mytrplseven

This is a re-post. My alternator won't energize (excite) until I rev the engine up after start-up. I checked the tension on the belts and they're tight and there's no oil on them. Does this sound like a voltage regulator or alternator wear issue, i.e., brushes, bearings, stator? It happens every time after I start the engine and then works fine while driving. I did notice a while back that the voltage meter was indicating variations between 12 and 14 volts and after driving for a while, it never came back. Figured there was some debris on the brushes. Could be I'm just ready for a new alternator.

sweetskillsrk

I'd say voltage regulator. It's super easy to get to, wouldn't take more than 20 minutes to check and 10 minutes of that is getting off that freakin rear alternator cover. If you're not getting any bearing noise/squeak or pulley wobble I'd say its the voltage regulator. Also, while you're in there, loosen up the alternator belt, slip it off, and give the alternator a good spin to just see how it feels.

mytrplseven

Just curious. Does the regulator regulate output voltage or does it have something to do with when the alternator starts putting voltage out. I understand electronics but this part has me a bit baffled. If the input voltage is ok and the VR is defective, could it need more input from the alternator before it switches to an "output" mode?

sweetskillsrk

The voltage regulator is the brush assembly that regulates the output voltage. It's mounted into the back of the alternator with two small screws and 9 times out of 10, that the little thing is the issue.

MAGK944

Are you absolutely sure it's not just a bad wire or connection on your alternator? The small wire is the exciter wire and should have battery voltage without the engine running, key at #2 position. It doesn't play any other function after the alternator starts to generate power.

If not, the regulator and diode pack is an easy swap, but I'd check that wire first.

mytrplseven

I guess if I'm going to go so far as gaining access to the rear of the alternator to check the wire and it's connection, I might as well replace the regulator anyway. Thanks for the info, friends.

joejoe

On the 928 there is a resistor on the guage cluster that can go bad. Without it symptoms are as you describe. Also does the warning light come on for the alternator? The charging circuit goes through this. No light with key in on position = no charging until rev past 2k or so.

f1rocks

My guess would be something wrong with the exciter wire as well. I don't have one on the race car. Just let it warm up a bit and blip the throttle and it starts charging no problem.

mytrplseven

The master warning light would come on until I rev the engine and it starts charging. Interesting idea about the resistor. I wonder which one (if applicable) it would be? I'll have to check the manual to see if there's a specific light for the alternator/battery volts. Later: I checked the operators manual and it refers to an alternator warning light but doesn't show an icon on the guage face pictures. Also couldn't find any references in the Haynes wiring diagrams that point to a master warning light and the volts gauge wiring looks like both sides go to ground (go figure).

MAGK944

The master warning light coming on really doesn't help you much for the diagnosis. That light has +ve from the battery on one side and +ve from the alternator on the other side. If battery or alternator fall to zero it will cause a voltage difference and light up. All it's telling you is the alternator is not putting out power, but you know that. Unfortunately it doesn't tell you why.

TexasRider

I vote for the brushes as the most likely spot, or at least the starting spot .

Arominus

I had this problem with my 924s after i put an LED bulb in the warning light for low voltage. It wouldn't charge until the car hit 1200rpm after startup. The lack of resistance was messing with the DME and IIRC the DME uses the light to check voltage at startup? I remember reading it somewhere but don't know exactly where.

Anyhow i put a normal bulb back in that spot and then it charged fine @ startup.

BlackOp

I have the same issue...I recently had my motor mounts done at Eurosport...and the mechanic said Porsche put a bulletin out about this issue...said something about adding a diode. I haven't seen anything about that though.. It's not your VR..I put a new one in and the problem still exists.

If you solve the problem..please post how. My feeling is the wire...

MAGK944

Yep, as I said above that bulb needs 12v on BOTH sides to go off. An LED in there, which is basically a diode that allows voltage to flow one way only, would really mess things up.

For those not to familiar with electronics, you need a voltage difference for a light to come on. Normally we are used to dealing with a difference of 12v on one side of a bulb and ground (0v) on the other. But on charging circuits it is common to have the battery 12v on one side of a warning bulb and the alternator 12v on the other. That is why when the engine (alternator output) is not running the light comes on and goes off as soon as you start the engine. Hope that makes some sense to someone.

BlackOp

Just to be clear in my situation...The mechanic said to ADD a diode to solve the start-up/no charge symptoms. I don't have any LED lights in my car....

To the OP...do you notice any flicker in your headlights/markers? I'm getting some and was going to check my grounds. I cant help but think these two issues are related...meaning not a ground issue.