Help w/ battery draining or alternator not charging

[D85James]

Hey Pelicans, need help with my 86' 911. I've read several articles on this but hoping the experts can chime in here.

Recently purchased this 78 911 with a 3.2L swap and the car ran good until i went on a 30-45 minute drive, turned the car off, tried to start and no joy. Took the battery to store said it was a dud, installed a brand new battery.

Went for a drive, started strong, ran strong, then while driving 30+ mph the car's RPM's fluctuated then car died in seconds, would not restart. Let it sit over night in a parking lot and it started back up next morning, weak but it started. Put tender and tester on the car and voltage was low <12. If i leave the trickle charger on the car overnight it starts fine each morning just afraid to take on long drives.

I pulled the fan and peaked at alternator, appears brand new, wiring appears to be installed correctly. I recently had car serviced and they stated the temp and RPM switches were bad causing the stalling and no restart after resting or cooling down. picked the car up yesterday from shop, drove it home, went to restart it 10 minutes later and no joy.

thoughts?
bad new battery won't hold charge
bad new alternator won't charge battery
bad ground
bad wiring back to battery to keep it charged
i've check all the lights as read on previous posts to make sure nothing draining while on/off

thanks in advance Pelicans!

James

[pampadori]

Many of us have had similar inadequate charging symptoms. One of the most common failures is the transmission to chassis ground strap.
From Pelican Parts site: "The infamous transmission ground strap is one of the easiest parts on the car to overlook, yet can cause so many electrical troubles. Since the transmission and engine mounts are insulated by rubber, the ground strap is the only significant ground to the engine."



On my 1983 model, the ground strap had so much corrosion from oil leaks and water splashing on it for its entire life, that it just couldn't let the angry pixies pass thru it enough. The alternator was charging but not getting back to the battery.

[D85James]

Interesting. Thanks pampadori for the input. will give it a check this evening.

[ed317]

I had a similar problem with a 91 964, since the battery was new I removed an amplifier and other aftermarket stereo equipment, the problem persisted.
I checked all grounds etc.....
Mark at Performance Independent Porsche found two issues
1) Bad Battery
2) Wrong size battery for the car
never had a problem since....
good luck, Ed

[D85James]

yea i neglected to mention the PO also had like 3 amps and big stereo in it. first day i had the car i ripped all that junk out thinking it was draining the system and plus it looked awful. thanks for info

[ChrisHoover]

Until you can find the problem, install a battery disconnect. I have intermittent current flowing from the battery when the car is off. When I start to trouble shoot it goes away.

[jpnovak]

First, The alternator can not handle to charge a dead battery. Use an external charger first.

What is the voltage while the car is running? Does the alternator seem to be charging? It would be regulated about 14.3V above 1500rpm or so.

Does the dash gauge alternator light come on with the key? Without this light there is no exciter current to energize the regulator and tell the alternator to charge.

With the car off I would measure the drain current. Connect an ammeter between the negative terminal on the battery and the ground strap. Measure the current and see if there is anything excessive.

[T77911S]

make sure all terminals are clean,

just take it to advance and let them test the charging system......its free.
battery needs to be charged to do the test. oh, they test the battery too while in the car.


I do electronics for a living also did auto electric for a living, I don't even screw with it anymore, its too easy to let someone test it...properly

[D85James]

Update: the ground strap appears brand new. I went ahead and took the nuts off the sand paper the grounds making sure contact is good.

When i turn the key to accessory i only get a green light then when i turn key to start the car the red ligh that has a G and circle around it in the gauge lights up red during start up then turns off. Is that correct?

With the car running i get 12.0-12.3 volts. After a good 10-15 minute drive i test it and shows 11.5-11.8 so still draining. I pulled the fan and alternator all appears to be wired correctly and grounded properly. Alternator is brand new too.

Besides the light to complete the circuit wonder if i have a wire unconnected under dash or any good spots, wires, colors to double check?

Many thanks Pelicans

[D85James]

below are pictures at start up. Turning key to accessory the top light goes green, when turning key to start the G or lower red light comes on during start up then goes off once car is running. is that the proper light sequence if the alternator light bulb is completing that circuit?
what wires/colors are connected to fuse box from alternator or dash light if any that i could double check?
Thanks,

[jlex]

With the car running, you should be measuring about 14 volts. At rest, a fully charged battery should register 12.6 volts.
Doesn't sound like a parasitic drain... more like the system isn't charging.
I located a parasitic drain on my BMW over the weekend. Was draining a fully charged battery down over a week of not driving. Drain should not be more than 50 milliamps. Had to disconnect the circuit fuse for the sunroof to stop the drain.

[Lyle O]

Please do as T77911S suggests and take the car to Advance or Autozone and have them use their professional equipment and properly diagnose your system for free. If they confirm the charging system is the issue, then you can focus on that (essentially either alternator or voltage regulator, or or the wiring, including to/from the dash bulb).

[timmy2]

New alternators can have bad voltage regulators. Check the bell curve for failure rates of new electrical devices and components. Real high when new or really old.
Have it tested at a rebuild shop....
Example of new failing below:
Valeo Alternator Break-down

[Bigtoe32067]

Why waste time taking it to Advance or wherever. He already said it’s only showing 12 volts or less when driving so it’s obvious the charging system isn’t working.
Tony

[911pcars]

Before adding a VReg and alternator to your discarded battery list, confirm the charging system is intact. That is, all components are operational and the electrical path to/from the battery is good.

Otherwise, you might possibly end up replacing more car parts than needed.
Start with the electrical path. Make sure all wires and connections are intact and clean. Dirty and/or loose connections raise circuit resistance and reduce system voltage and current. You will need a multimeter and some cleaning tools/sprays, etc.

Look on the internet or the PP archives for topics such as:
How to measure voltage drop with a MM
How to measure/find parasitic current draw with a MM

Only then, pull out the CC or checkbook, although some folks prefer to do that first.

Sherwood

[Walt Fricke]

The alternator/fan belt warning light should come on when you turn the key to the run position. After the car starts, it should turn off, because the alternator is generating voltage. The light is also connected to the battery, so when the voltage is about the same on both sides of the bulb, no current flows and it doesn't light up.

Your bulb obviously isn't burned out, so you don't have a bad bulb.

The 3.2 has an internal voltage regulator. The 78 still had the external regulator. But whoever did the conversion took care of that, because your light goes out when you start. If the voltage regulator was bad, it would stay lit. Same if the alternator wasn't putting out any power at all. Of course, the conversion might have involved switching to a later tachometer. Or an earlier alternator with external VR might have been used (not as likely - these swaps usually involve just putting the assembled used different motor in, and dealing with what wiring changes are needed).

However, your alternator obviously isn't putting out enough voltage. You should be able to purchase a digital voltmeter you can plug into your cigarette lighter. That way you can see if you are getting any charge, or are just running on the battery's charge (which seems to be the case). If the gauge is a bit loose in the cig lighter socket, just shim it with something thin and conductive (US and Euro cig lighters had different IDs, at least back when).

The G (alternator) light should have a blue wire attached to it on one side, and a red/black attached on the other side of the two spade lug bulb holder. If the conversion was done right, there will be a 91 ohm 5 watt resistor connected across these two lugs. The internal VR alternators needed a 4W bulb for excitation, and the external VR cars only had a 2W bulb, and no 4W bulb would fit - hence the resistor for the conversion.

The green low oil light should have a green/white wire to it, along with a red/black.

Our '84 had the VR go bad. One of the insidious indications of a bad VR is that the bulb never goes on - when it fails it doesn't provide a ground. If you don't notice this when starting the car (who notices the dog which doesn't bark in the night), you aren't warned. Stranded us once. I put in a new VR. A month later it stranded my spouse - new VR was faulty.

But your symptoms are a bit of a mystery. I don't suppose the green light and G red lights have been reversed on the back of the gauge?

[Walt Fricke]

Oh - while the transmission to chassis ground wire is important (and it appears that was dealt with), so is the ground wire from the battery to the chassis. Easy to get at to check and clean up if indicated.

It is also not unknown for the big red positive battery wire to get corroded inside the clamp which goes on the battery terminal - and this can cause an intermittent issue - like sort of OK when warm, not when cold (or vice versa).

[timmy2]

New battery tested to see if it is good?
It happens....

[tdw28210]

My bet is a bad VR.... but,
Buy a new style battery jump pack and throw it in the car. Like 80 bucks. That way you can still go on drives until you figure this out. Just don't stop / restart more than 3 or 4 times without recharging.

Buy the digital voltmeter for the cigarette socket and monitor.

Watch a couple videos on replacing the 3.2 VR.

Good luck.

[gomezoneill]

That alt light issue is your problem. My guess bad voltage regulator. The light should illuminate with the key in the run position not the start position. I'd replace the ground strap that the guy's were talking about and pull the alternator out and have it tested. Make sure you disconnect the battery before you pull the alt.