I have a 1980 W150 Power Wagon. I drive it daily. The other day I went to start it. It would start then when I let off the key it would die.

I figured it was the ignition switch because that has always been the case in the past with other vehicles.
So... I got NEW ignition cylinder/key and ignition switch. I even installed a new starter relay since I was replacing things associated with starting.

After installing it all.... the same issue remains. The truck starts up fine but as soon as I let off the key it dies.

Does anyone know what the heck is wrong with my truck?!?!

Ballast resistor usually causes this.

what Supercuda said, turn key "on" & with your meter see if you have fire to/thru the ballast & then from that point work forward or back (bulkhead is a frequent culprit) till you find the open

Roger that.... I will give that a try. Thanx!

At least I have some new stuff installed now Lol!

As a temporary test you can jumper out the ballast and see if it stays running, if so it's the ballast, if not it is elsewhere.

Under dash ignition switch to harness connector another possibility. One of the 'males' on the harness side may be pushed back out of position.

I'll jump on the ballast wagon you can check it with multi meter with key off I'm inclined to think it a dual sided ballast the current flows from post to post that are above or below each other you can check it for continuity with the ohm resistance setting on any cheap multi meter if you touch leads to pins above below each other # should drop or 0 if there is no change in the # when touching pins above/ below each other the ballast is in need of replacing or just do the previously mentioned plan b test jump top to bottom wires together

Replaced the ballast resistor and it starts great and runs great now!

Old ballast had no resistance.

Thanx!

They are cheap. Buy another one and put it in the glove box.