Odd Behaving 964 C4

[Porschestraße]

So I was driving my 964 home about two months ago, and during the ride back the vehicles characteristics changed vastly. It begun jerking and bucking back and forward quite violently. While this was occurring all dash lights were illuminated, and I had to inch it back to my garage.

So the vehicle sat and was finally flat bedded to the local shop my buddy owns, he found nothing off the top of his head, he's an early Porsche guru, but had a few ideas. So we changed the alternator, that cured all the lights except the lights for the Antilock, PDAS, ABS, and the blinking yellow light in the center above the diff control.

Now after a while some of the lights will fade, but the yellow blinking light above the diff control, and another will not go away. While test driving the vehicle, you will get one buck for every 1/4 mile or so.

We also already cleaned all the grounds we could locate.

[ToreB]

Maybe the 964 Christmas Tree Effect? Bad insultation on the high voltage cables causing sparks. The engine control unit gets hiccups due to this. Turn off the garage lights, open the engine lid and look for sparks, especially around the coil-distributor leads.
Cheers,
Tore

[Porschestraße]

Maybe the 964 Christmas Tree Effect? Bad insultation on the high voltage cables causing sparks. The engine control unit gets hiccups due to this. Turn off the garage lights, open the engine lid and look for sparks, especially around the coil-distributor leads.

Cheers,

Tore

Perfect, I'll have to give that a try!! Thanks man

[umgang]

Another simple test"
when it is sitting at idle turn on your light switch and see if the tach hiccups at all.
If so something is shorting/ drawing current.
I found the a power wire from DME CU to alternator was doubled over, like a paper clip before it was wrapped in the wire harness. You could see the brown burn mark in the wire insulation where doubled over. Over time, it built resistance, and started drawing power- resulting in erratic idle, surging, and poor performance.
The more options turned on to draw power, the worse it was.
hope this helps
Nic

[Garageboy]

Mine conked out right in the middle of traffic. Flatbed home, pulled ignition switch and it ohmed out nicely. Checked voltage to frame ground and the + terminal on the battery, OK. Turned on key and rechecked voltage again, OK. Started car. OK...scratched my head...damn $130 tow...engine then started to buck and the dash lights went Christmas. Turned off car, now won't start. Rechecked voltage at battery and for some reason put my hand on the negative lead from the battery; it was warm! Removed lead and ohmed out; un-insulated factory negative lead had accumulated some copper oxide and had become an intermittent resistor, sometimes passing current and if you twist a bit then not. Replaced with one from the local parts place and all good to go.

[bloomj]

Originally Posted by ToreB

Maybe the 964 Christmas Tree Effect? Bad insultation on the high voltage cables causing sparks. The engine control unit gets hiccups due to this. Turn off the garage lights, open the engine lid and look for sparks, especially around the coil-distributor leads.
Cheers,
Tore

The shop I went to said I had this issue and replaced the distributor caps and rotors. The dash lights used to light up like crazy, especially in the rain. I had not seen it again since...until last night. I was driving home with the headlights on and not long after I had been driving many of the dash lights flashed but not all of them. Then a few minutes later the same ones lit up and stayed on. I wonder if it may be related to running the headlights somehow like umgang said below. The lights I get are fuel low, washer fluid low, (!), Parking Brake, Brake Pads, and tow more in that area, on the clock I get the engine cover open light and the fan belt light.

Originally Posted by umgang

Another simple test"
when it is sitting at idle turn on your light switch and see if the tach hiccups at all.
If so something is shorting/ drawing current.
I found the a power wire from DME CU to alternator was doubled over, like a paper clip before it was wrapped in the wire harness. You could see the brown burn mark in the wire insulation where doubled over. Over time, it built resistance, and started drawing power- resulting in erratic idle, surging, and poor performance.
The more options turned on to draw power, the worse it was.
hope this helps
Nic

Umgang - where was this issue with the DME wiring harness? Can you point me to the area that I should be looking?

[Newsboy]

Failing coils are often the cause of "The Christmas Tree". They can have internal faults, even if they test within limits.