Where to begin-an electrical issue

jmshepard · 08-10-2008, 08:01 AM

The problem started about 3 weeks ago in a 1995 993. Driving at highway speed the car went over a small bump in the road. The jolt was not severe, but a sharp shake of the car. At the moment of the bump I noticed that the ABS light momentarily flashed. This same light flashed a couple of more times over the next several days when I ran over a bump. Then these events stopped and I forgot about the scare. Subsequently, I drove the car on an 800 mile trip with no issues. Last week, it again started flashing in synch with sharp road bumps. The difference this time; after the ABS light flashed, all of the dash lights flashed, and then the engine dies and immediately restarts. That evening I ran the car at idle on the driveway and tried a little trouble shooting. I used a rubber mallet and hit the ignition switch, fuse box, ABS pump and battery area; the engine never missed a beat.

More background:
The car was in a severe left side front end accident a little over 2 years ago.
Many months ago the car has seemed to have a starting issue. It is almost as if the starter motor is weak. I even bought a new starter a month ago to have standing by. (This focused my first troubleshooting effort to the ignition switch)

I think that something is momentarily grounding the system, but given the complexity of the “modern” electrical systems I don’t know where to begin.

Any ideas or theories?

Thanks,
Jim S.

tshih · tshih's Garage

Sorry to hear about your problem. The background accident probably caused some wires or electrical component to be grounding as you suspect. The main harness should be checked as a start. the ground may be rusty from the accident which is in the area of the battery (driver's side front). There may be poor grounding or just a loose battery ground connection (which would explain when going over bumps engine may lose power and lights or warning lights would flash). I would start by tightening the battery connectors. I think you can buy some circuit tracer equipment from Lowe's and look for shorts.

Tom

jmshepard · 06:45 PM

Thanks Tom. I thought I would give this a try. I replaced the ignition switch and followed up with an extensive test drive. So far, so good!

jimmcc · 08-21-2008, 09:39 AM

Jim,

Has the engine wiring harness on you 993 been inspected? This was a problem with the early 993s and Porsche has (had) a program to replace the harness. Your symptoms sound familiar. Have your mechanic or a dealer inspect the harnes on the top of the engine. The problem was a bad batch of insulation from a supplier that over time essentially disolved exposing the wiring in the harness. Any sort of bump could result in one or more wires shorting together with random results. I saw one attempt to start the engine when the tech moved the harness. This problem was more evident in the south and south west where the higher ambient temperatures caused the insulation to degrade faster.

If that update has not been performed I'd have the harness checked out ASAP. They have been know to ignite.

I got lucky and learned about the problem before mine experienced obvious problems. When the harness was opened it was evident that it was a problem in the making.

Jim

1995 993
1980 930

jmshepard · 08-21-2008, 03:55 PM

Thanks Jim, The engine harness was replaced under the recall notice a couple of years ago.

I've driven several hundred miles in the past few days. At this point I am going to call this mystery solved. The key to running this one was the behavior of the radio. When it would reset during the power failure, it indicated an issue with a major power source. The ignition switch harness was the ticket.

Jim S.