Audi A4 8K: Voltage Drop Troubleshooting

Hello,

On a friend's Audi 2.0 TDI (manufactured in 2008), I'm observing a voltage drop of up to 0.8V between the positive terminal of the alternator and the positive terminal of the battery.
There are very few losses on the mass side.
It's also interesting that the charging port in the engine compartment has a voltage level equivalent to the battery voltage. This means I'm losing the 0.8V somewhere within the engine compartment, and not between the charging port and the battery.

If I trace the cable from the generator, it seems to me that it first (possibly) goes to the starter and then somewhere else to the positive terminal or the battery. In the diagram, you can see the positive terminal. The cable marked in the diagram has the same insulation as the cable that runs from the generator to the starter, but I'm not sure if it's actually that cable.

To my knowledge, the wiring to the starter motor has been replaced before because there were starting problems. Therefore, the issue is likely a bad connection between the starter motor and the connecting cable.

Since it's extremely difficult to access the starter motor, my idea now is to run an additional cable from the generator to the charging port in the engine compartment.

Does anyone know if this is allowed? Is the charging port protected? And how does the battery management system react to it?

Thank you for your help.
Best regards,
Johannes.

Hi,
In principle, this is possible. The cable from the generator to the starter motor has a cross-section of 16 mm², while the cable from the starter motor to terminal 30 of the distribution block, along with the charging connection, has a cross-section of 35 mm². There is no fuse in between. The battery connects to the distribution block with a 30 mm² cable via the ignition system for battery interruption.
Regarding the conductor cross-sections, I suggest measuring the exact point where the voltage drops, and especially under what load.
Where was the ground connection located during your measurement?
hg
Herbert.

Thank you for the quick information.

I have marked the point where the voltage drops on the plan, and that's exactly where I would install the cable.
The measurement was taken at approximately 60A (high beam, blower, windshield heating).

What exactly do you mean by "mass contact"? On the mass side, I measured between the intake manifold and the negative terminal on the chassis, and in the trunk, between the negative terminal of the battery and the chassis. All measurements were in the 1-5 mV range.

Best regards,
Johannes.

Were the 60 amps measured as indicated?
"13 mOhms is quite high for the cable cross-sections; a value around 1 mOhm would be more normal. However, the contacts also contribute to the resistance in a similar order of magnitude. To rule out a contact problem, I would check and clean the cables from the generator to the charging connector, including the crimped connections."
5 mOhms would be normal for me.
hg
Herbert.

I measured the 60A using an inexpensive clamp meter.

The problem is that it's not easy to obtain replacement starters (which is apparently where the issue lies).
To do this, a motor mount or similar component needs to be removed.

The question was whether it was permissible to install an additional cable, as the starter motor is working fine.

Best regards,
Johannes.

If you're unlucky and the connection between the starter and the charging terminal is poor (or becomes poor over time), the starter current will flow through the additional cable and the 16 mm² alternator-starter cable.
Such loops should be avoided at all costs.
hg
Herbert.

Question about safety: Did you A) actually measure 0.8V directly between the generator and the charging port? Or did you B) measure once at the generator relative to ground, and then shortly afterward measure at the charging port relative to ground?

If B) is the case, then the measurements would not be simultaneous, and it could simply be that the load changed abruptly (e.g., end of afterglow, PTC resistor of the heater, etc.).

If A): 60A * 0.8V results in approximately 50W. The heating at the poor contact is clearly noticeable, so the fault should be easily localized by (carefully) touching the area. When the load is increased (e.g., cooler fan = disconnecting the temperature sensor, seat heaters, interior lights, RFS, NSL, NL), the cable at that point should also start to melt down after some time.

Most often, it's either the cable connectors or the crimp contacts.

Warning: A loose connection in the alternator cable can often damage the alternator diodes, which can then result in increased ripple and reduced current output. (I've experienced this myself with a Galaxy=Sharan).
If things go really badly, other control units can also fail (fortunately, that hasn't happened to me).

Greetings.
Sure, here's the translation:

"Guste" is a German word that means "gust" or "gust of wind." It can also be used to describe a sudden burst of something, such as laughter or anger.

Ich habe wirklich gemessen wie in der Skizze, also beide zur selben Zeit. Der Spannungsabfall ist wirklich vorhanden.

I'm starting to realize that my idea has a few weaknesses.
It probably needs the entire cable replaced.

Thank you very much for your help!
John.