Dauner

I've posted several times about my 98.5A41.8TQM having an annoying vibration that the car did not have for the first 100K miles.

I noticed something interesting the other day. The vibration gets slightly worse when I turn on the rear defroster (I assume decent current draw). It is almost the same as when I turn the AC on.

Any chance my engine vibration problems could be associated with my alternator? Car has ~ 170K miles.

Thanks in advance!

AudiMick

Putting more load on the alternator is going to change the vibration characteristics of the motor.

Somewhere one of the bearings on one of your accessories or tensioner maybe causing this.

Unfortunately vibrations such as this can be hard to find without good test equipment. NVH is an art unto itself. (I sell vibration sensors to automobile companies including Audi / VW).

You could unplug the alternator (not remove it) and see if the same thing occurs when running off the battery.

I would caution against repacing the alternator though, sometimes vibrations are caused by two or three different assemblies running at frequencies which are exteremely close to each other. You then get 4th, 5th,6th harmonic which is the differences in the frequencies.

So changing the alternator just might move the vibration to a different spot.

Are your engine, transmission and snub mounts in good shape.

Dauner

Thanks for the reply!

It gave me a vivid memory of my "measurements lab" class in engineering where I couldn't quite get the acellerometers to do what they were supposed to. Something about rotating shafts, and harmonics, but it just wasn't working for me... Maybe we needed your superior devices...

Anyway, I have replaced my motor and snub mounts, but not the tranny mounts. Primary reason for not doing the tranny mounts (last time I thought about it) was that my vibration seems to be extremely temperature dependant. I posted on the problem a while back, and got the same response about mounts. I like your idea of removing the alternator from the system, even though I may get inconclusive data. I'm actually fairly certain that I have multiple sources of vibration due to so many miles, I'm just trying to ID primary ones. Very frustrating, as every shop I bring it to just shrugs it off to being a 4 cylinder. I know the engine "can" run smooth, because it does during the right environmental conditions.

Other things for me to look at are the original fuel injectors, original O2 sensor, original suspension all the way around (although vibration exists while sitting in the garage), and original MAF. Anything else from a "non-invasive" standpoint that you can think of?

Thanks again!

Mark