alabbasi

I had an issue with my voltage regulator which would drain the battery. I recharged it but it would barely start the car. Even after a long run with the battery charging, it would still struggle.

So I pulled the battery out and put the battery from my 84 Mercedes benz 500SL (another group 48 battery) and it still struggled.

What's the story?

robot808

It's just you.

robot808

In all seriousness, I would look for another problem. Cables, grounds...
I've been running on a Autozone battery for like 5 years.

17prospective buyer

Pretty sure the alternators suck crap on 928's, test your alternator output, it could be like mine. I found out my alternator was a rebuilt unit, but it was so bad it didn't charge at idle, it would only charge at like 3 grand. Not only that but German cars are famous for electrical problems as they get older, don't know what it is.

alabbasi

Thanks guys, I'll check the alternator.

jpitman2

As usual, a model year would help here. Early cars had low output alternators....And follow Wally's annual Electrical heath check chart to ensure its not a ground or other poor connection problem.
Check for 13.8-14.1V at the hot post (engine running) or the cigar lighter. If you dont see that, check at the actual battery poles. If its low there, check and clean all connections, especially grounds.
Normally a good condition car will have fairly 'normal' battery life for the electrical loads involved,as long as the correct size unit is fitted. Mine takes the 88Ah unit, and is on its second battery in 12 years.
jp 83 Euro S AT 55k.

alabbasi

Apologies, it's an 88 S4. been sitting fo some time and I'm slowly getting it back on the road.

I need to update my profile.

jpitman2

OK, I believe S4 have smaller batteries - 65AH? This might mean there is less reserve than in an 88AH, so your charging circuit will need to be in pretty good order to be safe. At least you are not in a cold climate. Check and clean all the grounds, and it should help. If not, maybe check the starter, and look for a voltage drop between the battery + post and the starter input cable to the solenoid.
jp 83 Euro S AT 55k

jcorenman

The late alternators are fine (if they are working properly), it's just the result of leaving the battery sitting around mostly-dead. Conventional lead-acid batteries don't like that, and will sulfate and die.

If the voltage regulator was bad (I read elsewhere that you recently changed it) then the alternator wasn't keeping up with the normal loads, leaving the battery drained. If the alternator is working properly (see above) then fit a new battery, check the drain with everything off, and you should be good.

Bill Ball

In the highlighted sentence, if you mean a battery that worked fine in another car would barely crank over the 928, then don't blame the alternator or the battery. You have a problem with the starter or power to the starter. Start with cleaning the battery posts and main ground. Then do the same at the starter. It would be worth verifying that the ground cable is good too and that you see 12V at the front jump post. A bad starter can draw a lot of amps and not turn over very rapidly. If all else fails, have the starter bench tested.


So, does this happen when the engine is both hot and cold? If so, we don't have to bring up another more nasty possibility.

Mike Frye

+1 on JPitman's recommendation to follow Wally's annual electrical system check-up/refresh. Grounds and bad connections, along with relays will cause LOTS of the problems that look like something else in a 928. Voltage drains when resting are the real battery vampires.
One additional point of info: The voltage gauge on the dash is notoriously inaccurate. You should verify independently with a digital VOM at the battery and also at the jump post in the engine compartment. Use the ohm meter to verify connections for your grounds and the voltage setting to confirm resting, starting and running voltages.

Landseer

I like to replace the bat ground strap preemptively too.

MFranke

I used to have battery drain issues but no more. I cleaned all grounds and went around the car cleaning any oil or grime I found on electrical connections under car and in engine bay.

My biggest bonus came from cleaning up the alternator, which had been somewhat weak and lacking in output. Short of rebuilding it, I removed it and took the cover off. I removed and cleaned up the connections and thoroughly sprayed and cleaned the whole thing inside and out with a can of electronic cleaner spray. I cleaned all gunk off the harness wire and connections. I put in a new clean alt cover and gasket and new air ducting from Roger. Th alternator now has solid output right in the middle of the volts gage at idle. Again, the only thing I did was to clean everything.

I suspect over time road grime and oil can lead to high resistance shorts to ground and a slow drain.

Another thing to check is the door courtesy lights with the 3 position switch. I have found these can unexpectedly switch position via leg rubbing or door closing action leading to a light on while parked.

Chuck Schreiber

As most mentioned above, use wally's notes on cleaning all grounds.
A couple also mentioned (bill ball and others) checking ground strap. I replaced battery ground strap with one of Roger's aftermarket (cheap) ones. I was getting a ----- on my digi dash readout after letting the car sit for a week in the cold weather. Replaced w Roger's unit and it cured the prob. I'd also say replace the ground strap near the pass side exhaust manifold to chassis as that can get WAY corroded. Clean the contact area around the body as well.

alabbasi

Thanks guys, I have a bigger thread that I'm trying to keep up regarding this car. Please see https://rennlist.com/forums/928-foru...t-going-2.html for the latest update.