Hi All. I have a CPO 2004 CTT. I bought it about 1.5 years ago and it has 30K miles on it. About 2-3 months ago, the battery died and the vehicle was towed to the dealer. They said "that happens" and replaced the battery (at my expense). The other day, it happened again. I drive the car everyday, but only on short trips (1-5 miles each way). The dealer put a new battery in the car and the battery was drained within 2 hours (while with the dealer). They are trying to figure out what is going on. Any thoughts would be appreciated. Also, should Porsche Roadside have offered me a loaner car? Thanks.

 

Looks like the dealer found a short circuit in a cable under the battery. Seems rather odd? Anybody have any thoughts? Thanks.

 

If it were a true short circuit, then I hope it was fused. Otherwise you are lucky you didn't have a fire.

 

OK, so your battery keeps dying and they say it was a short circuit.
Well, if I dig up the dead guys of electricty (Kirchoff, Ohm, et. al) and ask them, I'm pretty sure I know what they would say.....

Bu!!**** (you can tell them I said so)

IF you had a short circuit, then, by definition, it would draw LOTS of current, for less than a second, followed by the fuse blowing, hence making the circuit OPEN, hence setting current flow to ZERO, hence preventing this from draining the battery.

So, you don't have a short circuit problem.

What you likely DO have is a module that, for one reason or another (i.e. H/W failure or S/W induced H/W state) is drawing excessive 'dark current' or 'quiescent current'....which over a matter of hours/days, WILL render your battery useless.

Somewhat tedious to track down, very labor intensive, not what a dealer would like to spend their time doing, especially if they don't know how much they will be reimbursed by PCNA.

 

Thanks ltc. After a week and a half, they still have the truck. Should I get ready to bail? As I stated in the original post, it is a 2004 CTT CPO with 30K miles. I've never had a car take this long to fix (except for a grey market/euro 1983 BMW 323i -- great car, had an Alpina cam and other mods). I suspect I'm not going to get too much sympathy yet.

 

I can't tell you if it's time to bail, that's up to you.

The theory of tracking down excessive dark/quiescent current is fairly straightforward......you simply pull each and every fuse, replace the fuse with an ammeter, measure the current and if out of spec, start tracing that circuit to see which modules are connected, disconnect them one at a time, remeasure the current until you find the defective module(s).....simple circuit theory.
(it's been discussed for just about every Porsche model at one time or another)

Easy on a bench or a mockup or a test mule, a bit more difficult in a vehicle when you have to start removing panels, seats, carpet, dashboards, instrument clusters, etc to get at the module(s), only to find out it wasn't that one......

Labor intensive, a high probability of having squeaks, rattles, loose fasteners, etc once it's all put back together again.

And no, the factory manuals' wiring diagrams aren't exactly the best you'll ever see and no, it's not what a Porsche technician really wants to spend his time doing, and no, it's not what PCNA wants to pay big $$ for.

 

I doubt you'd have a fire on a fused circuit in this vehicle.....that's what the fuse is for in the first place to prevent the "F" word.

 

BTW, there are other methods that can be used to follow quiescent/dark current besides an in circuit ammeter; non contact probes (Hall effect or equivalent)...you just slide the probe next to the wire and measure the resulting magnetic field (which is proportional to current).......ah, never mind.

 

Or, an in-line shunt and measure voltage generated proportional to current draw.

I had to do this on an old 928 to find a stereo draw problem.

 

An ammeter (analog or digital) is an inline (fused) shunt which generates a voltage proportional to current....thank you Dr. Ohm.

 

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