The Motorcycle Electrical Diagnosis Page...
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
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
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
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
- Trouble Shooting (Part B) -- Standing Battery-Circuit
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
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
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
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 &
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
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
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
(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
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
always follows the path of least resistance. In real-world
terms, this means electricity prefers to following items in
(updated source: Wikipedia: Table of Resistivities, thanks to Aaron Dakoff for
pointing it out):
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.