996 Alternator check ? HELP !

[britdave]

My C- 2 2002 996/ 911 seems to have weak starting issues,so decided to do some electrical checks. I want to put my meter leads on the alternator, but it seems almost impossible to get to them without removing several things ?

However, what if I hook the leads up to the positive terminal thats located in the engine compartment, and the ground lead to the chassis somewhere, will that suffice to check alternator out put ?

Yesterday I checked the battery, with engine off, and my leads right on the battery, and it was putting out approx 12.5 volts. Then, keeping the leads on the battery, and then starting the motor, with + and - leads still attached to the battery, and at about 1500 rpm she was putting out about 13.5 to 13.7 volts, is this about right ?

What has been happening ,is the car usually starts ok, I drive it 10 or 12 miles, come back out ,and several times she sounds very weak , turning over, and has now and again failed and I had to jump it.

I'm trying to diagnose what the problem ? Could be but don't want to buy a new alternator if I don't need one ? Perhaps it's the battery leads or a corroded ground strap ?

Anyhow, how would my original idea work, hooking up at the + terminal in the engine bay ? Any help or ideas, comment, solutions , would help. This has been going on for months, and I'm hesitant about even driving now, due to let downs ,and occasionally, a dead battery ? Thanks Dave.

[Ahsai]

How old is the battery? Sounds like a weak battery to me if your voltage reading on the instrument panel reads 13.5-14v whenever the engine is running. 12.5v reading although sounds fine that alone can't guarantee enough current.

You can measure the + jump start terminal in the engine compartment for alternator output voltage more closely since that's closer to the alternator.

Oh yea, you can take the battery to autozone or equivalent to test its health. Also make sure battery cables have tight connection to the batter and check the ground strap from engine (right rer wheel well) to chasis to make sure it's tight and there's no corrosion.

Edited April 28, 2013 by Ahsai

[britdave]

Thanks, I'll do the + jumper terminal test in the engine compartment, it appears to be as close as I can get to the alternator + side ?

But does that particular + jump start terminal go directly to or from the alternator, or to the battery ? Anyone got a schematic diagram, of that area ?

The battery is fine, I tried that before, but may get another one again, who knows ?

Tonight, whilst driving, I plugged a digital volt meter into the cigarette lighter, at first I got about 12.9 - 13 .2 showing, but later down the road and doing about 50 mph, the highest it read was 12.9 volts, average 12 .3 or 12.4 volts at the lighter socket ?

Also, is there a drawing to show where 'exactly' that ground wire is connected to the chassis ? Somehow, I feel it's still the alternator , or it's regulator ? Thats why I'm trying all different types of tests. To bad I can't hook up directly to the alternator ?

Anyone got any other ideas, before I get stranded again, or I spend a lot of money on an alternator I didn't need ? 12.9 volts at 3000 rpm . does not seem right ? But then again would a bad battery do that as I'm driving ? Any ideas , much appreciated, this is driving me nuts ! Thanks Dave.

[Silver_TT]

Sounds to me like it could be the wiring harness based on the fact that you said it starts up just fine (cold) and then after you drive 10-12 miles (it heats up) it starts exhibiting problems.. These wiring harnesses are known to do this as they are prone to corrosion and the resistance builds as heat increases. I believe there might have been an updated part for this or TSB (don't quote me on that) but the Porsche guys around here tell me they replace a fair number of them (put it this way: they stock the part). Usually, if this is what your problem is, after you take the wiring harness off you can clearly see the corrosion.

I would definitely check this before replacing the alternator as they will exhibit similar behavior. Dealer/indy should be able to diagnose this cheap, or if you DIY then you just need a simple ohmmeter. Conceptually this is very simple, although I found it to be pretty tricky in practice as everything in the engine is pretty tightly packed together. In my case it actually turned out to be both the alternator and harness, but I would say there's a decent chance it's just the wiring harness. Either way, this isn't a terribly expensive problem. Bosch alternator is only around ~$350 and the cable is ~$50.

[logray]

Just to clarify the "harness" the previous poster is speaking of.

It is not the "main engine harness" which costs many thousands of dollars.

It is the primary battery wire that runs from the alternator to the starter to the main jump point in the engine. There are a couple issues with this cable and it has been revised by Porsche a few times. The first issue is the point where it connects at the starter can become corroded and break. The second issue issue was the cable gauge was insufficient.

Of course, heat soak can compound the problems with an under-performing component.

There are many threads about that as well, here is one for example:

http://www.renntech.org/forums/topic/31797-strange-low-voltage-problems

This topic comes up very often on this forum (in fact just in the past week), and there are many threads about this as well, but you can also have the alternator load tested (a voltage test is less helpful in most situations). You can do pseudo load tests with the alternator in car, but a better way is to remove the alternator and starter (which is really not that difficult to do) and take them to a parts store such as Napa. They have expensive load testing machines that can give you a lengthy print out of each item (including batteries) and their estimated useful life remaining/pass/fail/ability to perform/etc. Most parts stores will provide this service for free.

[Silver_TT]

Yes, that's correct, I'm referring to the cable that runs from the alternator/starter/main junction. This is a very cheap fix as the cable is only about $50 and it's about a 2 hour job by the book.

Logray is one of the best posters around here so I would defer to him, but just my personal advice having gone through something similar not all that long ago:

1. It's often sufficient to pseudo load test the alternator with it still in the car and it will only take 5-10 minutes to do.

2. If you do have a place like Napa or Autozone test it, know that their equipment is "pretty good" but not the most sophisticated. At least in my case they diagnosed the alternator as "OK" but then it turned out it wasn't working properly at specific RPM ranges. This likely would depend on what the exact problem is with the alternator (bearings, voltage regulator, etc) if there is in fact a problem with it.

3. Without more information, if I was betting I would say that this is probably just the wiring harness. However, if your alternator is that old or if you really think it's the alternator, you could consider replacing it preemptively as it's only about $350 and the alternator is going to need to come out to do the wiring harness anyway (so it's pretty much no additional labor).

4. Conceptually taking the alternator out is cake, however I found it really to be crammed in there and difficult to remove. If you do this yourself, don't forget to look at the workshop manual steps and specifically don't forget to give the main bolt a good whack after you unscrewed it a few twists. This will knock the flange on the alternator back and loose from the engine. If you forget to do this it can be a real bear to get the thing out...especially as it was in my 2002 that had been sitting in there attached to the engine for the last ~12 years.

At any rate, the good news for you is that this problem probably isn't going that be that hard or expensive to fix.

Edited April 28, 2013 by Silver_TT

[logray]

Good information above. +1

The Napa near me has a large attached machine shop, and is quite large with lots of machines, computers, and other sophisticated equipment.

The machine they've used to test my alternator/starter/battery provides a lengthy 1-2 page report per item and is computer operated. It spins the accessories at various speeds and reports on parameters and offered a lot more than just pass/fail.

There are hand held units as well which can be less sophisticated.

Having the parts in car doing a load test can also simulate real world conditions (i.e. heat).

[Kim]

I second those who advise a power system check. Simply pull up to the front door of an Auto Zone, Advance Auto, etc ... and, for free, they will wheel out a machine that will stress test your electrical system: battery, alternator, etc. I've never had them give me an inaccurate report.

[pedefede]

I have just been through the "slow turning starter when hot problem solving" Here is my story.

Found out that one of my straps from the battery was so heavely corroded that I was sure it was the problem. Bought a new one at the fanciest car dealer in Denmark (Porsche, Lambo's, Bentley etc) Had a moment to remember when I had to ask the service manager at dealer to push my stalled car....left with the 50 $ cable and a smile. Unfortunately the cable did not do the trick. Checked the ground connection behind the right rear wheel - like new. Took the starter out, had it checked at a workshop - like new. Checked all wires and connections that I could get my hands on - like new. Then I could not see my clumsy self taking out the alternator, but I managed to get a wrench on the lower connection on the back side, and it seemed just a little bit loose, so I gave it half a turn. Put it all together again, took the car for fast 15 min run - and now it restarts with out any problem.

Now I will go to bed praying to the great porsche god that that was it.

Thanks for all the good advices in here, which really helped.

regards

John

[britdave]

Pedefede, in Copenhagen, are you telling me thats all it was ? :unsure: Ref your post above, and your quoted statement below.

And I Quote : "Then I could not see my clumsy self taking out the alternator, but I managed to get a wrench on the lower connection on the back side, and it seemed just a little bit loose, so I gave it half a turn. Put it all together again, took the car for fast 15 min run - and now it restarts with out any problem" Unquote.

So thats all it was, after all that hassle eh ? Could you please be a bit more specific as to what that "Lower conn" was ? : " lower connection on the back side" was ? :huh:

Was it the + positive terminal, - negative ground (earth) or what ???

Thanks a lot for all the advice everyone, this seems to be a HOT issue, even amongst my small Porsche club group, can anyone else relate to this problem ?

Thanks Dave.

[pedefede]

That is the ironi, I do not know what it was. I could feel a cable connection and a nut in the dark, and I was turning desperate - so I gave it a turn, it moved a bit and now the car starts :-)

I feel like a skilled mechanic.

John

[pedefede]

......I guess it was the positive connection going to the starter and from there to a small black box (marked with a + sign) next to the right side of the throttle house. The alternator is a far as I know grounded through the mounting to the chassis, but as you can tell I am no expert - but my car is running :-)

John