sws45

I have owned a 91 C-2 for about 2 months. As this is my first Porsche, I have been trying to familiarize myself with it, reading quite a bit about the 964 on rennlist. The car had been running perfectly. It wasn’t started for a couple of weeks while I dealt with some paint and interior issues. The battery died as a result and was recharged using a home charger with an auto shutoff. Over the next several weeks the car started and drove fine; however, the battery would not hold a charge for more than a day or so and was recharged several times. I checked the alternator and measured 13.6 v at idle at the battery. Based on this, I decided the battery was bad, when I went to replace it the real the trouble started…

1. I had recharged the battery the day before but still had to jump start the car. I decided to drive the car to autozone (a mistake). The car started fine and was running well but after 3 or 4 miles started bucking and lurching and then finally stalled.
2. Plugged in a 9 v. setting saver, removed the battery, and took it to AutoZone where it tested 0% charged.
3. Bought and installed a new battery and the car started right up but the check engine light was lit. It would not idle at all; just surged and hunted and stalled unless I kept the revs above 3K. If I kept the car running, I got a christmas tree effect with a buzzer. We turned it off and had it towed home.
4. I disconnected the battery and later tried again. Car started fine, and would now idle but hunted a bit and had a check engine light. I tried to drive the car. The check engine light went off after about a half mile then immediately came back on and the car started bucking, and stalling same as before. No christmas tree effect this time. I made it back home and parked it. I researched some posts and checked voltages as per the suggestions:
Voltage of battery with ground removed: 12.4 v.
Voltage at 1100rpm no load: 13.7 v.
Voltage at 1100 rpm with lights, radio, and ac on: 13.37 v. (should be higher?)
When the car “hunts” at idle, the voltage drops to 12.8v for a split second.
The alternator light comes on with the key but then goes off when started.
Ground wire looks o.k. no corrosion at either end.

I originally thought the “hunting” was due to a dme reset. I now believe I have damaged the alternator; probably the rectifier. My current theory is that repeated charging damaged it or the battery was completely shot and driving the car in that state blew the diodes. Is the bucking and jerking caused by a voltage drop? I don’t have a way to measure voltage while driving. Is my analysis correct or should I check something else? I am going to change the alternator myself based on the diy write ups and would hate to buy and install it and find the problem is something else. Any thoughts would be appreciated.

Scott

Indycam

The solder joints on the back of the clock are a known problem .
Pull the clock and resolder all the joints before doing a replacement of the alternator .
If the solder job fixes the problem , great , if not its still good to know the solder has been redone .

RichieRoo

Hi Scott

To measure the voltage whilst driving I connected a cigarette lighter plug to
a multimeter, then you can have it plugged in whilst you drive. I found the
voltage identical to that at the battery.

I don't think your voltages are outside factory tolerances.

I'm not convinced by the alternator theory. I'm sure more experienced
Rennlisters will chime in, but why not try a couple of things first. The bucking
and other effects you describe could be caused by a defective coil, have
you tried disconnecting each coil one at a time? (this procedure can be found
elsewhere on this forum).

Is the hunting intermittent or continually fluctuating?

Good luck. Keep us posted.

boxsey911

One simple thing to try is to disconnect the negative lead from the battery for a minute or so. Replace it. Fire the car up and let it idle for 10 minutes. Don't touch anything, especially the throttle and then take it for a spirited drive, going up and down the gears. This is basically a DME reset. It worked for a friend of mine who was having similar problems with his C2.

jimq

Try changing the DME relay

sws45

I did consider both the clock and the dme relay but not the coils. I don't have a spare relay ( I know, I know). My zeroing in on the alternator was due to this occuring with the bad battery combined with the voltage drop.
My initial thought was that the bad idle was from the dme reset, despite plugging in the setting saver to prevent this. When the car is idiling in the garage the hunting is intermitent. When I get it on the road, it feels like it loses power for a split second then jolts back. This happens several times then the car stalls.
I did attempt the dme reset drive and that was when I had to limp home. I will use the setting saver plug and attach clips to the battery attachment connections then to the multimeter and see what the voltages are if the car buck again. I will work on it this afternoon. Thanks for the input.

Scott

newsboy

Have the coils on your car ever been changed? Even if they test within spec's, they can have internal faults that can cause electrical problems.

Frank 993 C4S

Scott, as other have said, the sounds like a multitude of things and I would not start with the alternator. Check the "easy" stuff first: solder points on back of clock and the coils and try the DME reset before replacing the alternator.

Indycam

You don't need a spare if you know how to do the jumper .

RichieRoo

Sounds more and more like a bad coil to me. The intermitent hesitation and stalling is exactly the
symptoms I got when a coil went bad (it's happened to me twice).

Get back out there... with the engine turned off, disconnect the connector found at the base of the
coil (the wide plug that slips onto the module). Start the car and let it idle. Then turn off, reconnect
that one and disconnect the second coil. If the problem goes away whilst either one is disconnected,
you've found your dodgy coil.

Indycam

You can do that test faster and better by starting up the motor
and then reaching in and disconnecting/reconnecting the ignition switches while the motor is running . If you disconnect and reconnect while the motor is running you can hear what the effect is .

Richard Curtis

Even simpler fix before throwing parts at it: Disconnect the negative cable at the battery AND at the unibody. Clean the battery posts and cable ends, especially at the unibody connection. Reconnect everything. I'll bet this fixes your problem.

sws45

I tried again this afternoon. I let the car idle for 10 minutes. No check engine light until I had to restart it. Attached a multimeter to check the voltage while driving. The car started bucking after about a block. As I drove a bit, the frequency of the bucking got progressively worse until it was undriveable. It started off about once a block then ended up every couple of seconds. There was no change in the voltage, producing 13.3 volts steadily.
I will try and check the coils and the ground wire tomorrow between turkey and football. I think I understand the principle; turn one coil off to check the other. Is there any problem driving with one coil off? I have to drive for a bit to get the problem to start. Thanks for the feedback.

sws45

I tried the ground wire. Cleaned it up with a dremmel and contact cleaner. That was not the problem. I will try the coils tomorrow.

Indycam

You can run around on one coil / spark .
Don't run the motor hard .

Do you see how to reach in and pull the connector/ from the ignition switch/s ?