User 52121

Ok looking to solicit opinions (again) from everybody

I had my suspicions, and searches over the last few weeks (I thought) confirmed them. But it turns out I was wrong.

The voltage gauge on the dash reads low - and seems to change with the car's temperature. When I first start it in the morning (so the car is cold) I'm at ~14v. Drive the car for a half hour, and the gauge on the dash is just barely hovering over the 12. Get it hot, run the A/C, have the fans kick on, run the headlights, or any combination of that (where I really put a load on the electrical system), and I can get the voltage gauge down to pretty much square on the 12. I can also make the voltage gauge change by hitting the brake pedal (so the brake lights go on), by hitting a power window switch, etc. Voltage dips until the load goes away.

Most of the searches here pointed to bad grounds. So all dash grounds have been cleaned. Battery to body ground cleaned. Fresh (extra) ground wire added between block and battery ground. Grounds on the backside of the motor (near the reference sensors) also cleaned. Fresh interstate battery.

All of this resulted in no change. So I did some more searching, and the last thing I came upon was a bad/dying alternator and/or voltage regulator. Seemed to make sense. As alternators wear and internals oxidize, they don't work as well once you get some heat in there. I got a pretty thick stack of receipts from the previous owner, went through them, and none were for an alternator. So I figured there was a good chance it was original and probably just due to be changed.

So over the long weekend I put in a brand new Bosch alternator (which included a new voltage regulator.)

Still no change.

Using a multimeter at the battery, everything jives with what I see on the voltage gauge in the dash.

12.90v when car off at the battery.
13.90-14.00v when running and cold.
13.50v when warm. 13.30v with fan on. 13.20v with fan on and lights on.

If I were to get the car really warm, then flip on the headlights, A/C, etc. I'm sure I could probably get it below 13.0v.

So what am I missing here?

supes6

I get the same readings on my gauge but eventually I just gave up worrying about it...

TurboTommy

I think those readings are consistant with other 951s.
The electrical system doesn't run as high a voltage as most other cars, for some reason.

pontifex4

That's what I see with mine, as well, but all my readings on the dash gauge appear to be about 1V lower (in very extreme cases of heat and load, my meter reads just slightly above the 12V line).

I did notice a small improvement when I cleaned the front left frame rail ground, but no change from doing the others. I have yet to actually try a meter right on the battery.

User 52121

Well.... okay then.

Thx for the replies! Guess I just won't worry about it then. Kinda wish I would've known this BEFORE I dropped the coin on a new alternator! But at least now I have peace of mind in knowing that it's new (along with a lot of other parts on the car...)

pontifex4

OT: Thomas, that wasn't you at the St. LAC day at Shannonville yesterday, was it?

savvas944

voltage drops with the temp of the alternator,as it increases the resistance in the windings.
Have you checked if the back shroud and the air intake is intact and if the seal
between the shroud and the alternator is there?makes a hell of a difference.
Also the steel fan is very inneficient,i replaced mine from a 90 amp
bosch alternator from a scrapped volvo 240gl,and the readings jumped up
by 1.5 volts.

Willard Bridgham 3

Actually, outdoor temperature decrease causes decrease in electrical resistance in alternator windings.

User 52121

Ok so the shroud is there, but there is no ducting leading up to it.

Looking at the diagrams on the official Porsche parts fiche (http://www.porsche.com/all/media/pdf...91_KATALOG.pdf) in section "901", my shroud looks nothing like what's shown in the diagram (item #38.) Mine points downward. However I have A/C so that must be the image for the non-A/C car as I see separate P/N's for M573 cars.

I have never seen anything under the hood of my car that looks like the rest of the ducting they show there (#s 34, 35, or 36.) Are these parts supposed to be there on a turbo? Or is this perhaps just N/A cars?

At any rate it should be easy for me to just fab up some cheap dryer tubing to get some air ducted up to the alternator. Fingers crossed that takes care of it...

User 52121

I stand corrected.

The other ducting IS there. What's missing is the flexible hose section to connect the shroud to the ducting that runs over near the radiator.

Off to the parts store I go....

TurboTommy

No, but I was at the ST. Lac double header autox Saturday and Sunday, at the Picton airport.
I was in Kingston Friday and Saturday night.
Hmmm, a bit of a coincidence!

JDS968

Definitely the hotter the alternator gets, the less power it can put out...but I was under my hood yesterday and my instructor (specializes in electrics) was pointing out that these old Bosch alternators just don't put out a lot of power, even though they're pretty big. As long as you replace it with the same part, you'll probably get the same readings.

If you're not having any power problems, I wouldn't worry about it, but if you do (especially if you have mods that drain the battery faster than the alternator can recharge it) you might want to put in a different alternator.

User 52121

Ok hopefully most of you scroll down to the new post here (since I can't edit the freakin' original post....)

I've installed a Nissan alternator. I hoped/expected that I'd see a constant 14v on the gauge. Instead, it almost seems to have gotten slightly worse.

First few times I started and drove the car with the Nissan alternator, needle would sit at 14 on the gauge when cold, and once warmed up would sit halfway between 12 and 14 on the gauge. It was relatively cool outside for this though, highs only in the mid 50s. But I was hoping that's what it would ALWAYS be.

This past weekend, I installed a LC-1 WBO2 sensor. Shouldn't matter, but I'm listing it anyways.

Yesterday, we had our first warm day here in Chicago, upper 70s. I drove the car to work. Upper 50s in the morning, didn't notice anything odd with the voltage gauge. Driving home though, warm air, stop and go traffic, voltage ran low... from juuuuust touching the line for the 12 when cruising, to being square pointing dead on at the line for 12 after sitting still for a bit, or creeping.

Drove it again today - much warmer this morning (mid-60s), and started off with the gauge mid-way between 12 and 14, quickly fell to right above the line for 12 and fluctuated between "just above" and "on the line" at 12.

Got home tonight, took apart all the grounds and cleaned them. All were in good shape except the one by the driver's side headlight - that was pretty nasty. I must've spilled coolant on it. Cleaned it all up (and went over the others again), and added an extra ground strap from the block (water pump) to the chassis (where the coolant crossover pipe mounts to the frame rail).

I should note I already have a super duper ground strap (thick braided wire) running from the battery over to one of the fuel rail mount bolts on top of the cam tower.

So... grounds are clean. And I just took it for a quick ride - and NO difference!

Now, the gauge on the dash appears to be off a little... with the needle sitting just above the line for 12, my voltmeter on the battery says 13.0. Revving the motor, turning accessories on and off, headlights on and off, made no difference really. It'd more or less fluctuate between 12.95 and 13.05. While sitting in the garage I could not get the dash gauge needle to drop as low as it was in traffic today so I don't know what the *actual* voltage is.

I can't see anything wrong. I disconnected the LC-1 thinking perhaps it was an odd draw or something, but no difference. Shutting the car off, the voltmeter on the battery quickly fell to 12.6, so the alternator IS doing SOMETHING. I imagine once a bit of the surface charge wears off the battery will probably be closer to 12.2-12.4 in the morning.

I'm just nervous about letting it go... have an autocross this Sunday and the wife is going to be driving the car (I have to drive my truck) so the last thing I need is for this thing to puke on her while she's driving home.

Do you think 13.0v is enough to keep the battery charged? Am I just being picky and searching for problems here?

refresh951

Strange, I saw pretty much the same thing on my way down to FL with my new Nissan alternator. I was concerned about it but never had an issue with the battery. What is the logger showing?

User 52121

Don't take this the wrong way.... but that makes me feel a lot better, knowing you noticed the same thing.

Good call on the logger. Duh! I took some logs yesterday when messing with the WBO2. Lemme dig them up. I have to reboot my laptop (Mac w/bootcamp) to get to them though.