Oops! Battery, alternator, VR, flux capacitor?
 

Well I kinda screwed up…

I haven’t been driving my 78 911SC as much as I should be and the battery was dead a few weeks ago (slow drain somewhere). I put a jump pack on it and fairly quickly realized I put it on backwards. After that it would only run if connected to the jump pack but would die as soon as I pulled it off. Hoped I just fried the battery.

Got a new battery and drove it for a few miles and all was well (so far). I got busy with hunting season and didn’t drive it till today. Well the battery was dead again so back on the boost pack. Started quick and I was off down the road, but the tach and speedo were going crazy reading 7K RPM at idle (and jumping around 5-7K) and maybe 60 mph at 10 mph or so. Then it died when I pushed the clutch in at a stop sign. Jump pack again and I drove the block back home. Died again as I pulled into the driveway. Crap.

Ok so troubleshooting so far…

- Battery reads +6v.
- Cables disconnected from battery read +14v when running.
- Everything connected reads +10v at idle.
- If the boost pack is connected the tach reads correct idle, disconnect it and the tach reads. 6K, reconnect it and back under 1K.
- It will idle fine for a min and then die like you turned it off.
- Boost it again and it will start great and idle fine once again till it dies a min later.
- No trouble lights are on.
- No wires obviously burned
- No fuses blown

Unless I hear otherwise, I’m going to order a voltage regulator and hope for the best (cheap and easy). Any inputs on a possible alternator or other problem?

Yes, voltage regulator may have suffered when you plugged the jump pack backwards.
Alternator diodes too. They are soldered at the back of the alternator and can be replaced individually. But they are cheap and it's worth replacing them all.

If I read this correctly, you disconnected the battery while the engine was running? Did you have the charge pack connected at this time?

You may have an electrical drain path as both times you had low battery voltage you had let the car sit for weeks? months?

I wouldn't change any components until you do a deep charge of the battery with a battery charger. Then drive the car and measure voltages. If you have +14V when running then it would seem the VR is doing its job unless that was the voltage of the charge pack.

The charge pack I think is meant to jump a car, not charge the battery. I may be mistaken on that. Since the charge pack already got you into trouble, I'd keep that thing away from your 911 until you figure out what is going on.

A very low battery will kill an alternator and or voltage regulator! Make sure battery is fully charged then do voltage checks. Jumping tach indicates voltage spikes.
Resting voltage at battery should be 12.5 volts, charging voltage with battery connected should be steady +14V.
If you have to take alternator out to replace regulator, take it to a local rebuild shop with the pulleys and have them check the alternator.

The boost pack is only used to start the car, however, the tach will go wild and the car will shut off when I disconnect it. Everything was fine when I replaced the battery last time I drove it. It seems like the charged battery/boost pack is overcoming the erroneous voltages (although they seem to measure fine?) put out by the charging system. Then when the battery is low or the boost pack is removed voltages go wild.

Put the battery on an overnight charge with a basic battery charger (not trickle charge) and see if the battery voltage gets to 12.6V resting. Then fire up the car and measure the battery voltage.

A while back my 930 started not turning over so I charged the battery and it was still low (12V) fired up the car and the battery voltage would barely get above 12.5V. I thought the VR was bad. I took a chance and put a new battery in it and the charging voltage went to 14V and the car ran good for several years until the new battery crapped out.

I had asked you if you disconnected the battery when testing things out. Not sure if you answered. In general it is very bad for the alternator to disconnect the battery when the engine is running and could damage the diodes in the alternator.

I have the battery on a charger now, but I am almost positive the issue is in the electrical system. Like I said, I screwed up and reversed the polarity when I connected the boost pack, which then led me to replace that battery because it wouldn’t run without the boost pack. The current battery only has about a half hour of runtime on it, but the slow drain for the last month depleted it down to 6v. It likely was only running properly because of battery power before, but now that the battery is low, it doesn’t run. With a fully charged battery it should behave fine (like it did with the new battery and now with the boost pack) but that won’t solve the underlying issue.

Thinking about reversing the voltage on a battery, it is likely the battery was damaged. I would think the massive current that would flow from battery to booster would cause the booster to go into a protection mode and shut off.

Here is another thing to check, while you are at it. It sounds like you have a current drain somewhere you ought not to have. With the key off, connect an ammeter between the battery ground terminal and, after removing it, the ground cable. Or do the same with the positive side. See what it reads. It should be milliamps. On my '77 I could see a tiny jump as the clock solenoid occasionally wound the main clock spring.

May not fix your immediate issue, but might lead you to look for something which is drawing too much current when the key is off, meaning the battery, if not left on a trickle charger all the time when the car isn't being used, is going to run down way faster than it otherwise would. On our older cars, the battery ought not to lose much charge over, say, winter storage.

You can use a test light or 12v bulb between the positive terminal on the battery and positive wire and actually see the pulse of the clock glowing, if its bright start isolating circuits by removing fuses one at a time. Make sure hood light is disconnected.
You can have the battery checked at your FLAPS after it is charged.

To close this one out, got it all fixed today. Well, except for the slow drain that is.

I didn’t have much free time back when I posted this thread, so I went for the easiest fixes first. The fully charged battery didn’t help, so I ordered a new voltage regulator. This fixed the main problem, but the alternator light stayed on after I shut the car off, so I figured the alternator diodes were shot. I just unplugged the battery and figured I’d pull it out one of these days for a rebuild. Well, I ended up finally just ordering a new one last week and just put it in today. It was the new internal regulator style so I referenced the Porsche technical bulletin and it was an easy conversion. Nothing mentioned the cup washer (or whatever you call the big washer that presses on the shims) would be to small so I had to open it up with a step bit.

Just went for a drive and all was well. And since I don’t need it anymore, anyone need a brand new voltage regulator? It only been used for about 1 mile lol.