Diagnose blowing fuses

The basics of BMW Motorcycle electrical troubleshooting

by Duane Ausherman

This applies to the /5 BMW but is useful for many problems on other vehicles.

I try to use as much knowledge of others as possible.  I love giving credit.  Here is an example.

Jim was nice enough to write this for one owner to use in troubleshooting his fuse-blowing problem on a /5 BMW.  He did it so well that I asked if I might use it here.   Jim has expanded and altered it for public consumption.

By Jim Franzen

This is a basic guide to finding the problem when your motorcycle is blowing fuses.  Diagnosing such a problem is easiest if the fuse blows as soon as you install the fuse or turn on the bike’s electrical system.  If your fuse blows after a few hours of riding, or maybe when you hit a bump just right, you’ve got an intermittent problem. Intermittent problems can be very difficult to find, but if you vary this technique a little, as explained later, you can find them.

Please read the text all the way through before beginning to diagnose your motorcycle, as some important safety issues are mentioned. I’ve tried to keep this elementary for the benefit of more readers, but you must have some knowledge of proper wiring techniques and electrical safety to proceed.

When a fuse blows, an excessive amount of current (measured in amps) is drawn through the fuse by some electrical component.  The fuse is a piece of wire engineered to self-destruct if too much current (an amount over its amperage rating) passes through it.  The fuses exist to protect the motorcycle’s wires from burning up (literally) if some electrical component demands more amps than the wire can safely deliver.

Instruments that can measure amps (an ammeter) in the quantities running through motorcycle fuses are expensive, and the least expensive ones require that you place the instrument along the wire to measure the amps as they go by.  Even if you owned such an instrument, the problem is such that when the fuse blows, it stops all the amps from flowing (just like turning a switch to the “off” position), so having an ammeter to measure amps would be useless anyway.

So the problem is how to find the electrical component on the motorcycle that’s asking for too many amps from the fuse.  The problem is compounded because you can’t measure amps when they are not there, as is the case with a blown fuse.

The component could be any number of electrical things gone bad, such as a light bulb, a radio, heated grips, or a fuel injection module, or it could be something obscure like a piece of the wiring harness that’s been damaged.

It’s best to try the easiest things first, so if you’re blowing fuses, you most certainly know this because some electrical component on the bike goes dead.  Any electrical component is considered a “load” on the fuse.

Since you know which electrical loads go dead when the fuse blows, disconnect the loads one at a time to see if your fuse blowing problem disappears.  Please don’t drive around with your brake lights or headlights disconnected.  You’ve probably found the problem if a certain device blows the fuse whenever it’s connected. Swap in a known-to-be-good replacement device and retest.  If the problem persists, keep looking by disconnecting other loads on the fuse.

If you’ve disconnected all the electrical loads but are still blowing fuses, there may be a load on the fuse of which you’re unaware.  If you have the motorcycle manufacturer’s wiring diagram (also called a schematic diagram) and can read it, check it to be sure that you have disconnected every load from the fuse.

If a previous owner has added any aftermarket electronic device, there’s a chance that he wired the device in as a load to the fuse that’s giving you problems.  Most aftermarket devices have their own fuses, but that doesn’t mean that they are not wired onto your motorcycle’s already fused circuit.  You might try disconnecting any aftermarket devices, especially if you’re unsure of where they get their power.

If you’re lucky, you’ve found some device that is blowing the fuse, and you can choose to have the device serviced or to replace it.  You’re even luckier because you can stop reading right now.

Maybe you have no wiring diagram to tell you about all the electrical loads the fuse is going to.  Perhaps your problem is a bare wire that’s touching something metal.  Now you’re going to have to have to play detective and try to find and follow the wire that the excessive amps are taking all the way to the fuse-blowing culprit.

Let’s start with a crude but very effective way to follow a blown fuse’s circuit to the source of the problem.
Start by removing the fuse from the fuse holder where the fuses keep failing.

With the bike ignition on and the fuse removed, using a DC voltmeter or a test lamp, you’ll see 12 volts (test lamp lit) on one side of the fuse holder and 0 volts (test lamp off) on the other.  The side of the fuse holder that measures 12 volts is the “un-fused” or “hot” side, and the wire attached to the fuse holder side with 0 volts is what is protected by the fuse (the fused circuit.)  When checking the fuse holder, the black wire of your voltmeter or the alligator clip of your test lamp should be connected to the motorcycle’s frame (which is electrically the same as the battery’s negative terminal.)

Now that you know exactly what wire (fused circuit) the fuse is working to protect, inspect the wire for damage while you follow it away from the fuse holder.

If you come to a splice or a terminal block that serves to split the wire into two or more wires going to different electrical components, the circuit is effectively being branched into multiple circuits, all protected by that same fuse.

The fuse-blowing fault could be down any one of the circuit branches, so you will need to disconnect each branch one at a time to determine which branch has the fault. After disconnecting a wire, you’ll put in a good fuse and see if it blows.   If the new fuse blows, reconnect that wire and try the next wire (circuit branch.)  When a good fuse doesn’t blow, you’ve identified the disconnected wire as the circuit branch with the fault.

Now that you’ve isolated the wire with the circuit fault, pull out the still-good fuse and save it for later.  Re-connect the wire with the circuit fault and follow it until it either branches or arrives at its electrical load (light bulb, ignition module, etc.)  If the wire branches again, you’ll have to disconnect each one of the branches and re-test by installing more good fuses to determine which wire branch to continue following.

When you finally get to the electrical load at the end of the wire, disconnect it, and the fuse should no longer blow.  Try replacing the load device to solve your fuse problem.

IMPORTANT: Turn the bike’s electrical system off as you disconnect and reconnect wires. You’ll probably be using metal wire strippers, metal screwdrivers, extremely hot (and metal) soldering irons, etc.  Be extremely careful not to create new and possibly worse problems while diagnosing the original problem.  Follow good electrical practice when reconnecting wires if you’ve had to cut them, and don’t leave any bare wires exposed after you’ve reconnected them.

Here are some special cases that can occur when you’re following wires:

1)  If you’ve followed a wire to a switch, then it’s more than likely the switch is in the “on” position, and the circuit continues along another wire coming out of the switch.  Turn the switch to the “off” position, and if it stops the fuse blowing, you’re on the right track.  Continue by following the wire from the switch to the electrical load.

2) If the wire you’re following goes into a bundle of wires (a wiring harness), you’ll need to take a good look at the insulation color of the wire you’re following going into the harness.  Most wires are color-coded by the insulation.  In a simple system (just a few wires), the insulation of the wires will be different in solid colors.  If the manufacturer needs more wires than there are solid colors available, they’ll add colored stripes to the wires.  Look for that same colored-coded wire to exit the harness somewhere.  This is where those electrical diagrams called “schematics” come in handy.  They can help you to know where the wire is supposedly headed, which will also help get you to the correct harness exit point sooner.

If the fuses continue to blow after you’ve disconnected the load, you’ve either followed the wrong wire, or the problem is somewhere in one of the wires leading to the load.  This is usually the result of a ‘pinched’ wire, which is how most people refer to a wire whose insulative coating is damaged.  The insulation could have been punctured by being routed tightly past some sharp metal piece of the bike.  The insulation could be abraded
from repetitive friction against metal caused by bike vibration.

The idea that the fuse-blowing fault could result from a pinched wire and not an electrical device on the bike is why you’ve been examining the wires for damage as you’ve been following them.

In some extreme cases, you may find insulation that’s burned off. The insulation could be burned from being too close to a high-temperature engine part or a high-temperature electrical part.

Insulation can also be burned off from the inside of the wire due to excess current in the wire itself.  This is a very dangerous situation and usually indicates someone has placed an incorrect and too high an amperage fuse in the fuse holder.  Perhaps a previous owner did so in an unsafe attempt to prevent fuse failures.

Another remote possibility is that someone has used too small a wire size (also called gauge) when wiring some aftermarket electrical device, making a wiring harness, or repairing the existing one.

It is EXTREMELY dangerous to substitute fuses whose amperage is above the value specified for the circuit.  Don’t do it. Electrical fires can ruin your bike.  Electrical fires near gasoline tanks can destroy you.  Only use the fuse value that the motorcycle manufacturer specified for that fuse location.

If you see a pinched wire, that’s the problem, you can either reposition it, wrap it with electrical tape, disconnect the wire on either side and route around it with a new wire.

If you suspect a wire is pinched and is making contact with metal on the bike somewhere, but you can’t see it, disconnect the wire at both ends and loosely run a temporary wire of the same size (gauge) or larger to bypass the original wire which is now non-functional.  If your fuse-blowing problem goes away, re-route the temporary wire to make it permanent.  If the faulty wire is too hard to remove, you can leave it in place once it’s been bypassed.

So far, we’ve followed a course of action requiring very little in the way of test equipment, but it has caused you to blow up many fuses as you’ve progressed.  You’ve also had to exercise a lot of caution by turning the bike’s electrical system on and off many times during testing so as not to be working with metal tools in an electrically “live” environment.

Now we’ll introduce a similar but safer and somewhat more technically proficient (difficult) way to pursue the problem of fuse-blowing.  Most of you that have the DC voltmeter mentioned earlier have a multi-function meter, which includes an ohmmeter and the voltmeter.

An ohmmeter measures how much resistance the circuit is going to put up to the flow of amps in the wire.

It is possible to diagnose a fuse-blowing fault using an ohmmeter.  This is done in much the same way as previously outlined, but with the motorcycle electrical system off for more safety.  Using this method, you are following the fault by means of the “ohmic” value your ohmmeter displays when measuring the wire of the faulty circuit.  This method is difficult because you have to interpret if a low ohmic value you see on your meter is from the fault you’re chasing down or if it’s just the correct ohmic value of a light bulb or some other low-resistance electrical load at the end of the wire.

Just like the first method, you will follow the “fused circuit” away from the fuse holder.  When you get to a terminal block or splice, you’ll disconnect each wire (circuit) one at a time, just as before.  But instead of powering up the bike and blowing fuses, this time, you will use a low ohms range of your multimeter to measure how many ohms (how much electrical resistance) each circuit presents to the fuse.  You should always see some amount of ohms, although it may be rather small.

The black wire of your multimeter should be connected to the motorcycle’s frame (which is electrically the same as the battery’s negative terminal) as you are trying to measure how many ohms of electrical resistance the wire is offering to the flow of amps through the fuse.

No electrical resistance (zero ohms) is what is commonly referred to as a “dead short” or “short circuit.”

Remember, you will measure ohms with the motorcycle’s electrical system off.  Ohmmeters don’t like to see voltage on the same wires as themselves.

There are no hard and fast values I can tell you to look for on your ohmmeter when chasing an electrical fault.  Clearly, a value of zero ohms (absolutely no resistance) on a circuit branch is very bad and should be the most likely suspect.  If none of the circuits reads zero ohms, one of the circuits’ ohmic readings will be significantly lower than the others and somewhat close to zero. The majority of the current flowing through the
fuse (and probably blowing the fuse) is going to go down the wire with the least amount of ohms.

Just be sure that your meter is set to read a very low amount of ohms (the unit of electrical resistance.)  On a high resistance range, say one that reads up to 10,000 ohms, a good circuit with just a few ohms of resistance might look like a zero ohm circuit, and you could follow the wrong wire.  If your ohmmeter is an “auto-ranges” type, you won’t have this problem.

Disrupt the electrical load when you’ve followed a wire (circuit) with a very low or zero value of ohms to a load.  Now is a good time to put in a good fuse and power up the bike to check if you’ve found the problem.  In the event the fuse still blows, you’ve either somehow gotten off track, or the problem is a pinched wire leading to that electrical load.

A pinched wire, or any wire whose insulation has been compromised and is touching metal on the motorcycle, will have a reading of zero ohms on the ohmmeter.  The wire itself may have some resistance, but that amount would be negligible, maybe one-tenth of one ohm.

Turn the power off to the bike to check for a pinched wire. You’ve already removed the electrical load at the end of the wire, so go back one step and disconnect the wire at the previous splice or terminal block.  Measure the resistance of the wire.  It should be infinitely high because there is no load at the end of it.  If it reads zero ohms, you’ve found the wire that’s pinched.  If the wire checks out OK, but you’re sure that the fault is along
the wire you’re chasing, you’ll need to go back and check each branch of the wire back to the fuse in the same way.

One of the things that makes this method so difficult is the nature of the electrical loads you’re going to encounter.  You should know that light bulbs contain wires called filaments that connect the wire (circuit) from the fuse to the motorcycle frame (electrical ground) in such a way that the filaments glow from the current (amps) flowing through them.  When you are measuring the ohms of a wire leading to a light bulb, it may measure just an ohm or two.  That’s because the ohmic value of the filament wire inside the light bulb is very small.

Heated grips use conductive material that may also present a low ohmic value.  A relay’s main component is a coil of wire that may not have a lot of resistance.

To make things a little more difficult, when there are two or more light bulbs on the same wire (like parking lights or instrument cluster lights), they work against you because they combine together to make the measured amount of ohms on the wire looks even smaller (closer to zero ohms) than that of a single bulb by itself.

Now you’re aware that many different electrical loads will make the wire look almost like a pinched wire to the ohmmeter.

In the case of a single light bulb at the end of a wire, you can pull the light bulb out of its socket and measure how many ohms it has to know how many ohms you should see on the far end of the wire going to it.

Effectively, we’ve discussed following the short circuit using two different methods: The first method was disconnecting and reconnecting wires and switching the power on and off to follow the fault to its source.  This method is difficult and a little costly because you are constantly blowing fuses.

The second method had no power running through the bike as you followed the fault by means of the ohmic value seen on the wire of the faulty circuit.  This method was difficult because you had to interpret whether the low ohmic value you saw on your ohmmeter was the electrical fault you were hunting down or just some light bulb at the end of the wire.

The diagnosis still gets harder if you’re trying to trace an intermittent short. Visual inspection for potential pinches of wires leading away from the fuse is extremely important.

One way to incorporate the methods described here for finding intermittent faults is to follow the fuse wire until it branches.  You’ll need to wire one side of a temporary fuse holder to a circuit branch that you’ve disconnected for testing. Splice the other side of the temporary fuse holder to the “hot” or “un-fused” side of the original fuse holder that you’re diagnosing.  Put the same amperage fuse into the temporary fuse holder as the original fuse that you’ve just bypassed.  You can buy a fuse holder with lengths of wires coming out of either side from most automotive stores.  Also, be sure that the wire size is the same or larger as the circuit branch’s wire.

IMPORTANT: When you are wiring the temporary fuse to the original fuse holder’s hot side, ensure there is no voltage at the hot side.  Turn the ignition key off, measure the “hot” side with a DC voltmeter, or use your test lamp.  If there is still voltage present (12v or lamp lit), you will need to disconnect the battery before wiring in the temporary fuse holder.

With the temporary fuse in place and the wiring safely routed, continue riding. The next time your intermittent problem occurs, one of the two fuses will have blown.  If the temporary fuse blew, you’ve identified the faulty circuit branch.  Reconnect the wire to its original position and follow the wire to the load.  If it branches again, relocate just the “fused circuit” end of the temporary fuse to one of the circuit branches to further isolate the fault’s path.

If the original fuse blew again, you’ll restore the original connection that the temporary fuse was wired to and then move the “fused circuit” end of the temporary fuse to another circuit branch and try again.

Intermittent problem troubleshooting is very time-consuming, but if you’re only blowing a fuse once a day or you have to hit a good size bump in the road to blow the fuse, then there’s little hope of finding the problem quickly anyway.

This is version three. If anyone uses this guide and finds it in some way flawed, I would appreciate hearing back so I can either fix the text or scrub the whole idea.  I apologize in advance for its length.

Jim Franzen

Updated 30 March 2023