Should the car idle if the battery is removed?

[TeeJayHoward]

I'm trying to track down an issue with my '81 911SC where it dies on decel. It fires right up with zero problems, idles well, and has adequate power. But if you're cruising along at around 3000RPM, approach a stop light, and shift it into neutral, it will drop RPMs all the way to zero instead of stopping at a bit shy of 1000 like I'd expect.

It started after I fixed a vacuum-leaking brake master cylinder, but I'm not sure if that's related or not. I noticed that idle voltage across the two battery terminals was only 11.59V. I tried disconnecting the battery for a second or two while the vehicle was running to see if it was the alternator or battery. The car died immediately, so I purchased a new alternator and installed it. It now idles at 12.78V - Better, but still far less than the 14V or so I've learned to expect from my other vehicles.

1. What is the expected idle voltage for these cars?
2. Should the car still run if the battery is disconnected, or is there some special design in the 911 that prevents this?

[RSTarga]

Good way to burn out your alternator

[TeeJayHoward]

Originally Posted by RSTarga

Good way to burn out your alternator

I get that the voltage would be unregulated. I know the battery acts as a buffer for the whole electrical system. It's not like I drive it around like that. It's just a quick test to see if the alternator is working. As I understand it, a vehicle with a functional charging system SHOULD be able to run off the alternator alone... At least until the crazy spikes kill all the electronics in the car.

[911obgyn]

Idle voltage should be above static battery voltage which should be 12.5
At 2-2.5 rpm voltage should be 13- 14 V
At battery

[911obgyn]

You should not disconnect battery while engine is running !!!

[TeeJayHoward]

Originally Posted by 911obgyn

Idle voltage should be above static battery voltage which should be 12.5
At 2-2.5 rpm voltage should be 13- 14 V
At battery

Why is idle voltage so low on these cars? Any other vehicle I've tested, it's around 14V at idle.

[faapgar]

12.5-13 volts at idle.A porsche is not like other cars.

[Porsche 935]

If engine does not settle to normal idle rpm, it is normally too rich.

[pkabush]

Originally Posted by 911obgyn

Idle voltage should be above static battery voltage which should be 12.5
At 2-2.5 rpm voltage should be 13- 14 V
At battery

This^^^

[911pcars]

At a minimum, an almost free digital VOM from Harbor Freight should be in your tool box. Connected to your battery posts, you should be able to monitor alternator (charging) output, at idle speed and above.

Yes, This^^^

Sherwood

[TeeJayHoward]

Originally Posted by Porsche 935

If engine does not settle to normal idle rpm, it is normally too rich.

Sure enough, this was the issue. Turns out I was running 11.4 AFR. Twisted the CIS screw until that hit 14.6 at idle then took 'er for a drive. Problem gone.