1971 FJ40 'lectric
 

Hi, I was in the process of figuring out why my passngr side tail light goes dim when the brakes are applied with the lights on. I noticed that it goes completely out when the lights are not on. So I'm figuring this out and I choose to remove the ground cable on my battery, just to watch it run on the alternator and admire it :) haha.
While looking at it run, I decide to continue to troubleshoot my light prob. I turn on the lights, mind you with the battery disconnected(Black). Immediately when I do this, the thing shuts off.

My question is: Is my alternator not putting out enough power, or is this ground connection needed for the lights to work while running. Any help would be greatly appreciated. Thanks in advance.

71 FJ40

Hi O2runner,
First of all, NEVER unhook the battery cable to "check" an alternator system. This is an old test from generator days. An alternator can produce a lot of voltage very fast when it is run disconnected from the battery. Older cars just blew out light bulbs, newer ones can blow out computers. Where I work now, a mechanic did this on an older transit bus and blew out every light bulb on the bus. The bus is 24 volt with a 275 amp oil cooled alternator.
At idle, your alternator puts out very little, so turning on lights probably dropped the voltage enough to kill the ignition.

I also have a 71 cruiser. Checking for a bad ground is the correct idea for your problem. I would make sure the socket is grounded to the light housing, and the housing is grounded to the frame.

Thanks Yoda, the housing was bad, all corroded up beyond any clean up help. I replaced it.

The lights work the way they're supposed to. Thanks!