About half the time I start my 1993 968, there's a loud screeching sound that is exactly like a belt is slipping. The screeching lasts for about 30 seconds, reaching a crescendo and cutting off like someone flips a switch. Both belts (power steering and alternator/air conditioning) are good and snug. It's not related to the A/C, because it will screech with the A/C off.

Today, I was driving the car while it was doing its 30-second screech, and the screech got really loud and cut off. When it cut off, I could feel a distinct increase in power from the engine, as though whatever was screeching was loading the engine. Since I've ruled out the A/C, I figure that there's a problem with either the alternator, power steering pump or water pump. (The water pump is driven off the back of the timing belt.) Everything seems to work okay while the car is screeching -- there are no dash warning lights for a weak alternator and the steering feels fine. I can run the A/C or not run it, and it doesn't affect the screeching.

I have a few ideas about how to troubleshoot this but wonder if someone out there has experienced the same thing and can point me toward the problem. Thanks!

Bob Kovacs

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I have a screech too but not 30 seconds long...just 3 seconds at most and only during start in the morning, when things warm up no more sound during restart. I will try to tighten the alternator belt and see what happens:)
Reading your initial post again, and it is an odd problem. Some components are similar to mine (cold only, location), but I do not have any power loss sensation although I have a Tip, not sure if yours is manual and maybe you can sense it more than I.

Does yours run smoothly during the noise, even if with less power? Did you get a chance to try to pinpoint or at least narrow down, the source?

Is the alternator a weak point, do they tend to fail?
I've driven the car several times since my initial post and it only did the screech thing once more for a couple of seconds. It was too fast to test what was going on, unfortunately.

I have a manual, but the power loss was pretty substantial. The power loss I got during a long screech was WAY more noticeable than the drag from the A/C (for example). I just wish the problem was more consistent so that I could figure it out.

Bob Kovacs
I saw a post somewhere else about an idle control valve or switch that also has to do with the air conditioning that can cause a huge drag on the engine when the air conditioning kicks in. I wonder if that's something that might be worth checking out? Good luck.
Just an idea, try recharging the freon of your AC system and it might be the clutch/pulley of the compressor that its about to give up.

I have no screech while moving so I guess its different from your issue.
There was a good screech yesterday and I'm now convinced that the problem is the alternator. The voltage gauge in the dashboard read 12 Volts when the screeching occurred, and immediately jumped to 13 Volts when the screech stopped. I'm sure it's not A/C related in my car, as the screech is unaffected whether the A/C is on or off. And the A/C seems to work normally.

I guess I'll look around for a new alternator...

Bob Kovacs
I think its just your belt as they are driven by the same belt, just have it tightened first and test. I also read in other forums that the gauges on the car is over sensitive so unless it really shows less than 12v for a long time you shouldn't worry.
sounds like it stops rotating, does it not? That would explain the voltage drop to battery level and of course the belt noise and power loss (dragging the belt around the stationary pulley). It is still a strange problem to be so intermittent. If a bearing were going/gone, it would be a constant problem of some sort.

My noise only happens at low engine speed, basically idle, and is louder when the A/C cycles on, but just because the idle speed slows a bit. As soon as I start to move, the noise quits A/C or no A/C.
Yes, that's exactly what I thought... the alternator stops rotating for some reason. The odd thing is that the alternator shows no other sign of failure -- there's none of the usual noises or electrical anomalies. I will try tightening the belt a bit more (it's pretty tight now) and see if that makes a difference. If the problem persists, I'll get a new alternator.

I have to say that one of the major pains with the 968 is that so much has to be done from below the engine, which is covered by the big tray. I have taken off and re-installed the tray eight times in the past four months, usually for things that would be much simpler on any other car. (For instance, replacing the alternator in my Subaru took 20 minutes from the top of then engine, on a sloping gravel driveway on a freezing winter night. I had only flashlights for light and mostly wore gloves.)

Oh well... the joys of owning a Porsche!

Bob Kovacs
Howdy Bob,

That is similar to what happened to me. I finally had to replace the alternator on mine when it gave up the ghost one night. Just as we made it home to our house, everything quit. We were very lucky.

Good luck with your situation.

It's called "elegance of design" or something such. We can't be bothered with such vulgar things like mechanical problems or, perish the though, actually touching the engine....
Bob, just remember to tighten the belt correctly. If the belt is too loose, it will cause a squeal, too tight and it puts strain on alternator, A/C compressor and crank pulley, causing a leak at crank seal.
Let us know abt. finding an alternator. Maybe a good alt. starter rebuild shop can refurbish it.
good luck, Ken


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