I was on the freeway, coming home from a gig, and the car started to loose power. Not wanting to be stuck on the side of the highway, I figured out I could limp it home if I kept feathering the throttle. So I did, and drove about 10 or 15 miles. When I got home, I saw that the negative battery cable had come off and that's what was causing the problem.
I reattached the cable. Then the car sat in my garage for a month or two. I pulled it out Wednesday and the car is running like $hit. It has no power, then surges, it pops and misses. I feel like it's ignition related, but I'm getting spark. What could I have fried driving with the ground cable off, that could be causing these problems?

Samson
76-2056 with D-Jet

Get a volt meter and check the voltage between the battery posts with the car off and the engine running. Report back here.

with the engine off. I'm reading 12.6 volts
with the car idling I'm reading 13.8 volts between the battery terminals
at the coil I'm reading 12 volts
and I already replaced the points with no luck

Hows the cap and rotor look.

Pull the belt off the alternator ,and make sure it still turns freely. May have been some welding going on there.

did you get a new cable or did you use the old one?

i had a similar problem and it turned out to be the cable itself was bad ...
Andy

alternator turns fine, no luck there.

no, still the original cable. It wasn't bad, I just hadn't tightened the bolt on the battery post and it popped off.

was the engine warm when you took the voltage at the coil???

if the engine was hot the voltage at the coil should be way less than 12V.
as the ballast resistor heats up it decreases voltage to the coil so it doesn't overheat.

yes, the engine was cold, and I checked it according to the pelican artical, I removed both the green wire from the distributor and the tachometer signal wire from the coil minus terminal and connected the coil minus terminal to ground and got 12Vs

Your numbers look ok. The one thing that repair and owner's manuals warn against is running the car with the battery disconnected which is what happened to you. This will cause the alternator to supply all current needed to run the car. This could cause higher than expected peak currents and could kill the alternator or its diodes.

The 12.6V is a normal at rest battery voltage. Looks normal.

The 13.8V while running looks fine. This means your alternator and voltage regulator are working and busy charging the battery.

Coil at 12V shows the voltage drop in the wiring system of your car. This seems a bit low but not out of the range of possibility.

The circuit that was affected by the cable off is the charging circuit. Nothing else on your car should be affected. Not sure why it doesn't run good.

I suspect that without the ground that the alternator output would not be properly regulated. This could have caused high voltages to appear in the computer and/or other engine electronics. You might want to try swapping parts with someone else to see if any of your engine electronics got damaged.

I may be mistaken, but I believe he tested coil voltage with the engine off.

could be alot worse.
the battery is designed to absorb many of the voltage spikes created by the alternator during normal operation.
w/o the battery in place those spikes have to be absorbed by the vehicle itself.
so it could have spiked your pcm. frying the pcm itself.
i dont know necessarily about the bosche pcm but in most cars the pcm works on a 5V reference signal. so the 14V could have done serious serious damage. the only realy way to check though is to take voltage readings certian points relative to the pcm and ofcorse youd have to know the expected readings and those will be very very hard to find.

good luck.

other thing would be to have someone do a "vat" test on the alternator and chargins system.
this will check the alternator condition as well as the diodes in the rectifying bridge(the part that changes the AC produced by the generator principle to the DC that the vehicle works on.).

so...
get a vat test done then based on those results you might wanna take it to a vintage porsche specalist.

good luck sorry i cant offer more help.

You drove the car for awhile when it was not runny well. Limped it home. Before you go too far, check the plugs. Keep it simple at first.