After the car has been left for a few days in the colder weather (2005 vRS) and I start the car I get a shriek noise that lasts around 5 to 7 seconds. On popping the bonnet I can then feel the aux drive belt is hot.

 

I fitted a new belt and tensioner but no change. 

 

I have also replaced the alternator pulley which was jammed. 

 

Both alternator and air conditioning compressor pulleys move freely when I remove tbe belt and spin them. 

 

After the screeching has gone on a few seconds it goes away and is gone for the rest of the day.

 

Has anyone had anything like this? Any thoughts appreciated. 

Did you replace the alternator pulley with a new one of the correct type with the internal one way sprag clutch?

 

Alternator pulley should spin freely in one direction and in the other you should feel it take up the drive of the rotor and have more inertia.

 

Do you have the belt running in the correct path? It's quite easy to refit it on the wrong path which looks to be correct and to be the only way of fitting it but is neither.

 

The correct path will have it driving the alternator pulley through 180° of contact, the incorrect way has a lot less contact and the tensioner will have less reserve travel.

Thanks JR. I'll double check routing later but I'm 99% sure its fine. The alternator pulley is freewheeling and locking fine.

 

Looking through old threads could a battery that's failing be a possibility? But even at full charge rate the alternator shouldn't slip surely? 

 

I don't think it's an AC compressor problem as that's working fine.

 

 

I got the belt routing incorrect but could feel when I released the tensioner that it was not as before, it took a lot of head scratching to find the correct but not obvious to me route.

 

It was only when @Breezy_Pete posted some photos of the right and wrong belt routings on another engine that I was able to see and understand the difference, the difference being the much reduced torque being able to be transmitted to the alternator pulley which is why it sounded to me likely to be your problem.

 

The alternator is under its maximum load in the first few seconds after start up, I'm fairly certain from your description that is where the slippage is coming from, you could disconnect the wiring plug to be 100% sure

Good call on the plug! Something to try. 

 

You wouldn't happen to have a link to the thread with different routing of the belt would you? Correct and incorrect?

No, it was almost certainly a different engine but for most of the diesels with the alternator at say 14.00 and the AC pump at 17.00 the run is similar, the best explanation that I can give you is the one I did before:

 

On 17/11/2023 at 19:31, J.R. said:

The correct path will have it driving the alternator pulley through 180° of contact, the incorrect way has a lot less contact and the tensioner will have less reserve travel.

 

https://www.briskoda.net/forums/topic/437831-fabia-vrs-auxdrive-belt-replacement-how-to/?do=findComment&comment=4936037

Thanks both. I've confirmed everything is routed correctly. 

 

The only thing of note is the tensioner, whilst free and working, is one of the plastic pulley ones. Littens I think. I just replaced it with genuine VW with the metal pulley. 

 

The spring on the Littens is very different to the VW one so let's see how this works out.

Ok update time. It turns out that the belt fitted by my garage and the belt suggested by the motor factors are wrong. 1180 is what it is supposed to be but 1183 is what was fitted. 3mm too long. This doesn't sound much but it's a proper difference on the tensioner. With the 1183 it's much further around and noticeably lower tenson on the belt. This causes it to slip and polish and then slip more.

 

Fitting the correct belt has solved the problem.