911 SC went dead while driving - no juice?

[moparrob]

I was driving through the canyons this morning in my 1978 911 SC and the car's electrical system suddenly shut off mid-turn and left me stuck.

I pulled the battery and had it tested and it is still good/passed the cranking draw test.

Nothing in the car works at all, lights, horn, starter, etc.

Checked the ground strap and it is still connected to the body.

Any clues on what may have caused this or places to start looking to figure this out?

Does this sound like a dead short in the alternator?

Thanks in advance for your help.

[lindbhr]

You might double check the ground strap for corrosion of some sort. Also check the black or red round fuses up front. Good luck...

[brianlay]

Get a DVM or a test light and check methodically

[QueWhy]

Take a look at the fuses, they may be fine but the wires going into the fuse blocks may be corroded/brittle/broken.

[moparrob]

The ground strap is worth checking but it seems solid.

When you say the 'red and black fuses' up front I assume you are talking about the round relays on the fuse panel. The relays are fine.

In my situation I have no power to ANY circuits - as if the battery was out of the car. So it is not a bad fuse situation.

I appreciate the input but am wondering if anyone has experienced a TOTAL failure of power to any circuit when the battery is still good.

[Rodek]

Check to make sure all of the rubber coated pin connectors are plugged in to their corresponding receptacle under the dash on the steering wheel side.

[moparrob]

Update – still no power anywhere in the car. Here is the situation:

The ground strap is connected to the frame and has continuity. The battery is good, passed a load test and has 12.6 V.

the heavy-gauge black wire that runs from the battery to the starter solenoid is connected. All these fuses are good.

Nothing inside the car works, not even the clock or the radio. Turning the ignition switch does nothing and it is correctly hooked up. I have read the schematic several times over and cannot see anything that would cause a complete failure of power, but I am not that versed in reading schematics.

I am baffled. Can anyone else think of anything that would cause the car to have a sudden, major electrical failure while driving? Thanks for your help.

[brianlay]

Could be the ignition switch failed. Do the parking lights work? They don't depend on the ignition switch.

dismount the ignition switch and check for voltage coming in to it

[moparrob]

Thanks for the tips. I do not believe the parking lights work. I will check tonight when I get home.

[Marc Bixen]

Double check the accessory wires that go to the 6mm stud on the battery positive terminal. The nut is 10mm. If this is loose or corroded, you found the problem.

[moparrob]

I did check that last night but everything appears to be intact there. I am curious why one of the small diameter accessory wires on the terminal could asfect the entire electrical system though?

[timmy2]

3 red wires from battery positive terminal carry all power to the system.
2 go directly to the ignition switch 6 pin connector, one goes directly to the front fuse panel.
Take them off the positive terminal, clean and reattach.
Positive post terminal clamp cracked perhaps?
The large cable to the starter only goes to it, then the alternator, then the 14 pin connector and the rear relay panel to meet constant power needs there.

[moparrob]

So I got home and checked. The parking lights do not work. Nothing works.

The three red wires coming off the positive terminal are intact so I’m not sure why I have no power anywhere throughout the car. This is frustrating.

[pete3799]

Use a test light ( or volt meter ) and touch the neg. battery clamp (not the battery post) with one end and the pos. battery clamp ( not post) with the other. Got juice?

[boyt911sc]

Originally Posted by moparrob

So I got home and checked. The parking lights do not work. Nothing works.

The three red wires coming off the positive terminal are intact so I’m not sure why I have no power anywhere throughout the car. This is frustrating.

moparrob,

You have done nothing except look at your battery. They may look good to you but the fact is the current (I) is not flowing through. You need to test and confirm. You have been given many sage advises to try. You are hoping everything is fine. And they are NOT. Otherwise, you won’t be here seeking help. And do something to find the culprit.

Tony

[moparrob]

Originally Posted by timmy2

3 red wires from battery positive terminal carry all power to the system.
2 go directly to the ignition switch 6 pin connector, one goes directly to the front fuse panel.
Take them off the positive terminal, clean and reattach.
Positive post terminal clamp cracked perhaps?
The large cable to the starter only goes to it, then the alternator, then the 14 pin connector and the rear relay panel to meet constant power needs there.

Thanks for the input. Unfortunately mine is a bit more complex because the prior owner installed a headlight relay "update" which includes a fuse block and relays.









I spent 3 hours with a multimeter and cleaned all the grounds and cables /wires attached to each of the battery terminals, including the ground strap to body bolt connection. They were DIRTY, but that did not solve the problem.

Even with EVERY wire disconnected from the battery, except the heavy gauge black wire that goes to the starter, I am still getting a .4V positive reading on fuses 17 through 20.

I can't imagine how there is voltage leaking from the starter back up to the fuse panel.

Any more ideas?

[tirwin]

Originally Posted by pete3799

Use a test light ( or volt meter ) and touch the neg. battery clamp (not the battery post) with one end and the pos. battery clamp ( not post) with the other. Got juice?

Did you do this test?

[moparrob]

Yes. There is strong voltage at the terminals and at the ground location. They were dirty, and I did clean them extremely thoroughly, but this did not change anything.

The most pressing question is why am I getting positive voltage at the fuse block with only the starter connected?

[timmy2]

0.4 VoLTE could be coming back through the alternator blue wire, light and red/black wire back to fuse panel. Ignore it.
The terminals marked B and C look to be the originals that are normally the only wires besides the black cable going to the positive clamp.
Try ignoring the PO crap for the headlights and just connect those wires.
If you have power like it should be, then go back in and do the headlight relay kit correctly.
Any chance there is a disconnect switch on the negative cable?

[boyt911sc]

Originally Posted by moparrob

Yes. There is strong voltage at the terminals and at the ground location. They were dirty, and I did clean them extremely thoroughly, but this did not change anything.

The most pressing question is why am I getting positive voltage at the fuse block with only the starter connected?

Rob,

With the black starter cable still attached to the battery, there will be power supplied to a couple of fuses namely S23 & S24. But your main problem is that you are not getting power to ignition and headlight switches. Is this correct? If yes, then the problem occurs somewhere between battery (+) terminal and ignition switch @ 30 terminal.

1). Use the test procedure suggested by Pete using the clamps as contact points. Use a TL for this test. If the test shows power, proceeed to the next point.
2). Use the next test points at:
a). S18/S17.........there should be power at these fuses. If no power, the problem is @ ignition switch terminal #30.
b). There are three (3) red wires at IS@30. Two (2) coming from battery (+) terminal. The other one is going to fuses (S17/S18). If you are not getting power at terminal #30 (IS), the problem is somewhere between IS and battery (+).

If you have any question or I was not able to deliver concise and clear instructions, please do not hesitate to ask. Keep us posted.

Tony