Diagnosing a Blown Car Audio Amplifier Fuse

What causes a car amp to keep blowing fuses?

When a car audio amplifier fuse blows, getting to the bottom of the problem starts with figuring out exactly which fuse bit the dust. There are two or three different types of car amp fuses in a typical installation, so finding the root cause in any given situation hinges on figuring out which one blew and then narrowing down the possibilities of why it did so.

In cases where a power cable connects the amplifier directly to the battery, and it's wired properly, then there will be an inline fuse that can blow in addition to the internal amp fuse. In other installations, power is drawn from a distribution block with a fuse of its own. So, depending on how an amplifier is wired into the electrical system, you can find yourself dealing with a variety of different fuses.

Two people looking under a car hood
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In any case, the main reasons for an amp fuse to blow include a short to ground somewhere along the power supply line and internal amplifier faults. In order to track down the exact source of the problem, you’ll need to break out a voltmeter.

Basic Amplifier Fuse Diagnostic Steps

  1. Locate the blown fuse.

  2. Replace the blown fuse with everything turned off.

  3. If the fuse blows with everything turned off, there is probably a short between that fuse and the rest of the system.

  4. Replace the fuse again with the amplifier disconnected.

  5. If the fuse still blows, there is a short somewhere in the wiring.

  6. If the fuse doesn't blow with everything turned off, but it does blow when the amplifier turns on, there is probably an internal problem with the amplifier.

Finding a Bad Alternator Fuse by Checking Voltages

The first step in figuring out why a car amp fuse keeps blowing is to determine which fuse is actually blown. If you’ve already replaced the fuse in question, and you know which one it is, then you can skip this step.

If you haven’t already replaced the fuse in question, note that while you should never replace a blown fuse with one that has a higher amperage rating, you’re actually safer using ones with lower amperage ratings while diagnosing this type of problem.

Fuses blow when more amperage flows through them than they can handle, and a hot fuse can handle less amperage than a cold fuse. Since the original fuse was almost certainly hot when it blew, putting a new fuse with the same rating in may allow a malfunctioning amp to draw even more amperage than it did prior to blowing the old fuse, which could result in further internal damage.

If you use smaller fuses during the following diagnostic procedures, you’ll still be able to determine where the short or malfunctioning component is located, but you’ll be less likely to see further damage to the amp.

In any case, you want to identify how many fuses the power distribution line contains and check for voltage at both sides of each fuse. Some amps are wired directly to battery positive with a single inline fuse, and a fuse that’s built into the amplifier, while others draw power from a distribution block that, in turn, is connected to the main fuse.

Although you can technically check for blown fuses with a visual inspection or test light, a volt or ohmmeter is the more precise way to go about it. You need to check the voltage at both sides of each fuse, starting with the main, or battery, fuse.

If a fuse has the same voltage on both terminals, that means it’s good. If it has battery voltage on one side but not the other, that means it’s bad. After you have determined whether you’re dealing with a main, distribution block, or internal amplifier fuse, you can move on to the next step.

Diagnosing a Blown Car Amp Battery Fuse

If you determine that your main fuse is blowing, then you’ll want to pay attention to the timing. Try inserting a good, properly rated fuse with your head unit — and amplifier — turned off. If the fuse blows immediately, when everything is off, then you’re probably dealing with some kind of short in the power cable between the main fuse and the distribution block, or between the main fuse and the amplifier if there is no distribution block in the system.

You can check for continuity between the dead side of the amp fuse and ground to be sure. Under normal circumstances, an ohmmeter should read “overload” on this type of check. If it shows continuity, you will have to check the entire run of the power cable to locate where it is connected to ground. In some cases, a chafed power cable may only make contact with the ground when you’re driving, resulting in a fuse that blows when you run over speed bumps or rough terrain.

Diagnosing a Blown Distribution Block Amp Fuse

If both sides of the main fuse have power, and one side of the distribution block has power but the other side of that fuse is dead, then you’re either dealing with a shorted power wire or an internal amplifier fault. There are a few ways to determine which one is the culprit, depending on how your amp is installed and where the wires are routed.

The first step is to check if you can see a power wire that connects the distribution block to your amp. In an ideal situation, you’ll be able to see the entire length of the wire, even if it means pulling back the carpet, panels, or other trim components, which will allow you to check for any damage to the insulation that might allow it to come into contact with the ground.

If that isn’t possible, then the next best thing is to disconnect the power wire from your amp, make sure that the loose end isn’t in contact with the ground, and check whether the fuse still blows. If it does, then the problem is in the power wire, and replacing it will almost certainly fix your problem. Of course, you’ll have to take care when routing the new wire so that it doesn’t end up shorting out as well.

If the fuse doesn’t blow with the power wire disconnected from your amp, then you have an internal amplifier problem, which is far more difficult to diagnose — and may be impossible to fix yourself. Unless you’re fairly confident working with electronics, you’ll probably end up having to take the amp to a professional, or just replace it altogether. If it’s relatively new, it may still be under warranty.

Diagnosing a Blown Internal Amplifier Fuse

Many amps have built-in fuses that are user-serviceable, but tracking down the reason this type of fuse has blown, let alone fixing the problem, is a little more complicated than simply looking for a shorted power wire. If the amp has power, and one side of the built-in fuse has power but the other doesn’t, then you’re typically dealing with an internal fault in the amp.

If you can determine exactly when the fuse blows, you can get pretty close to figuring out why it’s happening. For instance, car amps have two power sources: a main source of power from the battery that is available whenever the ignition is in the accessory or run position, and a “remote turn on” voltage that comes from the head unit.

If the fuse blows while your head unit is turned off, meaning no power was ever applied to the remote turn-on terminal, then you probably have a problem with the amp’s power supply. This can be caused by hooking up the power backward, connecting a speaker or speakers with impedance too low for the amp, or simple component failure arising from time and normal use.

If the fuse only blows after you turn your head unit on, and power is applied to the remote turn-on terminal, then you’re probably looking at a problem with the amp’s output transistors. However, there are a lot of different internals — like the transformer winding, rectifiers, and other components — that may have gone bad. In fact, bad speakers or speaker wiring can even cause this type of fault — if the fuse only blows when the volume on the head unit is turned up.

Repairing or Replacing a Broken Car Amplifier

Fixing a grounded power cable or wire is pretty straightforward: install a new one, route it so that its insulation won’t chafe or rub on anything, and you’re good to go. If you determine that you’re dealing with an internal amplifier fault, however, the situation is a little more complicated.

Out of all the various reasons an amp can fail, the most common is bad output transistors. This failure is also one of the less expensive amp repairs, so if you determine that you are dealing with an internal fault that only blows the amp fuse after remote turn-on voltage is applied, and you have a relatively expensive amplifier, then it’s probably worth taking it to a professional amp repair shop or attempting a DIY repair if you’re comfortable with it.

You may find that the power supply is bad, though, which is typically more expensive. In some cases both the power supply and the output transistors may be damaged, in which case you’re often better off simply replacing the amp.

Of course, it’s also important to fix any underlying problems before you buy a new amp or reinstall your repaired unit. For example, if the power supply failed because the amp requires an 8-ohm load and it’s connected to a 4-ohm load, the field-effect transistors will most likely fail all over again, resulting in another costly repair bill.

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