Turning on my headlights killed my truck!
 

Last night after work, I started my 89 full size, went to pull out the headlight switch and 'poof' the whole truck shut off except the radio. Engine died, CEL didn't even come on, just darkness.

As long as I left the lights off I could start the truck (although it took extra cranking and pumping the gas on a TBI?) and eventually after screwing around, I tried the headlights again and everyting seemed o.k. so I drove home.

Once home I couldn't repeat the problem by turning on the headlights. But, get this, if I turned the dimmer all the way to the interior dome light position, the CEL would come on and the engine would start to stumble!!

What on earth??

Other wierd things going on recently included volt meter randomly dropping to 12 volts (like I lost the charging system) the coming back up to 13V after a few blocks.

Also the coolant tempt guage has been showing really hot coolant temps randomly then dropping back to normal...I suspect this is electrical problem becuase it is 15 degrees outside and I have totally new cooling system that has worked fine.

Any ideas?

I agree it sounds electrical.

The lights affecting causing the engine shutdown sounds to me like a weak alternator. But a faulty relay could be shunting too much power as well.

I'd try to verify the alternator output with a multimeter instead of the interior gauge. If possible watch both at the same time and see if both the meter and gauge readings fluctuate synchronously.

Could be weak or corroded connections at the battery or even a failing battery.