.
Checking and Cleaning Motorcycle Wiring Harness
Old motorcycles that have been sitting for a decade or two or that have been stored improperly are notorious for all sorts of electrical issues.
Expert tip: Before you get too deep into any electrical problem, be sure to check the easy stuff first:
- Are fuses blown?
- Is the kill switch on/off?
- Is the ignition in the right position?
- Are the plug wires connected?
- Is the battery charged?
- Is the headlight switch on?
- Did you turn the fuel tap on?
Once you’ve ruled out the simple issues, then you can move into a more thorough inspection and check for specifics. Again, it’s useful to still keep the simple things in mind:
- Does the wire have continuity?
- Is the wire’s ground solid?
- Is voltage getting through?
Sometimes a thorough cleaning and inspection of your bike’s wiring system can be the difference between a motorcycle that run and one that doesn’t.
Overtime the bike’s electrical connections may have come loose or have been exposed to moisture and corroded.
Weak, loose, and corroded connection can all lower the voltage the flows through the electrical systems on your bike. Your old motorcycle may not start, the lights may be dim, the engine may turn over slowly, and the spark may be weak – all contributing to an unreliable and poorly running machine.
What You Need To Check and Clean Your Motorcycle’s Wiring Harness
To start checking and cleaning your old motorcycle’s electrical system, you’ll need the following:
- The bike’s wiring diagram – found in the service manual.
- Digital voltmeter/ohmmeter.
- Electrical contact cleaner that’s safe on plastic.
- Needle-nose pliers.
- JIS screwdriver to remove headlight bucket.
- Brass wire brush.
- Small cleaning tools – carb cleaning wire, toothpicks, cotton swabs.
If a part of the electrical system on your motorcycle is not function, you can first check the connection with your digital ohmmeter to check for resistance. You’ll be able to rule out any poor connections and disconnected or broken wires.
- Refer to your motorcycle’s wiring diagram to check which color wires go where.
- Disconnect the wire(s) you want to check.
- With the bike off, place the positive connection of your ohmmeter at one end of the wire and negative at the other to check for continuity.
- If the ohmmeter does not move, the wire may be broken somewhere.
NOTE: When you are checking voltage with a multimeter, make sure the meter’s range is set above the voltage you are measuring, otherwise you may blow a fuse in the multimeter.
If your wiring harness is in really bad shape, you can create your own simplified version.
Check the Battery & Charging System
If electrical problems persist, check that the battery is charged and holding a charge.
If your battery is fully charged it should read 12 volts or higher with the motorcycle off. With the bike running and under load, check the battery voltage again – refer to your shop manual for the volts it should be putting out – if it’s reading too low, there’s likely a problem with your battery.
If voltage is too high and the battery is getting hot, there’s likely a problem with your charging system.
Check that the main fuse is not blown. Open up the fuse box and check that the main fuse is not broken or loose. If the bike is old and/or new to you, it’s probably a good idea to replace the fuses anyway.
Refer to your shop manual to check what voltage should read at certain components when the bike is on or running, and test that they’re working properly.
How to Clean and Repair a Motorcycle Wiring Harness
To clean your motorcycle’s electrical connections, begin at the front of the bike and unscrew the headlight to expose the wiring in the headlight bucket:
- Unplug one connection at a time so you don’t mix things up.
- Spray the connection with electrical contact cleaner and brush or scrape off any corrosion you find.
- Wipe the connections clean and reconnect them.
- Make sure that the cables and wires don’t feel brittle or hard when you move them – this may be an indicator that they’re corroded on the inside.
- Check that battery terminals are clean and have solid, tight connections.
- Check that all frame grounds are clean and tight.
- Clean fuses and fuse holders, replacing if necessary.
- Check for connections to the alternator, points plate, and neutral and oil lights – they may be hidden behind engine cases of under the engine.
- Check for solid, clean connections and any loose wires to the ignition coils and spark plug wires.
- Unplug and reconnect brake light connections.
- Clean turn signal and tail light bulb terminal sockets.
If after thorough cleaning, de-corrosion, reconnecting, and voltage/current checks there are electrical parts that still don’t work you may need to:
- Check that no wires are being pinched and that there are not severe bends in the routing of wires – either under seat hinges, gas tanks, or when the handlebars are turned.
- Inspect and clean the handlebar switches.
- Be careful when opening switches, as small springs and bearings may come loose.
- Spray with contact cleaner and remove any corrosion you find.
Check connection as per the service manual with your digital ohmmeter
With some patience, a simple cleaning and inspection might be all it takes to get your old motorcycle running better again. If electrical performance improves, but overall performance is still lacking, you may just need to go through and perform your bikes standard service/tune up items – valves, timing, carbs.