944 Turbo low voltage output
 

My alternator was only putting out 12.8-13.0 volts at the best of times. Given that the car has 140k miles I decided to replace the alternator with a rebuilt Bosch but I'm still only getting 13.1 V. How can this be?

Voltage Regulator.

Haven't dealt with this problem on my 944 "yet"... But one of my old motorcycles had this problem because of a bad Regulator...

Try searching "944 Voltage Regulator"

Check your voltage at the battery with a multimeter, sometimes the guages read low, maybe your original alternator was ok. was it having trouble starting?

The alternator came with a new voltage regulator. The in dash gauge only registered at 12V most of the time and the low beams were quite dim at night, wipers working slowly etc. Given the mileage I thought it would be advisable to replace the alternator and I'm almost certain it was the original one. I guess I should clean all the grounds. What ones would cause such a significant voltage drop? I wouldn't think it could cause a 1.0 V + drop in voltage.

Mine starts fine and the gauge reads in the middle or a bit above when I start driving then falls near the lower "normal" area of the gauge. I just checked it and it reads 13.2v running with the ac on. No issues starting or with headlights ect. Is this out of spec? I was going to leave it until something goes.

time to clean the little football looking things on the back of the cluster....and all your grounds...

Grounds cleaned...what are the football things?

you guys also notice your temp gauge go up as the voltage goes down?

Not mine it is consistantly below half when warmed up

I'm not sure 13.1 is bad but you could take a peek at the battery cable connections at the stater and the big ground lug at the back of the engine. If you're getting 13.1 at the battery with the headlights on and maybe the flashers too, then I guess I would think you're OK. The dim headlights and slow wipers may be a separate problem (pretty common really)...Bruce

I've never trusted my temp or voltage gauge. My voltage gauge has always laid flat on 12v and there's no way that's possible. New battery, new negative cable, cleaned all the ground points, new voltage regulator (stock alternator) and no change. It's got to be something in the console - one of these days I'll fish it out of there and clean it up (and put in new lights), but for now, I'll deal with it.

There was an interesting discussion on a different forum about measuring voltage on running cars with digital voltmeters. I haven't checked it myself, but apparently the small amount of AC ripple current in alternator output confuses the typical digital VM (even a quality unit like a Fluke). Cheap DVMs are supposed to have a real problem in this respect. It was suggested to find an old analog meter and check with that. Several of the people who had "problems" found they really didn't when they used the analog. Sounds suspicious but I intend to try it.

Analog meters "average" better because of needle swing. If you are getting a ripple that is faking out a digital meter (its actually just settling on a voltage that may or may not be the one you want to see that coincides with its sampling rate), chances are good your alternator or regulator is on its way out. That's the kind of thing that happens when you have dead spots on the alternator and the voltage regulator is wildly trying to keep up with it. The way you prove that for real is to put an oscilloscope on it. I'm guessing no one in the conversation you are referring to did that.

That ripple is the kind of thing that causes hums and buzzes in audio equipment. I don't know how kindly the limited amount of electronics on a 944 will take to it either.

Yes, the coil in the analog meter movement tends to dampen minor variations that are inherent in any alternator, even one performing properly. The chances that all the diodes have the exact same forward/backward characteristics is pretty slim. I would assume that anything like a DME that needs "clean" power has appropriate filtering circuits.

The diodes aren't the issue. It's the frequency. All you have to do is change the RPMs to change that and see if it makes a difference.

As far as filtering, sure, it has some. But a charging system that is exhibiting behaviors like this is pretty far out of spec and indicative of a problem.