Slight jerking during RPM drop from 1200 to 900 RPM

When I'm driving my car, and I'm in gear approaching a traffic light, and the engine speed goes from 1200-900 RPM, my car vibrates or jerks slightly! You can feel it mostly through the gear shift lever and... well, you know!

The same thing happens when the engine is idling, and even if I briefly accelerate it to 2000 RPM, the revs drop. While idling, I can also hear a slight metallic rattling sound in that range.

I'm at my wit's end! Everything is stuck in the engine compartment, but otherwise, the engine runs perfectly fine.

It's a chipped Audi A4 from 1996 with 214,000 km on the odometer.

I recently had the clutch replaced, but that probably has nothing to do with it, right?

Maybe you can help me!

Best regards and thank you.

Manuel.

Sure, here is the translation of the text from German to English:

'Addendum:'

I just figured out, with my wife's help, that this rattling noise is loudest at exactly 1000 RPM and it's coming from the lighting system!

I'm now suspecting that the alternator is faulty, based on the premature wear of the V-belt.

But can the alternator, via the serpentine belt, have such an influence on the engine?

Thank you.

Manuel.

Hi Manuel,

If you had used the search function, you would have found the article by Ulf about the point machine. At the end of this article, it mentions a tendency for jolting. The opposite effect suppresses this jolting.

I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.

The second article addresses exactly the problem you're facing.

Okay, I understand. Please provide the German text you want me to translate into English. I will only provide the translation.

I did read that!

It's hard to explain; it's not a real jolt, more like small vibrations!

Since this issue has only appeared in the last approximately 3,000 kilometers, and my alternator is making noises that are almost as loud as the engine itself, I think the problem might be related to the alternator.

Background information:

In 2001, I took my car to an authorized Audi service center for a water pump replacement, and when I got the car back, one of the fan blades on the viscous coupling fan was broken!

Of course, I wasn't told this, and the mechanic probably didn't dare to report the damage he caused, nor did he install a €20 replacement part.

Less than 10,000 km later, the car was back in the workshop because the imbalance of the viscous coupling had caused all the bearings driven by the ribbed belt to fail!

Everything was renewed, except for the generator, as it was still considered to be in good condition at the time!

Therefore, I'd like to reiterate my question: Can a faulty alternator bearing have an impact on the engine?

Regards,

Manuel.

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Hi Manuel,

Manuel Thomas wrote:

Everything was renewed, except for the generator, because it was still considered to be in good condition at that time! Therefore, I'd like to reiterate my question: Can a faulty alternator bearing have an impact on the engine?

Since every amount of resistance puts a strain on the motor, even a faulty alternator can cause jerking. However, you should definitely notice it if the free-running alternator is malfunctioning.

I just remembered something else:
With my first rally car, after replacing the clutch and driving about 500 km (on public roads, not in competition), I experienced an inexplicable jerking. It turned out that the clutch had completely failed. Subsequently, the KW bolts came loose, which crushed the clutch disc. This was a result of a faulty installation. I hope you don't experience something similar.

Okay, I'm going to replace the alternator now! I'm getting one from a salvage yard with a one-year warranty and 100,000 km mileage for €100!

Even if that wasn't the problem, at least I'm rid of that annoying running noise! And now I can enjoy working on my car again, instead of dealing with that irritating and embarrassing scraping sound!

However, you should definitely feel that with a free-range hen!

According to E**, I also have a free-running pulley! Could you please explain that to me briefly?

Jerking. According to my definition, jerking occurs when smooth driving is no longer possible within this RPM range!

That's not the case; it's more like a slight vibration! My wife doesn't even notice it herself, but I have a very sensitive 'butt meter,' you know!

Regards,

Manuel.

Manuel Thomas wrote:

Jerking. According to my definition, jerking occurs when smooth driving is no longer possible within this RPM range!

Agreement. That's also how I define stuttering.


When I say "free-running alternator," I mean: remove the belt, manually rotate it (in the direction of rotation), and assess the smoothness of the rotation. Normally, the alternator should spin freely without any noticeable jerking or resistance.
However, if there's an electrical fault in the alternator that's causing the jerking, you obviously can't diagnose it like that.

I won't be able to check that until tomorrow!

But I would also like to commend the engineers.

Thanks for letting me disassemble half the car to change the headlights!

Vehicle front in service position
Loosen the V-belt.
Remove the viscous fan.
Replacing the alternator.

Okay, I guess I'll have to take it to a professional since I can't remove the front panel myself.

Regards,

Manuel.