1999 BMW 328 Stereo

The fuses are in the under-hood fuse box, left rear corner of the engine compartment. There's up to four of them, depending on which audio system you have. They're fuse number 9, a 20 amp, fuse number 11, a 15 amp, fuse number 13, a 5 amp, and fuse number 33, a 10 amp.

I'm not sure what is different about your fuses. Look for two tiny holes on top where you can place the voltmeter probe. Voltage readings are a faster way of testing fuses than removing them to do visual checks.

The common spade-type fuse has two small slots or holes on top. Turn the ignition switch on, then measure the voltages on both terminals. If the circuit is powered up, you'll find 12 volts on both terminals. If you find 0 volts on both, that circuit is turned off. You're looking for a fuse that has 12 volts on one side and 0 volts on the other. A test light is even faster than reading a voltmeter.

BMW also uses a narrower spade-type fuse and on older cars they used an inch-long round ceramic cylinder with the fuse element exposed. On both of those you can easily see when it is blown. If you need to check a cartridge-type fuse, many of those designs hide the element, so all you can do is remove them and test them with an ohm meter.

All four fuses are shown as being in the box under the hood. That's always where the major fuses live. The fuses inside the car protect smaller circuits that branch off those under the hood.

I've never worked on a BMW radio because that manufacturer is real selfish with radio service manuals, just like GM, Ford, and most other imports. I CAN tell you that the common things I've run into on other brands are broken solder connections inside the radio and defective switching transistors. Those are in the circuit that tells the amp or the amp relay to turn on.

I also had one amplifier with a dead turn-on circuit. Amps can draw a lot of power and that is too much to be switched on directly through the ignition switch and radio's power switch. Instead, the high-current 12 volts is supplied directly to the amp, and a smaller, low-current circuit from the radio tells the amp when to turn on. This is why there's so many fuses involved as there's multiple circuits.

The bottom line is the problem could be in the radio, the amp, or the wiring in between. I DO have a somewhat basic wiring diagram that might help if you want to try to diagnose this yourself, but BMW really has their cars designed to force you to go back to the dealer.

Nope. The same can be said of every electrical problem on every car since the beginning of time. It worked at one point, then it didn't. The only variable is what happened in between. The ignition switch could have been turned off overnight. The car could have hit a bump. A transistor shorted while a comet passed overhead. (Obviously the two are not related). A weak solder connection let go just as you completed a right turn. (Also not related).

The ones that concern me are, for example, a wire rubbed through and shorted to the body, and that caused a fuse to blow, and it took five years, three days, and two hours for that to occur. Where was the car when that happened? If the problem showed up while you were driving on the highway, you'd take it in to be diagnosed. BUT, ... If it happened while the mechanic was driving the car into the shop for an oil change, you would unfairly blame him for causing the problem.

There is no correlation between how the problem occurred and how serious it is. You can have an intermittent misfire due to a bad connection inside a $1200.00 Engine Computer. I'd call that pretty serious even though you can still drive the car. You could have a no-start condition and have to walk to work caused by a failed $25.00 crankshaft position sensor. That is not at all serious but it too can occur all of a sudden, without warning, while you're driving or first thing in the morning.

In your case, if you DO find a blown fuse, I doubt we're looking at a bad solder connection because those almost always stop a circuit from working but they don't cause excessive current flow and thus, blown fuses. The fuses are just the place to start. If they're okay, we can move on to some other tests as long as I can get some useful information from the wiring diagrams.