Bizarre electrical problem - help!

DFWX · 04-10-2005, 09:49 AM

While traveling on an 1100 mile trip at night in her 1984 928S (I have an '80 928), the gauge showed it discharging and it when dead within 15 minutes (dead battery).
A local shop (there) told her it was the alternator, so she stayed in a motel while one arrived the next day. However, that was not the problem or the cure.

The car charged fine on the highway the next day, until that evening when she turned on the lights - and it again began discharging - so she brought it in the next day without lights.

The car charges fine. But if ANY lights are turned on, it will show discharge (below 12 volts). If just the blinkers are turned on, the gauge needle will dip below 12v with each blink. Making it more bizarre is that turning on just the fog lights will cause it to show less than 12 volts (or discharging) also. Yet it only has to do with lighting.

This does not occur with any other electrical components (a/c, fan, stereo, wipers etc) and the car runs fine.

I dread taking it to a Porsche dealer with this puzzle.

Does anyone have any idea what would cause the electrical system to go to discharging due to any light going on?

I could REALLY use some help!!

Thanks.

Mark

928ram · 04-13-2005, 09:00 AM

Mark,

Were you able to get anywhere with this prob?

Reading your description, I'd have to guess that there's a short in the lighting system somewhere.

I'd also check the possiblity of faulty/dirty ground points or battery terminals and connections; if the battery isn't getting good clean current flow it may not be charging fully and may not be able to handle the load of the lighting. I went thru something similar about a year ago, car would be dead if I didn't drive it for several days. After testing and hunting down everything else for several days, turned out that the main ground point behnd the tool kit had some heavy corrosion; cleaned it up along with some other grounds and the car's been fine since.

carmen78sc · 04-27-2005, 12:34 PM

Bad grounds are common in the 928. I replaced my ground from the battery with a shorter one and just bolted it to an open hole on a bracket right in the batter compartment.

I'd also use an ammeter to check for current drain in the lighting circuit. If you have so much current that you're draining a perfectly good battery in 15 minutes, you have to have a short. And, you're probably pretty close to having a fire hazard. I'm surprised that no fuses are blowing, though, if this is the case. Good Luck!

DFWX · 04-28-2005, 01:53 PM

Thanks, I'll give that a try. I have 2 928s and they both have numerous electrical gremlins, though this the trouble one. You would think that a company that made a 4 seater luxury touring car that could outrun a $150.000 Lambo Contact could figure how to make power switches that work and relays that last over 10,000 miles... thanks again for your reply.

R.Serio · 05-12-2005, 11:19 AM

Clean ALL your ground points. Should be 9 of them. Then watch that needle stay put.

DFWX · 05-18-2005, 01:20 PM

Could you suggest where the ground points are (or as many as you can name)? I am aware of the battery ground, of course, but other locations? I have the full 928 Factory manuals including electrical, but they do not say where that I can find, only diagrams.
This would really be helpful. Thank you!
Mark

DFWX · 05-18-2005, 01:25 PM

Another problem area: Any solution to the window switches that do not work? Every one has one or more window switches that do not work. Any suggestions of other brand switches to use? Is the problem really in the switches?
(Mostly, though, I do need to solve the charging problem on the Mrs. car as it now is a daytime car only).
1979 928 5 speed (parts car)
1981 928S 5 speed
1984 928S Auto
1986 928S 16V 5 speed
1986 1/2 928S 32V Auto
plus a 16V Euro and a growing mound of spares and parts..

R.Serio · 05-18-2005, 01:35 PM

Where in the DFW area are you? IF not too far from Plano, I could come over on sat or sun to show/help. You have a couple on top of the engine, one underneath the engine, 2 near the fuse box & one under the driverside dash. One near the radiator also.

HTH

carmen78sc · 05-18-2005, 01:38 PM

I'm not aware of any other supplier of the switches. I'd check both the switches and the window relay with an ohm meter. Sometimes the problem isn't in the switch itself; the relay is also a common cause of failure.

Tom

DFWX · 05-18-2005, 06:09 PM

That would be great, but I am in a little town called Wolfe City north of Greenville. I go through Plano all the time on the way to and from Dallas. Could I swing by some Sunday with the car (it is ok to drive in day light) and you point it out?
My personal email address is dfwx@hotmail.com.
Thank you, it would really help just to have an idea where to explore.
Mark

928ram · 05-18-2005, 07:22 PM

The grounds should be shown in the manual, I believe they are a couple pages before the wiring diagrams. Brief list:
Engine to frame, pass side
Core support upper, both sides (may be cover by harness & headlight motor)
Steering column support, underdash
Fuse panel, behind/above
Rear body, pass side behind interior trim (big 1/4 panel trim)
I may have missed 1 or 2; remember, 928 grounds are brown.

On the window switches, alot of guys have had luck with popping them apart to clean them up, they get some corrosion and dried out lube in there.

DFWX · 05-18-2005, 07:38 PM

Thanks for the tips, especially about ground being brown.

Anyone out there going to the Irving Porsche gathering , Texas by USA Porsche Club tomorrow night (Thursday)?

DFWX · 05-18-2005, 07:39 PM

Oh, I did pop one apart. Easy to do. Not so easy to pop back together with the two bbs and spring to all line up... Mostly, I took it apart to see what was inside.

DFWX · 05-18-2005, 07:46 PM

Also, any suggestion as to best shop to service 928 in D-FW area - and specifically air conditioning. In part obviously for good prices but also moreso for trust factors.
Neither work, but by records (and appearance) my 1981 had a virtual total a/c system rebuild one year ago by a Porsche shop. Fella said worked until it just stopped, he figured it a relay. A/C is so critical in Texas my preference would be to have a shop check the A/C out before it tinker with.
The 1984 A/C worked on and off until stopping, but I also know it needs a heater valve - ie continually blowing hot air through the vents, which just came in.

928ram · 05-19-2005, 04:44 AM

A couple things you can do with the A/C before you take it to a shop:

Swap the relay with a known good one.
Test for current starting at the compressor clutch, the work back thru the pressure switches (2?) to see if it's getting power from the interior controls.(engine & A/C on).
If you can find one, there's a pressure tester that looks just like a tire guage that you can use to see if there's freon in there.
If it's been converted to 134, you can pick up a cheap guage/can recharge kit and throw a little more in there.

R.Serio · 05-23-2005, 09:08 AM

Update . . . Mark brought the 928 over. We cleaned every ground I knew & could find. I'll let Mark chime in for the results. It was noon when he left.

DFWX · 05-23-2005, 10:39 PM

You were FANTASTIC!
I really was just expecting you'd point 'here, here and here" on grounding points - not climb under the car and get greasy hands with me for 2 hours on a hot day! This was absolutely invaluable as I have more than one 928 and pointing out how and why to check trans fluid probably saved me a couple grand!
I owe you an 8 mm socket too.
Oh, did you see the 1985 928S EURO on Ebay in Florida - only at $900 or so now. Its an automatic and rough overall, but wow would I like to have those heads, cams, throttle body and Euro spider manifold for my turbo 928 sitting in Florida. I'll be down Florida way end of June or so, probably driving one of our 928s. I might go for it, though some people might think having FIVE 928s is enough.
Never enough, no? I might go for it, but if you do let me know and I won't bid against you. Fair enough.
I can not tell you how much I appreciate your help and knowledge sharing.
Thank you again and have a good trip.
Mark

DFWX · 05-23-2005, 10:48 PM

...or we could split the Euro... My only interest is in the heads, cams, throtle body and mainfold - to the point of almost just leaving the rest along the road as an abandoned car. I have plenty of spares of everything else. Two parts cars is enough, no need for 3. I suppose the trans could come in handy as we have an automatic, but I'm not really worried about that and have a spare of that too. But the turbo sure could use that bigger intake system.
Would you want the rest of it? Spare shortblocks? Fenders, etc, everything but top of motor? Finding a 928 fender is a piece of cake, but a Euro fender? Depending where it is, it would maybe not have to be trucked, which is costly.
Just a thought...
Mark

DFWX · 05-23-2005, 10:49 PM

Sorry for typos - its late.

R.Serio · 05-24-2005, 03:52 AM

Mark . . . Your welcome my friend. The knowledge was showned to me & passed on to you & so on & so on. As far as that Euro goes, there is an 84 in Dallas. I saw it and it's a parts car. Missing right front fender & suspention but it's a euro. The engine is complete. Interior is shot. The guy wants a grand for it, i'm thinking $700. I'll try to get you info. Now back to topic . . . Did you run the car at night yet? Is it better? worse? Also, remove that old alarm system that is draining your battery. There might be a short that is causing your alt to over heat and burn out. I'd remove it ASAP. Take it to a radio shop where they install alarms and have them remove it. Probably worth it in the long run. Just my 2 cents.