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The Motorcycle Electrical Diagnosis Page...

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Electrical problems can exist in any motorcycle, new or used, and can be particularly daunting at first to try to diagnose. This page is designed to help you isolate your electrical problems and solve them. There is a separate page for headlight-related electrical issues.

General Electrical Problems

Electrical problems on your motorcycle can sometimes be easy and sometimes dauntingly difficult to diagnose and fix, depending on where the problem lies. We'll break down the possible issues into areas and symptoms and take a look at each:

1. Battery Will Not Hold Charge (Wiring issues):

  • Symptom: Bike will not start without a jump when left overnight or for a longer period of time. Will sometimes start again within minutes/hours of having been started without exhibiting the same symptom, but symptom will reoccur if left overnight or for a few days.
    Additional Symptom: Disconnecting the battery overnight, or leaving the battery connected to a charger when not in use alleviates the problem for that time period.
  • Probable Cause: wiring insulation is bare on some circuit that remains hot even with the ignition off, causing a short-circuit that drains the battery. Most common on motorcycles over 5 years old or with mileage in excess of 15K miles.
  • Trouble Shooting: With the motorcycle ignition turned to off, remove the negative lead on the battery, then use a volt meter (set to DC amps, range 12 or higher) to measure between the battery's negative terminal and the negative lead. If the measure is anything other than zero, either you have an accessory that is constantly on (like a fuel gauge or clock), or you have a wiring short (or both). If you do not have any constant power accessories, you have tracked down the preliminary source of the problem. If you do have constant-power accessories, disconnect each of them at the accessory, then remeasure at the battery. If the amperage draw has not dropped to zero, then you have again found the preliminary source of the problem. If the reading is zero, skip this section and go on.
  • Corrective Action: Warning: time consuming
    At this stage, the problem is locating the wire(s) that is/are bad. Sometimes you'll get lucky and visually see that a wire is chaffed or abraded. If not, you need to start by removing every blinker, brake, running light and headlight bulb on the vehicle, one at a time, taking another amperage reading at the battery negative lead between each one. If the reading drops to zero at any point, then you know that the problem lies in that particular segment of the wires (either going to or returning from the bulb). We start with the light bulbs, because they are easy to remove and are the items usually the furthest away from the battery (therefore the most likely wires to fail). If, having removed all the bulbs, you still have exactly the same amperage draw, then you know that the wiring to the lights are OK, and you can reassemble them.
    After having gone through the lighting wires, the other wires may prove more difficult (starter, starter relay, gauges, fuel pump if applicable, etc.) Repeat the same procedure -- disconnect one item at a time until the reading drops to zero.
    Once you have located the wire that is causing the problems, trace the wire all the way through the wiring harness, then find brand new automotive-rated multi-stranded wiring in the same gauge or a lower gauge (i.e. - 16 gauge wire can be replaced by 14 gauge, but not visa-versa). Cut out the majority the bad wire, leaving about an inch on each end for the new wire to connect to. Cut the old wire in various places and check for the existence of corrosion in the wire -- if present, check both ends of the old wire where you will attach the new wire to, to ensure it does not have any corrosion; if corrosion is present, you will need to replace the entire wire including both ends. If no corrosion is found, connect the new wire in the old wire's place. The preferred method is to braid the wires together and solder them, then cover the connection with heat shrink tubing to ensure no water can penetrate. If this is not an option, use butt-splice connectors at each end, liberally coating each end of the wires with dielectric grease first (to ensure the bare wire inside the butt-splice won't corrode later), and then wrap the connections with weather-proof electrical tape (i.e. - plasticized or rubberized electrical tape, not fabric-based electrical tape). Reassemble everything and retest.

2. Battery Will Not Hold Charge (Battery issues):

  • Symptom: Battery will not hold charge at all, or will not hold a charge for seven days, even when disconnected from the motorcycle.
  • Possible Cause: Battery is incapable of holding a charge for longer than a day & less than a week. Most common on bikes lacking maintenance for longer periods of time, motorcycles left standing through winters or vacations without a trickle charger, and bikes over 3 years old. Also occurs in motorcycles with problematic charging systems. Do all three troubleshooting tests if this section applies.
  • Trouble Shooting (Part A) -- Battery Acid/Water levels: NOTE: sealed, sealed-AGM and 'gel' batteries have solid black case sides and do not get water added.
       If battery case's sides are transluscent, disconnect the battery from the motorcycle (negative terminal first) and remove and set on a flat surface. Visually inspect water levels by shining a flashlight from behind the battery and looking at the battery from the front. If water levels are found to be low compared to markings on the case, use corrective action (next item).
  • Corrective Action: With battery separated from motorcycle and on a flat level surface, gently remove caps to battery cells, and using ONLY DISTILLED WATER (not tap water) and a small cup with a pouring spout, or a funnel, or just a straw (dipped in the water, cover the far end with your finger, then put it at the top of the cell and release), fill up the water levels to the recommended levels.
    CAUTION: Battery acid is very caustic and can easily injure you and damage your clothing and whatever else it contacts. Take extreme care not to over-fill the cells, nor to splash the acid out of the cells. If you get any battery acid in contact with your skin or clothing, remove the clothing and flush the skin for 10 minutes with tap water, until no sensation of burning is present. If any damage has been done to your skin, seek medical attention. If you get any battery acid in contact with your eyes, flush under running water for 20 minutes and then seek medical assistance. You are solely responsible for your own actions and safety (including wearing eye protection and acid-resistant gloves) is highly encouraged!
  • Trouble Shooting (Part B) -- Standing Battery-Circuit Voltage Measurement:
    Having already verified the battery's water levels (electrolyte levels) are filled correctly or that your battery is maintenance-free, set your multimeter to DC Volts (range 20 or higher), then measure between the positive and negative terminals of the battery (engine off, preferably battery out of bike).
      Reading should be at least 12.3 volts if charged at all. If below 12.8, connect the battery to a low-power charger (1.0 amp-hours or less; trickle or float charger preferred) and permit to charge until full (usually overnight) with the battery cell caps off non-maintenance-free batteries, in a well ventilated location free of any fire or spark sources (not around stoves, gas/oil/diesel furnaces, gas-powered water heaters, etc.).
    Retest. If after charging, with the battery disconnected from both the charger and the motorcycle, the battery voltage is not at least 12.6 volts, replace battery. Note that a new battery may temporarily alleviate symptoms without correcting the original cause of the problem, if the problem is a bad charging system (see next section) or a ground-fault short in a wire (see previous section of this document). A new battery should read approximately 12.8 to 13.2 volts when charged.
    If battery measures over 12.4 volts when fully discharged (too weak to start the bike) or over 12.7 volts when fully charged, battery is presumed good for our purposes.
    CAUTION: As seen in numerous action thrillers, a fully charged battery can easily electrocute a human. A large one can also easily fling you 20 or 30 feet. Be careful when measuring the battery to not touch the leads nor the contacts with your skin or metal attached to your body! Hold the test probes by their insulated handles only. You are solely responsible for your own actions and safety is highly encouraged!
  • Trouble Shooting (Part C) -- Standing Battery Voltage Measurement:
    Having already completed portions A & B above, measure the voltage and write it down. Then, without reconnecting the battery to the bike, leave it sitting for twelve hours (disconnected from everything - charger & bike), and remeasure the voltage again. A healthy, charged battery will not have any measurable voltage drop in a twelve hours, and may have a voltage rise. If the voltage dropped, replace the battery with a new one; this one is defective.
  • Trouble Shooting (Part D) -- Running Battery-Circuit Voltage Measurement:
    Having already completed portions A thru C above and taken whatever corrective action was listed, reinstall the battery in the motorcycle (connect positive terminal first, then negative terminal). Start the motorcycle (by whatever means necessary -- even push-starting if necessary). Park motorcycle in neutral with engine running, on center stand if present, on racing stand if no center stand is present, and on side-stand with a chock-block under the front wheel if neither other option is available.
    Set Multimeter to DC Volts, range 20 or higher, and then measure between the positive and negative terminals on the battery with the motor running, revving the engine from idle to 5k while measuring.
    If the reading is 12.7 volts or lower at all times, then the charging system is not charging correctly (under-charging). Go to section 3B marked "Charging System Under-Charging".
    If the reading is between 12.7 volts and 14.3 volts at all RPM's between idle & 5k RPM, the charging system is working correctly. Return to section 1 and retest.
    If the reading at idle is 14.4 volts or over, or the reading at any RPM is over 14.9 volts or higher, and the battery was fully charged before you put it into the bike, then the charging system is not charging correctly (over-charging). Go to section 3A marked "Charging System Over-Charging".

3A. Charging System Over-Charging:

  • Possible Symptom #1: Headlight and instrument backlighting brightens, dims or flickers with RPM, battery will not hold charge for normal period of time (overnight, etc).
  • Possible Symptom #2: Motorcycle blows fuses regularly.
  • Possible Symptom #3: Batteries fail regularly, must be replaced.
  • Possible Symptom #4: Known good, fully charged battery is hot to the touch after bike had run (hotter than the plastics & metals around it).
  • Probable Cause: Charging system is over-charging.
  • Trouble Shooting: complete Battery tests above first. If readings from "Trouble Shooting (Part D) -- Running Battery-Circuit Voltage Measurement" are over range, consider rectifier or rectifier pack bad.
  • Corrective Action: Replace rectifier pack and restart testing from step 1. If this does not cure it, replace alternator/stator/generator (depending on bike model -- whichever produces the electricity on your bike)

3B. Charging System Under-Charging:

  • Possible Symptom #1: Headlight and instrument backlighting dims or flickers with RPM, battery will not hold charge for normal period of time (overnight, etc).
  • Possible Symptom #2: Motorcycle sputters when running, misfires.
  • Possible Symptom #3: Batteries fail regularly, must be replaced.
  • Possible Symptom #4: Batteries have a lower voltage reading after riding than before riding the bike.
  • Probable Cause: Charging system is under-charging or not charging at all.
  • Trouble Shooting & Corrective Action: complete Battery tests above first. If readings from "Trouble Shooting (Part C) -- Running Battery-Circuit Voltage Measurement" are under range, proceed. Remove primary leads to alternator/generator and check for continuity as per electrical diagram. If continuity can not be established, replace. If continuity is valid as required, replace only regulator/rectifier pack if voltage is out of range while engine is running and battery is in range with engine off. Restart testing from step 1.

4. Bad Ground Circuit:

  • Possible Symptom #1: Blinker flashes cause other items to flash in turn (such as brightening and dimming of gauges, indicator lights).
  • Possible Symptom #2: One or more fuses fail frequently.
  • Probable Cause: Defective Ground Circuit Path.
    This commonly happens when a negative grounding point (the location where the negative leads on the motorcycle are routed to ground) no longer maintains a good contact due to corrosion at the connector, or in the wire leading to the connector. Since electricity always follows the path of least resistance, if it encounters resistance at a grounding point that is shared (or if the wire is spliced further up the line), the flow will follow a different path, causing other items in the newly formed alternative circuit to take the excess power. This overflow may be visible as brightening/dimming of gauges or other bulbs on the motorcycle.
  • Trouble Shooting & Corrective Action: Battery Grounding Cable
    Note this really falls under category 1, wiring problems.
    Examine the negative lead to the battery on both ends and verify that no corrosion is present at the connectors and/or within the cable. If connectors are corroded, sand with emery cloth to remove corrosion and coat with vasoline or dielectric grease. If cable itself is corroded (i.e. the wires within the insulation), replace the battery grounding cable.

  • Trouble Shooting & Corrective Action: Bad Ground Point(s)
    Note this really falls under category 1, wiring problems.
    Locate the item(s) which are causing other items to flash or operate intermittently, such as the blinkers. Remove blinkers' negative lead and using a temporary set of wires, jump the negative lead from the blinker directly to the battery's negative terminal. Activate blinker again. Has the problem with other items flashing disappeared? This means that the fault lies in the wiring to the blinker or the ground point that the negative lead of the blinker normally leads to. Locate the ground point and check for presence of corrosion and/or a bad contact (loose screw, bad connector). If corrosion is present at the grounding point, clean connector with fine emery cloth and coat with vasoline or dielectric grease before reassembling. If connection is bad, tighten or replace ground screw as necessary, replacing star washer also. If grounding connection is located in a plug-style connector and connector is bad, replace connector or jump connector, replacing it with an inline butt-connector instead. Examine wire to/from blinker for presence of corrosion in the wire. If the wire is corroded, replace the wire and/or blinker & wire.


5. Headlight failing intermittently or regularly:

Visit separate web page for diagnoising headlight issues.


The electrical system of a motorcycle can be thought of in very simple terms. It consists of three parts: an alternator/generator that is designed to provide electricity to run the motorcycle and charge the battery, a rectifier or rectifier pack that is designed to keep the power coming from the alternator/generator from over-charging the battery and from sending too much power through the circuits, and a battery to store electricity for starting and when draw exceeds demand (which should never happen).


The Alternator (on newer bikes) or the Generator (on older bikes) produces electricity by turning a magnet within a set of coiled wires. The turning of the magnetic field resulting from the spinning of the magnet induces an electrical current in the coiled wires surrounding the magnet, and from this all the electricity for the motorcycle is created. On almost all motorcycles, the alternator/generator (whichever is applicable) is turned directly by a shaft within the engine, and is usually located within the engine casing, within the engine's oil.
The primary difference between an alternator and a generator is that an alternator produces alternating current [AC] (which the rectifier pack converts into direct current), while a generator produces direct current [DC]. Because of the nature of their designs, alternators and generators normally fail for only one of three reasons:
(A) Most common: a break in the windings of the coiled wire that surrounds the magnet. A break in the wire keeps the electricity from being send elsewhere in the motorcycle where it's needed.
(B) Less common, but occurs on older generators: the brushes, a component within the boundary area between the magnet and the coiled wire wear out. Very cheap to replace (usually under $5), but the amount of labor involved in removing the alternator/generator and then disassembling it to replace them usually means it's cheaper and more reliable to simply replace the entire alternator/generator.
(C) Rare on post 1970 motorcycles: the shaft which holds the magnet becomes lopsided, or the bearings holding the shaft become bad (usually as a result of the magnet or it's shaft being lopsided).

The battery on motorcycles is a smaller version of the battery in cars, because the power draw of motorcycles is less than that of cars. It's purpose is to store electricity in a chemical form (when being charged) that can be recovered at a later time (such as to run the starter when you start the motorcycle). Motorcycle batteries are rated in the following terms: volts (usually 12 volts, although 6 volts was common in pre-1970 motorcycles), amp-hours (how much power it can produce under ideal circumstances), and physical size (so it fits within the required space/shape for the battery holder). Traditionally, the physical size relates directly to the other ratings, so a battery designed to fit in a specific space will normally mandatorily have the right other values.
Although batteries can summon forth less power when cold (like at freezing) than at room temperature, it is actually heat exposure that reduces a battery's lifespan. A cold battery will produce more power once warmed up again; an over-heated battery will never produce as much as it did originally. Thus, motorcycle manufacturers, to protect the batteries, have designed battery storage locations that are as far away from the exhaust and engine heat as possible.
Another thing that plagues automotive and motorcycle batteries is the fact that each time they are discharged (emptied), they decrease their ability to hold a charge by approximately 25%. That means if you run a motorcycle battery empty three times, it will only hold about 42% of the charge it was designed to hold originally, which is often not enough to start a motorcycle. This is why it is important to attach your battery to a trickle-charger if you are not planning on using your motorcycle for a longer period of time (such as during winter storage, or summer vacations away from home). The same is true in the automotive world, and a standard car battery that has been drained three times is usually no longer adequate for daily use any more. For cars, we recommend always replacing dead batteries with Marine-grade equivalent batteries, which are designed to lose only about 1% - 2% of their capacity each time they discharge. Unfortunately, there are no marine battery equivalents for most modern motorcycle batteries (please email me if you know of a source).

The wiring on a motorcycle is designed to carry power to the various devices that use it. That list includes the headlight(s), tail light(s), blinkers, brake light(s), gauges, horn, starter, starter relay. It also connects the battery and alternator/generator to the other portions. Wires used in motorcycles are generally identical to automotive grade wiring, but inherently different than wires used in home electrical usage. Part of that difference is the insulation around the wire: by design, it must be able to resist oil, gasoline, fumes, heat, sunlight and vibration in ways that home electrical wire never does. Additionally, automotive/motorcycle grade wiring is traditionally multi-stranded, to help ensure that the wire will not break when bent around various paths on the bike, and vibrated by the engine.
Unfortunately, over time, vibration and age can combine to break or rub-away the insulation covering the wiring on your bike. When this happens, the wires underneath become exposed, and if permitted to contact any bare metal (such as the engine, or the frame), they will permit electricity to make a full loop back to the battery, draining it. Furthermore, water can exacerbate the problem, by creating a temporary circuit for electricity to flow through from a bare wire to other metal on the motorcycle. The best ways to help ensure that your wiring lasts as long as possible is to cover your motorcycle from the rain and sun (either using an indoor parking spot, or a waterproof motorcycle cover when parked), and to never use a pressure washer on your motorcycle.
Additionally, since wire's leads are made of common metals, they are subject to oxidation/corrosion/rust, which can interfere or blocks their ability to transport electricity. Only exposed portions of wires (such as the connectors at each end) can become corroded normally, and that corrosion can then spread down the wire, corroding the parts under the insulation as well. There is a simple way of preventing this in advance: dielectric grease. Dielectric grease is similar to vaseline, and is a product which displaces water and air. You buy the stuff (it's cheap), and then disconnect each electrical connector on the bike one-at-a-time, liberally smearing the dielectric grease into the connectors, then reassembling that connector before moving on to the next. It will prevent water and water vapor from reaching the connectors, thus preventing them from corroding (only works if corrosion is not already present -- if it is present, you need to remove the corrosion, or more often, replace the wire or connector first).

Electrical Theory 101
Electricity always follows the path of least resistance. In real-world terms, this means electricity prefers to following items in order (updated source: Wikipedia: Table of Resistivities, thanks to Aaron Dakoff for pointing it out):
Unoxidized Silver
Unoxidized Copper
Oxidized Siver & Copper, etc. See above link for more details.
Thus, if electricity is following a wire and the wire has two spots in which it goes back to ground (one at the far end of the wire, and one in the middle where the wire's insulation wore down & is touching the grounded chassis of the motorcycle), the electricity will take the first available route. Additionally, if it encounters rust or corrosion in it's path, it will start to seek out an alternative path if available. Always ensure that both wires (the wire to a device and the wire returning from it) are in good condition, as well as all the connectors on it, in order to ensure good operation of the device.


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