Dead short blew the main fusible link???

[cygnusx1]

1972 240Z pretty much stock other than Webers, and a bigger fuel pump.

So my son and I were driving home tonight from a car show. The car was missing a tad, then my headlights went dim, then they came back, then dim. My fog lights, on their own circuit with relays, remained bright, thankfully. Then BAM total darkness, no ignition, instant rolling chassis in the dark. I managed to feel my way off the main road with momentum, and turn right about 20 feet into a dark side road. The smell of burnt electrical. With my phone's light, I found that the main fusible link from the starter lug to the main harness was melted. With the ignition off, lights off, all things off in the car, I tried to touch the burnt ends of the f-link together, and got a huge spark. Enough was enough, I had a flatbed take us home.

Tomorrow I will diagnose. Any guesses? The car has a 2 year old re-fresh, and all the harnesses were out inspected and cleaned. It has been trouble free. My first guess is that the alternator shorted internally or the starter/solenoid shorted. I will use my meter to look for continuity to ground tomorrow. Any suggestions or thoughts that could help me pinpoint the short?

Edited August 19, 2012 by cygnusx1

[SteveJ]

Did you connect the positive for your fog lights to the battery? If so, my first guess would be catastrophic failure of your alternator or regulator. Look for my PM.

[cygnusx1]

Thanks Steve.

[madkaw]

My money is on the starter shorting

[cygnusx1]

Pulled the +12 from the alternator and the dead short went away. The alternator is shorted internally. I have another one. I need to replace the fuse link and make sure that nothing else got fried in the process.

[SteveJ]

If you had a properly sized fusible link, you probably had little other damage. Check the wire connectors for the fusible link. Check the connectors for the voltage regulator and alternator. Check the connectors for the white/red wire at the alternator and fusebox. Also check the white and white/red at the ammeter. Those are the most likely areas for other damage.

If you replace the fusible link and alternator and the car still won't start, that would indicate you blew the ammeter.

[Zed Head]

That's a weird one. Do you have a 72 alternator and external regulator or a conversion to internal? Factory Hitachi or auto store "rebuilt"?

Do you think you could take the case off of the alternator and see where the short happened? Poor manufacturing or just normal wear and vibration? Stories like yours make me nervous.

[Mikes Z car]

On my car there are two connectors one for each of the two white/red wires above the passenger's right knee, '70 240Z.

[SteveJ]

Dave told me it was a rebuilt alternator. Overheating in the bearings could break down the insulation in the windings, leading to grounding of the windings. In power generation, monitoring bearing and winding temperatures is a critical function.

[Zed Head]

Thanks Steve. I'm still curious to see what he finds, if he looks. Brand and store it was purchased at would be even better. Looking for ways to avoid a similar problem. The rebuilt alternators, in general, are known for poor quality, I wouldn't be surprised if a part just broke or came unbolted and shorted something. I'd like to know,the more specific the better.

[cygnusx1]

This alternator was "rebuilt" by "who knows shop" in the city. I put in my spare alternator, checked the systems, built a 40Amp fuse link to replace the blown factory link, and she fired right up. All systems go! Now THAT'S why you should never eliminate, or skip fuses, when you work on, or modify your car.

The old alternator is in my growing scrap pile, and I will probably open it up this week to report. I think Hagerty Insurance will cover my flatbed fee.

Edited August 19, 2012 by cygnusx1

[cygnusx1]

I wouldn't mind having the original fusible link to replace the decimated one in my 72, if anyone has a source?