Weird Harmonic Balancer Squeal

[Minimim]

Hello! This past week i have been chasing down a belt squeal. I have one 1.9TDI AHU and one 1.9TDI AFN (my personal daily which was squealing) so it was easy to switch parts and know for sure what caused the squeak (or so i thought) since i did not want to spend money to replace a good pulley.
Anyway the squeak was kinda random and got very very loud when the car warmed up. This made me believe that the tensioner pulley bearings is bad. I took it off and replaced it with an INA pulley and tensioner that i got for 30 euros new. That solved the squeal for around a day or two until it started doing it again. I then switched the alternators between my cars which did not fix it either. Finally i switched the harmonic balancer (crank pulley) and it did fix the squeal. However on the AHU when i installed the "bad" pulley there were no squeals (except when the AC is on but only at idle and a very faint squeal that you can barely hear unlike when the pulley was on the AFN and everyone in a 50m radius could hear me coming).
So what is the deal that when i switched the pulleys it solved the squealing but did not cause the other car to squeal the same? The water pump or power steering is 100% not making the noise. The alternator freewheel works and AC compressor has no play. Do note that the AFN has a freewheeled 140A aftermarket alternator (which makes no noises either from the clutch or bearing) and a different AC compressor since its again aftermarket).
Do i play it safe and buy a 100 euro pulley or run it like this since its not squealing anymore.
Thanks in advance!

 

[Lug_Nut]

Make a mark, a straight line, from the center of the harmonic damper out to the edge. Start the engine. When you hear the squeal, stop the engine and look at the line.
The AHU and 1Z had a damper with a bonded rubber ring sandwiched between the inner hub and the outer flange. When the bond failed the flange (or hub) would slip on the rubber and create a squeal.

 

[Minimim]

Update: I installed the new pulley on my AFN and swapped the one on my AHU that i test fitted. The AHU now doesnt squeal which is expected as the pulley i used as a test came from it however my AFN squeals the same if its raining out. While its warm it doesnt squeal so atleast that is fixed. The belts both have under 20.000km on them and they are in good condition. Tensioner, idler pulley, harmonic balancer are also new. Alternator was checked and its also good. If i spray the belt with anything it stops squealing for a day or two.
Would my missing belly pan plastic cause the belt to squeal that much? On 3 of my other cars there are no belly pans and they dont make a single noise when wet.

 

[Lug_Nut]

A wet belt might squeal from the stick/slip action. What you want is constant 'stick' and not persistent nor intermittent 'slip'.

A cold damp belt might have condensation from overnight that might create the stick/slip situation that clears up after the alternator load drops and/or the belt warms up and dries. The belt spray would displace the moisture, but thas is addressing the symptom (noise), but not the cause (belt is slipping).
Turn off the electrical consumers (headlights, fan blower motor, seat heaters, air conditioner, rear defogger). Does the squeal stop? Turn all the electrical consumers on again. Does the squeal resume? The alternator load to keep the battery charged increases and makes the alternator more difficult to turn and can contribute to the belt slipping.
A warmed up car has had time to re-charge the battery from the starter operation, and the car interior is at a stabilized temperature so there's possibly less alternator load.

A belt that has been slipping for a long time will become 'glazed' and too smooth to retain grip on the pulleys. The rubber in old belts will become harder and less able to grip the pulleys. If the belt is original, change it. If the belt is recent, change it. At worst you'll have a spare belt.

Have you tried putting oil on the belt to stop the squeak? I jokingly advised a co-worker (40+ years ago now!) to put some oil on his Buick's V belt to stop the squeal. He did and thanked me profusely, until he tried to steer that barge of a car and found the power steering pump was not turning because the oiled belt was slipping.

 

[Minimim]

A wet belt might squeal from the stick/slip action. What you want is constant 'stick' and not persistent nor intermittent 'slip'.

A cold damp belt might have condensation from overnight that might create the stick/slip situation that clears up after the alternator load drops and/or the belt warms up and dries. The belt spray would displace the moisture, but thas is addressing the symptom (noise), but not the cause (belt is slipping).
Turn off the electrical consumers (headlights, fan blower motor, seat heaters, air conditioner, rear defogger). Does the squeal stop? Turn all the electrical consumers on again. Does the squeal resume? The alternator load to keep the battery charged increases and makes the alternator more difficult to turn and can contribute to the belt slipping.
A warmed up car has had time to re-charge the battery from the starter operation, and the car interior is at a stabilized temperature so there's possibly less alternator load.

A belt that has been slipping for a long time will become 'glazed' and too smooth to retain grip on the pulleys. The rubber in old belts will become harder and less able to grip the pulleys. If the belt is original, change it. If the belt is recent, change it. At worst you'll have a spare belt.

Have you tried putting oil on the belt to stop the squeak? I jokingly advised a co-worker (40+ years ago now!) to put some oil on his Buick's V belt to stop the squeal. He did and thanked me profusely, until he tried to steer that barge of a car and found the power steering pump was not turning because the oiled belt was slipping.

Thanks for the info. No the electrics dont have any effect on the squeal. In the rain if i drive into a puddle the squeal gets so bad you can hear it with the windows closed. After around 15km of driving it returns to a slight chirp. Until the rain stops that chirp is always there. Though after changing out the pulley its a different, more easy on the ears noise so thats why i bealive its the belt. I will steal the belly pan from the AHU and drive it a few weeks like this and report back.

 

[Minimim]

UPDATE: After installing a home made belly pan made from scrap plastic the squeal is now gone while driving. There is a slight squeal after starting the car that goes away after about 3 seconds which doesnt bother me.

 

[ZippyNH]

.....
Have you tried putting oil on the belt to stop the squeak? I jokingly advised a co-worker (40+ years ago now!) to put some oil on his Buick's V belt to stop the squeal. He did and thanked me profusely, until he tried to steer that barge of a car and found the power steering pump was not turning because the oiled belt was slipping.

Aww ... remember the days of "belt dressing, used only on V-BELTS...

 

[ZippyNH]

Make a mark, a straight line, from the center of the harmonic damper out to the edge. Start the engine. When you hear the squeal, stop the engine and look at the line.
The AHU and 1Z had a damper with a bonded rubber ring sandwiched between the inner hub and the outer flange. When the bond failed the flange (or hub) would slip on the rubber and create a squeal.

Do look at this...if it goes you will be stranded.
Other cars, like some years of MINI had the same issue. Typically there are different aftermarket designs that can a permanent solution, but if the OEM lasted a long time, it's often overkill