Problems diagnosing Alternator failure?

Sootywg

Sootywg

A2OC Donor
United-Kingdom
There are many posts relating to battery/charging problems, however what I am currently experiencing seems a little different to any of the threads I have read, so I thought it worth posting my experience so far in case it is helpful to others and of course I hope to conclude with identifying the solution. I would be grateful if anyone can add to my diagnosis as it progresses in the hope it may reduce the expense that I fear is heading my way. The car is a 2004 1.4TDI 75 (BHC engine code) with 140A Bosch alternator and 72k miles

The starting point is that all was fine on Thursday no known problems. On starting the car Friday morning a screeching noise was coming from the offside under the bonnet.

Whipped the bonnet off and it sounded as if the screeching was coming from the area around the poly v belt driving the alternator and A/C compressor. I had no choice but to continue my journey, with the sound varying in intensity as the speed varied. The only other thing I noticed was that the radio kept switching itself on and off. No warning lights on the dash. The battery charging light stayed out.
On returning I put a voltmeter on the battery which read 12.4 (ignition off) and 11.9 (engine running) revving the engine made no difference.

So dead alternator presumably?:(

I have read all the threads on faulty alternators. Failing clutch pulleys/ tension pulley


Questions I asked myself:
Why sudden failure, no advance warnings
Why no charging light on dash
Why the screeching if it is the alternator rather than the clutch pulley

Next steps:

Remove poly v belt (looks in ok condition no shredding just a little cracking on the ‘v’s)
Run engine with no auxiliary belt:- No squeals
Check alternator bearings and clutch pulley:- alternator spins freely clutch works correctly. Very slight axial play in pulley.
Check tension pulley:- spins quietly and runs true

Remove Alternator, check brushes:- not badly worn both of equal length but slip rings are quite deeply grooved.
Test voltage to the rotor field coils:- 12volts OK
Test diodes: low resistance in both directions (if I tested it correctly!) bad sign possible diode failure.

Initial conclusion:- Rectifier array appears to have failed. This might explain the sudden failure. However the question remains as to why the battery charging warning light does not come on? Also why the screeching.

Can I replace the diodes or the rectifier assembly? Regulators (with brushes) seem quite easy to find but rectifiers are more difficult to find.

Re-assemble and re-fit the alternator with new poly v belt to the car and start the engine. No change expected and none found.

I can only conclude that the screeching is caused by the clutch pulley because there being no load/resistance from the alternator it causes the alternator to intermittently spin ahead of the engine? Perhaps this pulley is also on the way out.

What I cannot understand is why the battery charge light is not on. I read in a different forum that some alternators are actively controlled by the CCU and what seems like an alternator failure could be a fault in the CCU (even more expensive I think!). Anyone on the forum know if this is true on the A2?

I will go to an auto electrician on Monday and see if they confirm my diagnosis, if they do I then have to decide whether attempting a home rebuild is worthwhile or bite the bullet and buy a new alternator. The 140amp is certainly a back order from Audi

140amp Alt 045 903 023 GX £357.54 +VAT
Clutch Pulley 022 903 119C £ 72.96 +VAT

Finding a 140amp version on Ebay is difficult, what are the consequences of fitting a 120amp? Is the higher output needed because of the electrical engine heater?

I will post any progress

Any input will be gladly received

Geoff
 
It would be nice to make progress before Monday but cannot think of anything constructive to try other than to make a swap of the alternator with a known working one.
Anyone local to Huntingdon happen to have a working spare lying around?

Geoff
 
Hi Geoff you sound like you have done very good diagnostics on this problem--it does sound rather strange. Sorry i can't help more but maybe someone else can shed more light.


cheers mike
 
Yep looks like the one and the numbers match up. The best price I have seen so if it comes to the crunch I will give them a call.

Thanks for the link

Geoff
 
The plot thickens and some expert A2 advice is definitely needed!

By the end of Sunday I had convinced myself the alternator was definitely the problem but decided to let my local independent do a 'test it in situ' Re installed the alternator and off I go not expecting them to tell me anything I did not know. Indeed their tester just said there was a low output at the regulator. Doh!
They could not tell me whether it was likely to be regulator, Rectifier, brushes or windings.

With a replacement being near £300 I found a specialist Auto electric place that claimed they repair if appropriate rather than replace (sounds my kind of guy) and can do an off vehicle test on a bench rig. So off with the alternator again (that tension spring and I have a strained relationship :mad:).

Took the alternator to the Auto Electric guy and guess what? He says it is all working fine 13.8V output and 120amps (running below it's 140amp spec though)
He says no problem with rectifier or regulator so he says it must be wiring on the car (or could it be the CCU not activating the field coils?) Clutch pulley a little suspect but not to the point of causing the symptoms.

As stated before, I checked the voltage at the 2 pin plug and it was at 12v (when disconnected) but the output is only 3v when spinning on the car so my current conclusion is that the field coil is not being activated.

If I believe what I am being told I could replace the alternator and still have the problem!

I will now check for wiring faults but if one of our resident experts have any suggestions on what the CCU might be contributing to the problem I would be grateful.

Would a VAG COM diagnostic pick up on a fault like this? I am reluctant to drive it any distance in the current condition.

All help gratefully received.

Geoff
 
Problem resolved... at least I hope.

I had hoped to have some words of wisdom and experience form forum members as I am sure I can not be the first to have experienced these symptoms. However I post my own experience in case it helps any other members.

Having had one garage confirm 'alternators dead mate' and an auto electrician bench test tell me all is working fine (alternator was spun from the spline bypassing the clutch pulley) I already had a new pulley on order. I went home to scratch my head, on the way home the battery light did flicker on and off every now and then, I could not determine the condition that caused it to come on.

I feared it could be CCU related not activating the alternator but after a nights sleep and the pulley arriving I decide to fit it anyway as the first symptom had been the screeching from the pulley belt area and it was the only part that could make all the symptoms tie up.


Old pulley off, there seemed little obvious difference from the new one, turns reasonably smoothly one way and locks the other, axial play not much more than the new one.
Fitted the new one anyway, reassembled alternator and belt not expecting much change other than maybe a quieter engine. Fired her up and.... 14.45volts at the output. Result.

Unless I have jiggled an electrical connection somewhere that is the real reason for the cure (unlikely as the alternator has been off at least 3 times and to no avail)
it would seem that a not obviously dodgy clutch pulley can cause the alternator not to activate. If this is the case then be warned that at the first sign of any noise from the pulley/belt there could be an imminent loss of charging. It seems yet again the intelligence of the A2 even at 10 years old is too clever for it's own good.

Interesting to note that had I replaced the alternator as advised, the problem would also have resolved because it would have been fitted with a new pulley but I would have been over £300 lighter in the wallet.

I am going to be monitoring the voltage at the cigarette lighter socket for the next few days just to re-establish confidence in the old girl

Geoff
 
well done matey, always helpful to post solutions. luckily ive had my pulley changed already but i suspect as mileages creep up this problem may crop up a bit more.
 
Great thread! This is something to bookmark for future reference.
Glad you got it sorted!
 
just read this and noted you have fixed the problem. i had the exact same issues and the screeching was the clutch pulley not engaging properley thus not spinning at the correct speed and only throwing out 11.8 volts. bench tested all semed fine as yours did. i suspect it may have something to do with the way the belt is attached to the pully on the bench, probably due to the tension applied being greater than when fitted to the car
 
georgem said:
i suspect it may have something to do with the way the belt is attached to the pully on the bench,
Click to expand...

Absolutely correct. The auto electrician span the alternator directly on the spline hence by-passing the clutch pulley.

This is what convinced me to change the clutch pulley before anything else. The screeching is the clue, I could not make the clutch slip by hand in the drive direction and it ran free on the over run. The activation must be very sensitive to the alternator speed.

Geoff
 
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