Voltage regulator or alternator?
 

The speedo in my 3.2 recently stopped working. Leading up to this, there were a few occasions while driving when the needle would go crazy for a couple of seconds, and then go back to normal. I sent the speedo out for repair, and the guy found several components on the board fried. This is making me suspect that my voltage regulator is going bad. I don't want to do anything more until I get that resolved. I tested the charging voltage at the battery at 4000 rpm and it looked normal, 14v or so. But the speedo needle issue seems to suggest that the problem is intermittent and could keep happening.

From the records I have, I know that a previous owner replaced the alternator with a new Valeo one in 2012. Since then the car has only had 18k miles put on it. To me it seems unlikely that the alternator itself could have gone bad in that time. Does it make sense to try just replacing the voltage regulator first?

The regulator is on the alternator. You will have to remove the alternator to get to it. If you have a local rebuilder just take it to them, or replace the regulator and maybe the diodes.

It is not an easy project to get the alternator out and back in. The air diverter on the back of the alternator leaves little room to work.

I just did the voltage regulator in my 86 930 a few weeks ago. I bought the inexpensive one from our host, it looks different than the original but it fits and works just fine. I’ve got 2.5 tanks of gas through the car since.

My guess is that you may have a damaged signal wire or poor connection at the transmission.

When the VR acts up, your tach starts to drop to 0 first.

I had problems similar to the OP's in my '87 930. For a few seconds, the tach would go nuts, bouncing all over the place while I was driving. I hooked up a multimeter to my battery terminals and saw how the voltage would spike up to 16 volts while this was happening. I was fortunate that nothing fried. I replaced the VR (and the alternator while I was at it, as they're piggybacked on top of each other and I didn't know the age of the alternator--it's about as much labor to replace one compared to the other).

A damaged signal wire wouldn't lead to burned components on the speedo PCB. One of the resistors on the board was visibly scorched. The unit is was definitely fried - I had good pulses from the sensor, but on the bench the speedo was dead. After repair, it works on the bench.

Yeah fair enough about taking it to a rebuilder, if I can find one. They would at least be able to test it before and after. I've watched a YouTube video on the replacement, it looks manageable - I have small hands :)

My main thing is that I'd rather not replace the whole unit, at a cost of almost $1k, if only the regulator ($120) is bad. I understand that I have to get it out either way.

I didn't find the overall job (of VR and alternator replacement) too difficult for even my own shadetree mechanic abilities.

I’m all for original parts, but I’d probably go with Valeo for $400 vs $1k if you can’t find a good local rebuilder.

Well even the Valeo is $725 USD from our host. Now that I live in Oz that's $1k AUD :) So that's more what I was referring to. I know of a good auto electrician shop near me, so I'll call them on Monday and see if they would be up for doing the rebuild. I can also ask my regular Porsche mechanic.

Ah. I didn’t see you were in Aus.

What was the behavior of other dash instruments when the speedo was acting? Did they also bounce? What is their behavior now?
Your alternator and VR may be OK. Before you ditch them you may want to do a few test runs. May be plug a voltmeter in the cigar lighter, preferably an analog meter with a needle and not a digital meter, as a DMM might not see very short pulses while an analog meter will bounce and let you know if your electric system is going mad. Do this while driving, with the engine at various RPM.
You may find out that your alt and VR are OK. Maybe it was only a speedo failure that is now repaired and you can plug it back in.

I have pulled the alternator in and out of my car enough times that I think it's pretty easy, and I have size XL hands. The first time you do it, pull the wires to the alternator into the shroud and much as you can, and you will get another inch or more of wire length. That doesn't sound like much, but it makes a real difference in working space. The ground wire/strap from the alternator to the engine frequently is the shortest, and you can replace it with a longer strap, or move the eye from one of the case bolts to a closer case bolt, which will give you more room to work "next time."

Agree with Gilles on using an analog voltmeter to check for voltage spikes. A loose blue wire might cause voltage spikes, because the alternator gets its field current through that wire. If the wire is loose, the field current, and thus the output voltage, will jump around. Therefore, when you first remove the alternator, check the connections to the alternator to see if any are loose or corroded. If so, you might have a no-cost repair.

Not getting the "difficult" job part.
Remove the belt, undo the strap, remove the air diverter, remove the alternator, replace the regulator.

The VR is sub $30