First project: find the electrical gremlin
 

Hi all -

I have a recently acquired 1988 Targa. It's mostly great, but there's some electrical issue:

- The battery charges while the car is running
- While the car is off it slowly discharges. After about 12-14 hours there's not enough charge to engage the starter
- If I put the battery on a charger, it'll charge and the car starts and runs great

This was sort of a known issue, and the PO had put a quick disconnect on the battery neg terminal, but recently it's gotten a lot worse.

I need to troubleshoot, and I'm thinking these steps:

1. Test the battery disconnected from the car to be sure it's holding a charge (eliminate bad battery from the issue). It looks new, but who knows. I assume I'll just charge it, test voltage w/ voltmeter, come back after 12 hours and check again.

2. Check the voltage from the alternator with the car running to be sure it's correct and not messing with the (new-looking) battery.

3. Good battery and good voltage I imagine points to a short. One candidate is the (not working) driver's door power window. Since I need to fix that anyway, I plan to pull the door apart and see what I can find.

4. Pull all the fuses, and, with the car off but battery connected, use a tester to see if there's any charge across the empty fuse terminals that would indicate any particular subsystem is draining power from the battery.

Other symptoms: the cruise used to work, then stopped. The switch for high/low beam headlights doesn't dim them in the low beam position.

Any other suggestions? I know this is one of the more time-consuming (read infuriating) things to find.

Gremlins.........
 

A couple of things:

Charge the battery and then take it to an Auto Parts store to have it load tested. Just checking the voltage won't tell you what you need to know.

The fact that the high/low switch is not working may indicate a short in the switch. Pull the headlight fuses (#5-8 counting from the front) and see if there is a drain on either high or low beam. Fuses #5 and 6 are for the low beams and #7 and 8 are for the high beams. Of course, you will need to have the battery connected to do this test. If the car has high wattage headlight bulbs, the extra current being pulled thru the switch could have cause a premature failure. Adding headlight relays is an excellent idea! With the relays added, only a fraction of an amp will be pulled thru the headlight switch and the high/low beam switch making both last much longer!

hmmm
 

you have an internally regulated alternator. if the diode (a one way gate of sorts) is bad, which is reasonably common, voltage will drain from the battery and ground in the alternator. key off. voltage flows downhill (path of least resistance) like water. I think the other symptoms are unrelated. cheers, geoff

I've got the same problem in my car and I'm working through the fuses to try and figure it out.

Does anyone know how to determine if the alternator is the cause?

If memory serves, put a meter across the battery leads and measure the current draw (car not running, all lights off). Then start pulling fuses until you no longer have the current draw. That helps you isolate which harness/system it's in.

You'll get voltage reading.......
 

Putting a meter across the the battery leads (terminals) will give you the voltage reading. You need to check the current (ampere) draw by connecting the meter in a series configuration using the negative pole.

Tony

Thanks all - great tips

To test the voltage draw, disconnect the negative terminal from the battery post. Set your multimeter to the 10 Amp unfused setting. Then connect one multimeter lead to the disconnected wire and one to the battery post. Then disconnect one fuse at a time and see it changes.

YES! Thanks for correcting me.

Oh - here is the article that Wayne wrote about it. That same article is in his 101 Projects for your Porsche 911 book.

You should see a current draw in the low milliamps range. I could see the tick of the clock (in a '77 a solenoid pulls a spring tight. When the gears the spring is loading turn enough, the relay reactivates and pulls again, hence the tick. Very low current. If it is more than a few milli amps, you have an unnatural draw.

Then you do the fuse pulling job. You don't have what we usually think of as a short, or you'd be burning something up (or blowing fuses). But current drain you certainly can have, the equivalent of leaving lights on (crawl into your trunk and have someone shut the lid to see if that light actually goes off).

My understanding is that when one of the alternator diodes goes (and there are what, 6 or 7 of them depending on the alternator model), either you don't make the voltage you need to keep up with electrical loads, or something else goes wrong with the voltage regulation system. A blown diode, at least the ones I have dealt with, won't pass current in either direction, same as a blown fuse. But I may have this wrong.

All cars should have a built-in voltmeter. An easy way to add one to a 911 is to get one which plugs into the cigarette lighter. Cheap, works great. Saved my bacon when a voltage regulator went bad and the alternator was putting out over 15 volts even when the battery was fully charged.

First make sure your interior lights work,
as in door open lights on door closed lights off.
EDIT. The window relay draws a lot of power.

Bad door switch can cause the problem. Power windows could still be live after engine off, door open and then closed.

Thanks all. I think the battery is toast - won't hold charge even with the negative terminal totally disconnected. But a drain in the car may have hurried its demise.

Well, the old battery went on the tester at the auto parts store and immediately lit it up all FAIL FAIL FAIL. I learned that the stock battery size shown by the store computer is a MAMMOTH huge thing, and that there are two mounting holes (? it's dark here now, so I can't check) for two size batteries in the car? If so, that's yet another cool and quirky Porsche thing.

PO had a smaller battery that fit with the mounting bracket in the "closer" hole, and I bought another that same size over mild objections from the store, who didn't like selling me something not in their computer :-) but eventually agreed to take my money.

Charging the new battery, then into the car and on to locate a drain, if any.

New battery in. Manufacturer claims it needs venting, which is fine, but how do most people do that? Couldn't find any existing vent line to hook into. Tips?

Also, meter inserted between the neg terminal of the battery and ground doesn't show any current when the car is off, which I take to be good news re: any slow drain. Maybe the problem was just the bad battery.

did you find this?
http://forums.pelicanparts.com/porsche-911-technical-forum/248149-turn-signal-switch-quick-fix-pictures.html

Ooo, great thread. Thanks!

Just get some tubing and route it to one of the body drains. Some batteries have an oblong vent nipple that requires an adapter. I just bought an AGM battery that doesn't need a vent tube.