Weird !!! Turning headlamps on kills ignition

[hemi68charger]

Hey gang.
Weird experience.. My buddy came over to pick up the '71 Challenger R/T. He drove around the block, came back and I noticed his headlamps were real dim.. mmmmmmm  ?? The battery lost juice over the last couple of days it was here.. Didn't seem to be charging. The car was idling in the driveway and we pulled it back into my garage to see if we took the positive battery cable off while running the motor would die ( I know, I know... not the safest thing to do ). Any who, it didn't die.. I turned on the headlamps on, wham !! it died... What the F?.... We started it again, let it idle... Seems the amp gauge was acting properly with a charge indication. I turned the blinkers on, no problem. Turned the running lights on, no problem.... Turned the headlamps on and they worked fine... What the F?.... We took the positive cable off again and it kept running... I turned the headlamps on again and this time,,,,, the motor died... What the F?...

Soooo, now it is troubleshooting time.... 

[73_Cuda_4_Me]

 

You said the the car died BOTH times with battery disconnected when you turn on headlights. Sounds like alternator is putting out enough charge on it's own to power ignition, but when loaded down with big load like headlights, it isn't... Does this vehicle have the alternator bypass wire added, so that the main current does not go through the firewall connector?

If not, then there could be enough resistance in firewall connector to drop voltage coming into keyswitch and going out to ignition enough to keep it from firing, especially when idling (lowest alternator output), and battery disconnected (no backup 12 volt supply).

Check what alternator puts out at idle... should be greater than 12 volts, and when above idle should be 14-15 volts.

Sounds like you are getting down to like 8 volts with headlights on... ignition no likey that low...

[dodj]

Start with checking/cleaning your battery/alt/bulkhead connections and your VR ground, engine ground, and rad support ground. You have high resistance connections somewhere. 
Then see how it behaves.

[hemi68charger]

The common point is the ground there at the core support; battery negative body ground, headlamp harness ground and it is what I used for the MSD 6AL box ground.

[HP_Cuda]

Sounds like the alternator died and the car is trying to run solely off the battery.

[roadman5312]

        Disconnecting the battery cable while the car is running is not a good thing to do.     Back in the generator days is was a quick accepted practice, not good with alternators.