Random loss of all power while driving - Help please
 

Hi - Have an issue I was looking for any guidance on pinpointing or maybe others have experienced. Background - car is a '72 with built 3.2ss, PMO carbs. Cars run really well though I did have a fried alternator about 1k miles ago. Made the conversion to internal VR on the new alternator. Just to give some background on recent work. About 1k miles on car since with no problems until recently.

The issue: Go out for a drive and car randomly dies. Losses all electrical power..so engine clearly dies as well. The only things that work are the lights, I guess bc wired directly to battery. But other electronics won't turn on at all such as air blower, parking brake buzz, no noise from fuel pump (I can typically hear it), and other dash board items don't respond like fuel level, etc etc. I might sit there trying to turn the ignition on (but not turn the engine over) to try and see signs of functioning electric. After anywhere from 2-10min off trying every 30 seconds or so I will all of a sudden get electric back - parking brake buzz, dash responsive, hear the fuel pump, etc. And of course the car will start up with no issues. Its been a real hard issue to replicate but its happening every drive now. Its happened cold and car fully warmed up. The only commonality I've been able to find is the complete loss of electric/power has only happened when moving slowly, like pulling up to a stop sign or into a parking lot, or backing up. But it also won't happen every time. So I could go out and drive slow, back up, etc...and of course car will run fine. Just saying of the 8 times this has happened in the last cal of days Ive not lost power cruising at speed down the street.

Battery is measuring 12.7 or so volts engine off, 13.7+ volts when engine running. So I feel like the battery is fine and new'ish alternator seems to be charging fine. Its not a fuel delivery issue. When I have power the fuel pump works. Its clearly an electrical issue, its just strange that it comes and goes.

Has any experienced this or maybe suggest the best places to start to try and diagnose. Unfortunately, the electrical system is my weakest knowledge base with these cars, but id like to try on my own before caving in for professional help. Thank you much

Check that you properly tightened the B+ cable on the alternator. I had a very similar problem years ago and it was a loose connection at the B+ cable, specifically my ring connector was loose on the wire and I suppose the heat and vibration would cause it to stop making a connection.

Thanks for the suggestion rwest. Does the B+ wire create a circuit that if disrupted would cause power loss to everything else but the headlights basically? Like I said, my battery is not getting drained. When it runs alternator is charging. I know the B+ goes to Battery. Guess I’m just not seeing how that temporary connection issue would cause all other items (fuel pump, blower, dash, etc) to lose power.

edit: nvm, re-read your post and my advice was bad

If it's behaving as you say and happening frequently It shouldn't be hard to troubleshoot.
Get a test light with a ground clip wire and a probe.
When it dies, open the front trunks and clip the ground wire to a body ground point and work from the positive battery post toward the fuse block.
Touch the post, light will light.
touch the clamp, light should light.
Proceed step by step through each connection until the light doesn't light.

CoUld an immobilizer security device cause it

Hi. Thanks for the suggestions. I always like to follow up with what the outcome was after starting a thread. This was obviously a big electrical issue, so after reading other post, and before amazon delivered a test light I was going to use as brianlay suggested, I went and cleaned the battery terminals/ground straps and connections. Everything seemed clean enough. I tightened every thing down really well...also everything did seem very tight. I unplugged and replugged in a few relays and connections like the the ignition connections under the dash and the 16pin connection in the engine bay. Just checking for any corrosion or loose connections. Nothing jumped out at me. That said after doing this I have not had any loss of power. A cpl short 15mind drives and then a couple hundred mile hill country drive this weekend without any issues. My guess was a ground strap or battery connection just needing cleaning or maybe one of the connections or 16 pin. who knows but all seems fine now. thanks again

If you haven't done it yet, pull apart all the connections and ground points you can reach, squirt with some kind of water repellent grease (white lithium, Boeshield T6, etc.) and plug back together several times. Should be good for another 40 years!

Sounds like what would happen if the keyed ignition switch/switch wiring was loose. Lights still work, most everything else dead.

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