Chevy G20 full size van, conversion, 1993 with 240,000 miles and a 350.
This is a weird one. Took a trip last weekend. First problem noticed was that transmission downshifted when headlights were switched to high beam. After this happened a couple of times we just left the brights off.
Next day, switching on dome lights would kill all power - engine dies, radio resets, everything! No fuse blew, and when ignition was switched off for a few seconds you could start back up. So, checked battery connections (OK) and left dome lights off.
Day after that, coming home, stopped for gas. Switching on headlights now killed all power. Further investigation found that switching on dome light, parking light, and horn would all cause this failure. Other things were ok (brake lights, turn signals, heater fan).
Tryed removing fuses, but couldn't find a combo that would leave me with headlamps - finally came home with 4-way flashers on.
So, what the heck could cause this? No fuse is blowing (that I can find), and everything resets. Right now I don't know what to look for and suspect that the whole thing could die permanently at any time.
Looks like this could have been in a better forum - moderators feel free to move it!
First noticed that some circuits were still working when failure occured (heater blowers, front & rear).
Figured this out - it was a failing fusible link. The link apparantly had gone out previously as it had been spliced, but that splice had deteriorated enough to cause an open under higher loads but worked OK with minor loads.
Traced it after I got a factory wiring diagram and saw that all of the failing circuits (horn, cig lighter, tail lights, dome lights) were fed via this circuit.