Hello everyone

I own a 2019 Karoq 1.6 tdi. I bought it 4 months ago and have a strange problem. Sometimes when i start the car I get a red light warning that the battery is not being charged. However, if i start driving, the error goes away. Additional problems are occasional high revs at idle, and start stop not working. Otherwise it drives fine. I bought OBDeleven to try and diagnose the problem myself, and ended up suspecting the battery sensor that feeds the computer data about the battery health. The computer shows values of -40 for battery temperature, 0A for current, charge condition 100%, internal resistanc of 51mohm and other rubish. Also, I have a constant fault code: U103000. Amps dont change when i turn on varios heating. I dont have an osciloscope, but the wire going into the sensor gives 12V and 0,96V on my multimeter, which seems off.

So i took the car to a specialist to check if the problem is actualy the battery sensor of perhaps some wiring, and his diagnosis was that neither is the problem but the alternator. According to him, the alternator, when pluged in, disrupts LIN communication and that disrupts all other sensors on the line and that results in no data being fed to the computer from the battery sensor and hence the high revs and start stop not working.

Can anyone give insight into this? Does this sound correct?

Thanx

Hello, welcome to the forum.

I'd go with your first suspicion of the battery sensor being faulty - minus 40 degrees clearly looks wrong and I cannot see an alternator fault causing that.

Try replacing that first, likely not expensive and an easy DIY.

With the engine running, what voltage are you seeing at the battery terminals?

Around 14V, if I remember correctly.

7 minutes ago, Warrior193 said:

Hello, welcome to the forum.

I'd go with your first suspicion of the battery sensor being faulty - minus 40 degrees clearly looks wrong and I cannot see an alternator fault causing that.

Try replacing that first, likely not expensive and an easy DIY.

Not cheap either. 200 EUR for the new part.

45 minutes ago, Dodo051 said:

Additional problems are occasional high revs at idle

Occasional high revs at idle could also be the normal DPF regen happening?

Since you can talk to the battery sensor then the alternator is clearly not blocking the shared LIN bus, so I go along with replacing the battery sensor.

It's not DPF regen, i've checked the status once it had high revs and it wasn't in regen mode. The values I am getting of the sensor are default rubish values. If I unplug the sensor the stay the same, so I dont think I'm getting anything of the sensor. However, I did read somewhere that both the sensor and the alternator wires use the same line to enter the onboard computer. Apparently thats how LIN communication works. If the alternator is feeding rubish through the wire then it might as well distupt the signal the sensor is sending. Unofortunatelly, the wire from the alternator is hard to reach without dissasembling stuff, so I cannot double check his claims.

38 minutes ago, Dodo051 said:

If I unplug the sensor the stay the same, so I don't think I'm getting anything of the sensor.

That extra information changes my view - so it could be the alternator blocking the shared LIN line.

As the commutator uses sliprings, it's little difficult for me to see how the alternator can be generating such interference while appearing to output correctly - Is there a smoothing capacitor anywhere in the alternator circuitry?

2 hours ago, Warrior193 said:

As the commutator uses sliprings, it's little difficult for me to see how the alternator can be generating such interference while appearing to output correctly - Is there a smoothing capacitor anywhere in the alternator circuitry?

The alternator also contains extra electronics communicating via a LIN bus which is used by the battery management system to control the alternator output voltage. This electronics could be faulty - it wouldn't be the first time a "smart" alternator has failed in this way.

56 minutes ago, PetrolDave said:

The alternator also contains extra electronics communicating via a LIN bus which is used by the battery management system to control the alternator output voltage. This electronics could be faulty - it wouldn't be the first time a "smart" alternator has failed in this way.

Yes, thats what the mechanic stated. He said that once the electronics was disconnected from the circuit that everything else started working. He also said he was not able to find only the regulator for purchase, and therefor the complete alternator should be replaced. The problem is also he quoted the valeo regulator at 600 eur. I'll have to check if he will be willing to do the work with a refurbished one and still guarantee for the work.

I bought a new alternator. Tee mechanic changed it and everything is working now. Start/stop operational, no strange high revs, unless dpf regeneration is in progress. Battery sensor showing normal readings, no red warnings 2 weeks after, so i guess it will be fine. The old alternator had a faulty LIN module after all. Thanx everyone for helping me out

Edited 4 May4 May by Dodo051