Audi A6: Troubleshooting Electrical Issues

Hello.

I have something that's currently bothering me about my Audi A6 (1995/AEL). I'm someone who occasionally likes to listen to music at a higher volume, so I have two amplifiers in the car and a few other things that go with it. The problem is that when there are bass pulses, the on-board voltage drops briefly (with the engine running and loud music playing, down to 12V). Okay, so far, not too dramatic. So, I installed an auxiliary battery. It's gotten a little better; the voltage doesn't drop as much anymore. You can definitely feel the extra power, because even the radio stays on when the engine starts. The flickering lights wouldn't bother me, BUT what bothers me is: these damn voltage fluctuations are affecting the engine's running .
'When driving at a constant speed and with loud music, it feels like someone is tightening the handbrake by one notch at the moment of each bass pulse. When idling, the engine even starts to sputter (or at least it seems that way, around 150 RPM). This starts to become noticeable as soon as the voltage drops below approximately 13.8V. I'm guessing that the system is drawing around 200-300 amps. I never manage to get the voltage stable enough to stay consistently above 14V.'

Therefore, the question is how do I fix this problem. It's a 5-cylinder TDI, so it doesn't have an ignition system. But what is causing these fluctuations in engine speed (e.g., at idle)? My suspicion is that it's related to the engine control unit (ECU) or the fuel injection pump. Would it help to 'buffer' the ECU, by adding a capacitor to it?
Is there anyone here who is extremely knowledgeable about vehicle electrical systems and diesel injection?

Regards,
P.S. The power amplifier units are wired with a minimum of 35mm². The cross-section of the ground wire from the battery to the chassis has also been increased. Something else I will try is to create an additional ground connection in the engine compartment, from the engine to the chassis, because my battery is located in the rear, under the back seat.

Hi,

Here's the translation:

"Firstly, a large buffer capacitor (>=1F) can maintain a much more stable voltage during very short current peaks, such as those produced by an audio system, compared to simply adding another battery."

On the other hand, the fluctuations in speed are likely not directly caused by voltage fluctuations, but rather by the load on the alternator. An alternator that has to deliver its maximum current will rotate much more heavily than one that is not under load.
To differentiate these symptoms, it would be helpful to temporarily disconnect the alternator. This will allow you to determine whether the uneven idle is caused by the alternator load or the fluctuating voltage.

Regards,
Guste.

Hello Guste,

Thank you in advance for your reply.
It had also occurred to me that the alternator might be turning more slowly under load, but I couldn't quite explain it. I'll be replacing the serpentine belt soon anyway, so I'll try it then (it would even be enough if I just unplug the connector - right? No excitation voltage -> then the alternator shouldn't charge either).

With an amplifier power of approximately 1.5-2 kW, using capacitors becomes quite challenging. I have installed a dry cell battery with low internal resistance, as well as a 1-Farad capacitor (which I happened to have lying around).

Best regards, Mario.

Hi there.
That's definitely due to the stress from Lima.
If the amplifier needs to draw 2kW from the vehicle's electrical system for a short period, it's essentially like briefly applying the brakes to the engine. In idle, the engine control unit then adjusts the engine speed. Because the load was only temporary and the control system is somewhat slow to react, this can cause a "sawing" or fluctuating effect in the engine speed.
Try activating the high beam headlights while the engine is idling, or run the power windows all the way up and down, and observe the tachometer while doing so...

Olli.

My experiences with the AAT and the Kadett were different: Both started to stutter when the LIMA (alternator) battery was running low. In the Kadett, with its undersized alternator, this only happened when the headlights were on *and* the (normal) radio was also running. When measured with an analog multimeter, the voltage was 14V. So, it was either a coincidence, or the control units can only filter or smooth out a faulty voltage to a limited extent.

Gottlike wrote:

...it would even be enough if I just unplug the connector - right? No excitation voltage -> then the alternator shouldn't charge either....

Okay, disconnecting the cables might be enough. However, depending on the alternator and its speed, it might be necessary to disconnect all the cables. Above a certain speed, the alternator becomes self-exciting.

christians wrote:

My experiences with AAT and a Kadett, however, were different.

Okay, practical experience is, of course, more valuable than my theoretical assumptions. That makes the test with the disconnected alternator even more interesting. I wouldn't have thought that the control unit would have such problems with that, but you never stop learning.

Regards,
Guste.

An installation with larger dimensions presents a minor challenge for the vehicle's electrical system. In your case, assuming a 120A alternator, you would have approximately 1440 watts available. If you run your system at full power, not only will the alternator be heavily loaded, but the battery will also partially discharge.
To eliminate the voltage fluctuations in your vehicle's electrical system that you described, there's a relatively inexpensive method. I wouldn't connect the auxiliary battery directly to the vehicle's main electrical system. Instead, use the battery only to power the amplifiers. The auxiliary battery should be charged from the vehicle's electrical system using a properly sized charger. This should eliminate the voltage fluctuations. However, there's one drawback to this method: if you play music at high volumes for an extended period, the auxiliary battery will discharge and the music will stop.
Here's just a suggestion, and it might work if you only listen to music "occasionally."

@wutz
I'm not sure if the idea with the charger is feasible. I don't know of any charger that operates on 12V and charges at >14V. The problem would likely be that the charger would never be able to maintain a voltage anywhere near as high as the alternator.


Okay, simply disconnecting the connector isn't enough to diagnose the alternator. I've been paying attention to the belt tensioner, and it's definitely moving noticeably with the music's rhythm. So, I also assume that the alternator will become significantly harder to turn under load. Today, I removed the charging cable, and I could no longer detect any fluctuations in the idle speed. I also measured the alternator, and it's delivering over 110 amps, and without significant load, the on-board voltage is also well above 14V.
The car now has 180,000 kilometers on the odometer. Will replacing the alternator improve the situation?

Best regards, Mario.

Hi,
An extra battery, combined with disconnecting the alternator via a charger with current limiting, is probably the only solution to meet the short-term power demands of the amplifier. The amplifier essentially acts as a temporary short circuit in the vehicle's electrical system. A new alternator certainly won't help. Even a 200A alternator would likely be overwhelmed. Alternatively, you could install a generator in the trunk and essentially run your music system independently }.

Best regards from Berlin.
Olli.

"You can find chargers like these in the model-making hobby. Of course, you can't continuously supply the same level of power. The charging current is naturally lower than the power output of your amplifiers. But you mentioned that you only listen loudly occasionally."
You could also install a charging socket in your car, allowing you to easily connect it to a charger, for example, overnight.
It's a bit complicated, but it would solve the problem.

Gottlike wrote:

...Today, I unscrewed the charging cable, and I could no longer detect any fluctuations in the idle speed.

Okay, so it's now clear that the fluctuations in speed are not caused by the fluctuating board voltage, but by the fluctuating load on the alternator, as I also suspected earlier.

I will continue to maintain my claim that additional buffer capacitors (deliberately plural) could significantly improve the voltage and, consequently, the load on the alternator. This is especially true when you write...

Gottlike wrote:

I installed a dry cell battery with low internal resistance, as well as a 1 Farad capacitor (which I happened to have lying around).

I assume that you only installed this one capacitor, and that it may also be of only moderate quality (e.g., due to aging). High-quality capacitors (such as those from Brax) often have a significantly lower internal resistance than lower-quality ones, even with the same capacitance. And, by the way, the internal resistance of any capacitor is lower than that of any battery.

Given the system power you specified, approximately 2kW (assuming you're referring to RMS values), there should still be enough budget for surge protection.

If not, then you're left with a makeshift solution: reduce the motor load! This could involve installing resistors in the system (e.g., thinner wires), or even more of a hack: installing a *smaller* alternator. The downside is that the system voltage will drop, resulting in a loss of sound quality and power. A larger alternator would actually worsen the problem (higher alternator output = higher alternator power consumption = higher motor load).
Or the sensible approach: Listen more quietly. (I know that's probably not what you wanted to hear ).

Regards,
Guste.