Alternator/VR problem......help requested

crockey · 09-04-2011, 10:27 AM

This is my first time tackling a problem on my 71' 911E with a 2.2. Trying to learn as much as I can about fixing problems on my own.

Went out to start the car one morning and it would not turn over. Checked the voltage on the battery, and it was <6V. Alternator light came on when turning the key, and always shut off after 2k rpm, so I figured the battery was due for replacement. Replaced the battery, and car started w/o issue. Drove it for a couple weeks, and same thing happened again. At first, I thought I had an alternator issue. After I trickle charged the battery, the car started right up. I connected the multitestor, and the battery was at 12.5V with the key turned. Alternator light is on. Started the car, and it dropped just below 12.5 before returning to 12.5 at idle. Alternator light still on. When I rev the engine up to 1.5k, the light shuts off, but the voltage does not rise above 12.5. In fact, it seems to drop a little. The fan belt is nice and tight and is not slipping.

I have had the car since April, and this is the first problem I have had with it. The only work that has been done is an installation of a MSD ignition. Here are my questions:

1. I will check the ground strap under the car to see if that is a problem. If that checks out, does it sound like a voltage regulator problem since the alternator light is not on and my car and lights function properly once the car is started when the battery is dead?
2. How can I tell if my VR if integrated into the alternator? (I included shots of my engine bay if that helps. I am embarrassed to say I don't know what everything is on the driver's side of the engine bay.)

I am prepared to take out the alternator to be tested after reading the tutorial on line and in 101 projects, but if the VR is separate, I thought it might not be needed.

Here are the engine bay shots:

Joe Bob · Joe Bob's Garage

Lot's of threads on this....but basically....charge the battery.

A fully charged and working battery should be 13+ volts at rest. Charging with the alt after a jump isn't a good idea.

Idle 13.5
2500 rpms 14+

Over 15 volts the VR is bad.
less that 13 the alt is toast.

I usually just alligator my DVMeter to the battery posts, prop it up on the hood and go at it.

crockey · 09-04-2011, 06:16 PM

I don't think it is the battery since it is brand new. Since the lights and everything else functions fine once the car is jumped, does that mean that the alternator is working properly? As I mentioned, the alt. light does not come on after 2k rpm? Also, can someone help with my question about the VR being integrated or not?

Joe Bob · Joe Bob's Garage

A bad VR can kill a battery by overcharging. External VRs are on the left side of the engine. Most have been replaced with internal ones when alts die and are replaced. One of those two silver cans could be one...or not. The first one looks to be an MFI speed control relay. Since you have carbs.....not needed.

You really should test the charging circuit for voltage.

Canada Kev · 09-04-2011, 07:48 PM

If your battery is dead, you gotta charge it anyway - not good to use the alternator to charge a dead battery. As Joe Bob said, running voltage should be about 14v, if it's not around there, then you have an alt or VR issue, indicated by JB.

kidrock · 09-05-2011, 07:05 PM

probably your alternator is going south. probably not your VR, or you would be experiencing some weird "spikes" in your electrical system....not battery drains.

RWebb · RWebb's Garage

"Went out to start the car one morning ..."

- how long had the car been sitting between that morning and the last long drive in it?

Porboynz · Porboynz's Garage

The voltage regulator is usually above and to the right of the 2 metal cans on the relay board next to the MSD unit. It looks like your wires have been reconnected to bypass the original external regulator indicating you now have an all in one alternator/regulator. Its interesting that the warning light goes out when you rev the engine, that usually indicates some sort of charging action but in your case the voltage does not rise above 12.5V so your alternator is bad. Get that new battery recharged asap, they should not ever be left discharged. Then remove the alternator, not a real hard job. I take a bet on the brushes in the alternator slip rings needing replacing, this is a low dollar fix. If the diode pack or built in regulator have gone bad its time for a new unit. Take it to an auto electrician if car electrics are not your thing.