Question pertaining to a Voltage Regulator possible malfunction?
 

Hey guys and gals,
My 79 911 seems to be having a bit of a tough time electrically lately...

I was driving it fine through most of the spring, then noticed a couple times the idiot light for the Alternator flashed on a couple times. It did seem to correspond to a drop in idle, and once I revved it, it went off.

Then all of a sudden I drove it to a store and after it sat about an hour, I went to leave and it needed a jump start.

Now this all happened a couple days after I picked it up from a shop that had replaced the rear window, trim and rubber for me. So I assumed perhaps they did not tighten the positive side of the battery cable as they most likely removed it for their work. (I find most shops disconnect battery when working on my car)

So sure enough I drove it home, noticed a large red cable going into the pos side of the battery cable was loose. I tightened it up and it seemed to correct the problem.

However, My concern was as I was driving it home in the heavy rain, with defroster on, lights on, the windshield wipers moved quite slow in high speed. I know that the defroster takes a lot of volts to run, so I am wondering if that is normal to result in a slower wiper speed? Or perhaps my battery needs to be fully charged again or could I even somehow have damaged the battery by having the loose red cable? or could that have damaged my VR?

The only reason I mention the VR is because directly before I tightened up the red cable I had a charging system test, and according to the local chain car shop my VR was testing inconsistent. Once I tightened the red cable I went to Napa and they said all looked great, but they were unable to test the VR as that was not an option on their device.

I am not getting a bouncing tach. I am getting a aftermarket stereo that will occasionally just shut off on me though. How else to test VR issues?

Loose cables often develop corrosion ... which insulates. (clean 'em)

stereo that will occasionally just shut off - that is classic over voltage.

+1 clean the connector.

I had a similar issue when my positive side connector was cracked due to over tightening.

If island is correct, that could mean your VR is going bad. I would remove it, bring your pulley halves and have the alt bench tested.

Age of battery? Naturaly make sure the belt is not slipping, and have the battery load tested to make sure it is still in good shape. Local garage tested my wifes battery, and it barely passed. Needs to be replaced before winter.

If you car sat for over a week the battery was low. Combine that with a loose cable I am afraid that you cooked your alternator. Remove it take it and voltage regulator (if external) to an alternator shop and after your wallet is 150 dollars lighter reinstall. But fully charge your battery. Also get a $10 float charger at harbor freight and connect it to your battery whenever you park it for more than a couple of days

Before pulling the alternator check the voltages at the battery with a meter. Off, running at 1500 rpm and running at 2500 rpm and report what you see. Keep the meter on for a while to observe what it is doing.
Could just be the remote VR needs to be replaced if the voltage is jumping all over from 12 - 15+ volts at 2500 rpm.
About $50 for a new solid state VR.

Ok, so here is the diagnosis so far.
Took the battery to Batteries plus since that is where I bough it. They said they charged it over night and could not get it to more than 50% charged. Said the CCA are only at 99. My battery is a 24 month warranty with a 65 month prorated amount. They are crediting me towards the Duracell battery (SLI41 Duracell Battery) It's a little lower profile by like 3 inches I think. and the CCA are only 650, and has a 24 month warranty as well.
So I'll do this and then run the test Timmy2 recommended. I'll also check the Harbor freight for float charger. The guy at Batt Plus said I most likely shortened the bbatery life by letting car sit with no trickle charge... Thanks guys!! Oh yea, last time I pulled the alt to repair the VR I think we learned I have no external VR only internal

well, so just got the new battery and car is back on the road. After the second week, I went to drive it for the day and at lunch noticed the red light come on again. I revved it and it did not go off. then after a few seconds it went away. It's a big pain in the butt getting the alternator out, but it seems this is going to have to be the route to go so I can check the VR. - Dang, was just hoping it was the battery...

I doubt that it is your voltage regulator. You had a loose cable the vr will not be damaged. Your alternator needs rebuilt see my reasons above.. If you want to test it the put a meter on your battery terminals and rev the motor and watch the voltage if it does not go up your alternator is toast. The loose cable caused it as the alternator was running flat out and they are not designed to do that. Like a welder they have a duty cycle if you exceed it then the diodes get too hot and the alternator shop gets 200.