dr90254

So I bought my c4s cab from the Porsche dealer with 15K on it, a little over 2 years ago. Car has had intermittent 'battery' problems. Basically, died every 10 weeks or so. Initially I thought it was the XM satellite aftermarket radio and had that explanted. Battery still died. So I brought it to dealer, they said I was only driving it once a week and that I needed to drive it more.

So I drove it twice a week. Still died. Brought it back to the dealer. Replaced the battery in May of last year, OEM battery. Then the NEW battery died. Dealer said I needed to drive it every 2 days, which I did, faithfully, and it still died. Brought it back to the dealer, they said that it was my fault because I was only driving it a few MILES each day, that I needed to drive it 10-12+ miles in each direction to really get the battery back up to charge, and that if I wasn't driving it every 2 days, to put it on the battery maintainer. (I posted my problem at this point, and appreciate all the input on the battery maintainer).

I did this, and it worked for 3-4 months, then died. Brought it back to the dealer, they said, as they have every time (the battery was a little low; it held the charge; there were no shorts on it; draw was only 25 mA which is within normal limits).

Brought the car home last week, and the VERY NEXT DAY, drove the car 18 miles to work, went 15 miles to the grocery store, came out and DEAD battery. Very bizarre since I came from the dealer with 'fully charged' battery. At this point, myself and the dealer are convinced this is not a problem with the battery. Now, the car has been at the dealer for the past 2 days, and they say they can't find the problem, they need another day, blah blah.

So I am appealing to you to help me. Please let me know your thoughts because I don't know what this could be--I asked them to check the starter solenoid and the alternator diodes, but any other ideas?

rusnak

Since it is not an issue of current drain, I guess you could do a google search on float chargers, and find one that you like. 25mA though? hmm

73-911E-Targa

Read pages 14-17 of this article: Battery Service. This is part of Ken Sullivan's excellent series Auto Shop 101 which should bookmarked by everyone who has an old car and any DIY tendencies! The battery article talks about all sorts of battery drains, including parasitic drains, and how to identify and test for them. Very well written and easy to follow.

As far as why your battery would suddenly die at the grocery store, that sounds more like a short causing very rapid drain (or inability to get current to the starter) than and kind of slow drain. That might actually turn out to be good news for you in that a minor or intermittent short - like frayed or cut wiring insulation that touches a ground - could have been your slow drain all along. If this wire frayed more and now (while at the grocery store) is completely grounded this would cause a rapid battery drain or current cutoff to the starter and may explain why your battery failed suddenly at the store.

Hope this helps. Good Luck!

dr90254

Thanks for the post about the Battery 101, that was actually very informative and now I understand what the service rep is talking about.

Update -- So I just picked up the Porsche from the dealer yesterday.

Turns out the problem was a parasitic drain coming from a corroded ground, to the convertible control module. They said they cleaned the ground, and the drain was significantly reduced. And the battery held the charge.

Car has started up fine the past two days, no dead battery so far. We will see how the future holds.

I asked the service rep -- how does this ground wire get corroded? But I didn't get an answer. Anyone know? I want to prevent this from happening again.

Also, I am a little frustrated bc it seems to me that this corroded ground wire was probably the culprit all along, and I likely never needed a new $400 OEM battery, lol.

Last question for you guys -- since I put the replacement OEM battery in, the car has obviously required several jump starts. Does that mean that the battery longevity is adversely affected? I'm sure it has to affect the life of the battery, I just don't know how many jump starts it will be before the battery needs to be replaced.

73-911E-Targa

That's an easy one! All metals will corrode over time from exposure to "the elements." In ground connections this will eventually inhibit their ability to make good contact with the surface ground (normally the frame or something directly connected to the frame). In order to slow down the ground problems that are quite common in Porsches, that a look at this thread: Grounding Points – Ground Stars on Older Cars which will tell you where the ground points are on your car. After you have located each one, clean it with 00 steel wool or a brass brush and then coat it with dielectric grease. This will make a world of difference in avoiding intermittant electrical problems!

dshepp806

Parasitic drain through a corrosive ground connection? mmmm.....

Doyle

ron mcatee

Why an OEM battery? There's plenty of good ones at less than $100.

dr90254

dshepp,
you sound skeptical? do you think that this was not the real cause of the mystery electrical problem I encountered? please expound.

and ron,
you are totally right, I should not have paid almost 400$ for an OEM. I just did what the dealer recommended, which was obviously way more than I needed to pay. next time, I would definitely not get an OEM battery.

autobonrun

Parasitic drain from a corroded ground sounds bogus to me; impressive but bogus none the less. A parasitic drain implies an unintended current discharge from a system when there should be no current flowing. On the other hand, corrosion actually increases resistance to current flow, which should reduce not increase the drain. In the case of your cab top for example, when it's not being used the controller probably should not be using any power or at the worst a very minimal amount.

Where cleaning a ground connection helps is in a situation where you've lost your ground because of the high resistance. Note that even though we use the term "ground" in DC circuits, it's really the return path to the battery negative. You've got to have the return path to complete the circuit. Think of corrosion as an impedement to that return path.

Here are some suggestions. Make sure you keep your battery maintainer on line when you're not driving. Using a voltmeter, take some voltage measurements to help figure out what's going on.

1) Battery Voltage measurement when system is charging (it should go up to around 13.5 v typically)
2) Battery Voltage measurement after your maintenance system fully charges the battery and maintainer removed (should be around 12.6 volts)
3) Remove the battery maintainer and remeasure the voltage after 24 hours. If the voltage has dropped below 12v you do have a parasitic drain, but it's not from a corroded ground cable. Something is shorted. A typical scenario is where a wire's insulation has worn off and shorted to ground.

Have you had any fuses or light bulbs blow lately? Anyone do any electrical work on your car before this started happening?

dshepp806

Autobonrun beat me to it...

Best,

Doyle

whalebird

I may have missed something, but year/make/model. I understand your car to be a C4S cab. The year may help. In later cars, the control units work differently than early ('80s-'90s) technology. Although it is a 12 volt negetive ground system, much of the cars controls operate off of lower voltage, and many control units work off of an "elevated" or controlled ground. You can forget about a test light and leave the voltmeter in the toolbox because neither is good at diagnosing the inputs/outputs of control units. Power and ground are not what they used to be. The dealer's finding are plausible, ground points do strange things. A voltage signal in the above mentioned power/ground path can "back feed" through the ground points sending parasitic voltage backwards into control units and consumers. These ground points in a modern car are more than just screws in the body; They are sensitive and can be hard to diagnose. As a side note, The factory advises one to drive a car for at least 20 minutes to restore the batteries charge. That does not mean let it idle for 20 mins - Drive. It sounds like you have driven the car more than enough to maintain a charge.
You have done the right thing taking it to the dealer and logging this problem. If the problem returns then you can take it back and have them continue the diagnosis. Stay after them until it is fixed.
Sounds like your well on your way. Good luck

autobonrun

That's a good point about the control units. I would not go so far as to say leave the voltmeter in the toolbox. What you a referring to is a floating signal ground. You are correct that signal ground can be different from chassis ground unless they are tied together. You can still measure the voltages across a floating ground system but you have to measure from the plus to the signal ground. I utilize several 10 volt systems that operate +5v to -5v and I use a voltmeter.

As far as the battery voltage loss you can still use a voltmeter on it as well because even the most modern cars connect the negative of the battery to the chassis, so it is at 0 volts. I would still check that the battery charges to 12.6 v and it maintains a reasonable level after sitting 24 hours with no charge.

Excellent observation you made though.

dshepp806

voltage/grounding methodologies aren't magic. A voltmeter is clearly usable, one just needs to know where the reference is taken' from ("common-tiepoint" or "earth ground", or a host of others, to include "floaters"). It's a matter of knowing (studying) the circuit presented (whatever year it may be...)

Many times (not always), one will find the signal ground (later) tied to common at some point, relative to the grounding design. (at least that's what I've seen in many electronic circuits of many types) Not up on current auto approaches (new cars)......still, voltage is voltage,..and will certainly need measurement at some point (you pick).....

..I'd keep that meter (very) nearby.,..as you will certainly find it useful.

Best,

Doyle

theiceman

Doyle makes a good point. I hav stayed out of this discusion as it may serve to confue the O.P. Floating grounds are used mainly as noise supresion as you can often raise the ground potential above the noise elliminating noise which can play havoc on digital circuits. That in itself however has it's own set of problems with ground loops and bridges.
I think cars of out Gen use one common Ground, go forth with this.

whalebird

So, like the OP states, a ground could be the problem. I don,t think that a cab top CU has any magic going on, but they do have lots of micro switches connected to flimsey linkages. Thats why I ask the year of the car. A 993 has a totaly different cab top control than say a 996/997. Lots of cab top problems almost always boil down to microswitch adjustment.
I would most certainly agree with the above posts that,given the complexities, proper voltage in and out of the CU should be easy to nail down. My point with the volt meter is that there is a fair amount of "micro" voltages doing the communicating inside these CU. ~5v is normal. If one was looking for 12v then... instant snafu. And even then lots of components use pulse width modulation(PWM) in which case the garden variety volt meter will average the signal. A good analog meter is better in these cases.
Anyway. Good info from everyone. I would like to know the substance behind the dealer's diag.
Oh, Dshepp: I hooked up my Von Schweikerts and listened to Power Windows-a gold standard in my CD player. So much style without substance, So much stuff without style...
Hope everyone is having a great holiday weekend.
Scott