I recently purchased an 88 928 s4 w/ 5spd and I can't find out what is draining my battery! First a little history. Bought the car in mid June. Had the timing belt and water pump done right away. That was the only thing I had somebody else do. Now what I've done, New stereo and speakers, AC System (new AC condenser, expansion value, o-rings and converted to R-134), Plugs and wires, Rebuilt steering rack, new motor mounts (solid rubber), solid steering rack mounts, new battery, new alternator (installed last night), and loads of smaller items.
Now to the problem. I am constantly jumping in the car and the battery is dead! I've gone through the fuse panel and can't seem to find the cause. I can't seem to find what is drawing the power and the volt gage is reading 12 or just a tick above it. I think that the volt gage changed after a mechanic used a long screw drive to get the R-134 ling onto the low pressure fitting and touched (sparked) the electrical post under the hood. Again, this is a guess on my part.

FYI, As long as the car makes it I will be at the Boston outing on Saturday the 9th.

 

It is possible he damaged something - esp. if the car was running at the time.

3 major possibilities:

A Your battery is shot and won't take a charge

B Your alternator/regulator is shot and won't charge properly

C You have a parasitic current drain that is draining the battery

OR

D Some combination of the above

You can jump to test for C by monitoring current drain when everything should be off - see many previous posts on this.

A - Take the battery in to be tested properly - charge it first overnight out of the car. Go to a place that can test it on a carbon pile tester.

B - If the battery is good with the car running see what voltage you get on the jump post with a real (accurate) multimeter if its <13.5v @ 2K rpm take the alternator/regulator in for testing...

This is a risky situation for a long drive if the battery or alternator/regulator are shot you may get stranded...

Alan

 

An easy way to check for stray current is to make sure everything in the car is shutoff, remove the positive lead off the battery and then take a test lamp, attach it to the negative terminal and give the positive lead a poke, if it lights you know it. There's been a fair amount of work done recently on your car, there could be a short. The alternator and battery have to be checked as well but at least you'll if the problems are compounded or not.

Mark

 

Thanks. As stated the battery (die hard) and alternator are new. I drove it about 50 minutes this AM with lights and radio on without a problem. Is that long enough to determine it I can drive it for the 2+ hour trip?

 

Its funny I just went through this with my wifes car. I would say the best test isn't a 50 minute drive but whether it will start after sitting for a day, or 2 days, etc. That is if its a stray current. My 928's currently behaving itself but I did have that problem in the past and in my case it was a short in the alarm circuit. My wifes car just needed a new battery. Everyone's different but these cars electrical systems are complex and sensitive. Before driving anywhere I'd find it. You could also check all of the grounding points on the car, make sure the connections are strong, no corrosion, etc. Do a search on that there's lots of info. Oh and do check the battery, sometimes when a mechanic accidently shorts something while working as in your case it can damage them. I'd check it just to be safe.

Mark

 

If you are charging the battery in between - this about 70 minutes too short to see if you can make it for 2 hours...!

I know the alternator & battery are new - but that's no guarantee they are working right - especially with the screwdriver short and the multiple full discharge cycle on the battery... Still worth checking as they are not only common causes but they are also relatively quick/easy to fix. Chasing down and fixing a current drain issue can be more challenging.

Alan

 

All of the above are great suggestions.I think 928seregreat meant to connect light in between neg. cable and neg. terminal on batt. this is the only way to complete circuit for that type of drain test.

 

If case C, attach an ammeter (preferably a digital multimeter) between the positive pole of the battery and the positive battery cable. Write down the current draw with key off. Pull fuses one by one and note when the current draw lessens. That will tell you what circuits are draining the battery (dome light, AC, radio, alternator excitation winding, whatever).

Also measure the battery voltage when you find it dead to make sure it is <10V (i.e. really dead) -- It could be the starting system (battery to starter cable, ignition switch, solenoid, bad grounds, etc.) instead of a dead battery.

 

Now I am really worried about drive it to beantown. Before I hit the raod this AM I put a multimeter on the battery with it running and it read 13.6. Is that a good sigh?

 

That means your alternator is charging (good), but it doesn't mean that the you battery is holding a charge. I think if you brought a set of jumper cables (just in case) you'll be fine. If you have time, I'd take the battery to a place to verify it's holding a charge. If it is, then bring a multi meter with you and have a little impromptu tech session - there will be no shortage of people willing to help.

 

Yes it is. I was thinking about your low voltmeter readings as well, it possible that your alternator is hard at work charging up the battery after that short and thats why it read low initially.These cars do that, on mine if the tunes are cranked, air conditioner is on headlights up and I'm motoring around town my voltmeter reads low. Its my excuse for taking it on the highway.... That said I'd still try to establish whether it had a stray before taking it on a long trip. If there is a stray and it was the result of work just finished then it could lead to other problems.

Mark

 

I've contacted athe manchanic that cause this and he said that he wuold take a look at it before I head up, but he asked if this car has a history of bad dash gauges. Anybody know the answer.

And by the way, thanks to all for the assistance!!!!

 

Never trust the dash volt gage - they have a tendancy to read low. But if you had a draining battery, that won't be caused by a gage. Not sure why he would ask this...

Most auto parts stores will test a bettery for free - only takes a few minutes. If he doesn't have a testor, take it somewhere else, get it tested and breathe easy.