2010 A3 S3. No start amd sudden shutdown when hot

[edmiri]

Grettings!

Im having an issue with my audi 2010 a3 s3, 2.0 petrol, s-line. 

When the weather is hot or the car gets hot the engine cranks and doesnt start. It also shuts down suddently and occasionally there are no dashboard lights at all. 

 

After it cools down it starts and run fine.

I connected my oschiloscope to the battery terminals and while engine cranks i noticed a drop in voltage everytime the engine rotates. It only does a few rotations and then stops. Max voltage drop i measured was 5v.

 

I also used a clamp metter during cranks and got 59 ampere. 

Any iddea why this happens.

 

I have tried a new battery.

 

 

[Steve Q]

This could be a starter motor issue or crank sensor issue. Or according to this thread the sensors that have decide on fuel delivery can be mapped out: 

https://www.google.com/amp/s/www.audi-sport.net/xf/threads/197826/%3famp=1

[edmiri]

On 6/21/2025 at 6:27 AM, Steve Q said:

This could be a starter motor issue or crank sensor issue. Or according to this thread the sensors that have decide on fuel delivery can be mapped out: 

https://www.google.com/amp/s/www.audi-sport.net/xf/threads/197826/%3famp=1

That doesn't explain the no dashboard lights issue.

[Steve Q]

30 minutes ago, edmiri said:

That doesn't explain the no dashboard lights issue.

Sorry if missed that bit. In that case I wonder if it's a problem with the imobiliser which is built into the instrument cluster. 

The clusters are known to fail but can be repaired for around £300

[edmiri]

10 hours ago, Steve Q said:

Sorry if missed that bit. In that case I wonder if it's a problem with the imobiliser which is built into the instrument cluster. 

The clusters are known to fail but can be repaired for around £300

This would be the only issue or its an issua in adition to the ones you mentioned? Could a defective imobiliser also prevent the engine from starting and also turn off the engine suddently.  Those happens when its hot weather only. 

[Steve Q]

4 minutes ago, edmiri said:

This would be the only issue or its an issua in adition to the ones you mentioned? Could a defective imobiliser also prevent the engine from starting and also turn off the engine suddently.  Those happens when its hot weather only. 

Yep imobiliser definitely can. 

[edmiri]

On 6/22/2025 at 6:37 PM, Steve Q said:

Yep imobiliser definitely can. 

I noticed something. The ingnito key is sending unregular voltage to the relay. When i feed the relay with full voltage during ignition the engine starts without cranking.

[Steve Q]

4 hours ago, edmiri said:

I noticed something. The ingnito key is sending unregular voltage to the relay. When i feed the relay with full voltage during ignition the engine starts without cranking.

So could be the key then. Have you got a spare?