Car woudn't start with headlights on?

[Fiat88]

Just came back from filling her up at Hess tonight. I shut the car off to fill up but when I turned the key, starter wouldn't work.Had the headlights on at the time. Then I shut the headlights off and it started right up. When I got back to my garage, tried it again with the headlights on,no good. Lights off, it started. Any ideas?

[joelbert2k]

I would suspect a dieing battery. Check the voltage at the battery with the lights off and then with them on. I'll bet there will be a significant drop. I'm not sure what voltage is needed to energize the solenoid but I think you're dropping below it.

Joel

[mbouse]

yup. clear indication that there ain't enough juice in the battery to crank the starter and burn the lights.

if the battery isn't giving it up, the charging system is.

when running, what is your voltage reading at the alternator? and what is it at the battery?
when not running, what is the voltage reading at the battery?

[smfinley]

I had this same problem consistently until I added a relay to power the starter solenoid. Similar concept to the headlight relay. Trying to apply as much voltage as possible to the solenoid. The voltage loss through the original electrical system plus the drop from the headlights appeared to be to much for my solenoid to operate. Now I can start with the headlights on all the time.