I have a 1970 442 that blows the tail light fuse (20a) every time I turn on my headlights. The only comman part I could find in the wiring diagram I have is the H/L switch. Any other possibilities??
Thanks Hugh

 

Welcome to CO. That might be it but it could also be any of the lamp sockets that are in that circuit; one of the four tail lamp sockets, or side marker sockets, or even a wire it rubbing against a part of the body. Someplace you have a short to ground that is blowing the fuse. Take every lamp out of every socket in that circuit, put a new fuse in and then turn on the switch. If it does not blow the fuse, slowly add one lamp at a time until the fuse blows. If it blows the fuse before you do all that then you have a shorted wire or a bad switch that is shorted.

 

I expect you have a bad socket or pinched wire somewhere toward the back of the car; could even be a bad bulb that's shorting internally. You should be able to unplug the taillight harness from the body wiring somewhere in the trunk. Unplug it and turn the lights on. If the fuse blows, it's in the body wiring between the headlight switch and the trunk plug, that runs under the carpet. If not, it's in the taillight harness.

Get a box of fuses, then take all the taillight, license and rear side marker bulbs out. Replace and remove them one at a time until the fuse blows again. The one(s) that blows the fuse will be where the short or ground is. You may even see sparks!


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I have a feeling many do not understand the guts of a GM car's headlight switch, so here's a quick tech school if anyone's interested. Helps to have a shop manual opened to the electrical wiring diagram as dinosaur act does not have a scanner.

There are actually three separate switches inside the headlight switch. One for the headlights, one for the tail, park and instrument lights, and one for the dome light.

Looking at the switch on your wiring diagram, the headlight section feeds from the junction block thru the red wire to the BAT terminal. Pulling switch to headlight position completes the circuit which feeds the headlights thru the dimmer switch. When the headlight switch is pulled out to headlight position, the contacts in the switch move to the light blue HD wire to feed the dimmer switch and thru that to the headlights. There's a circuit breaker (really a thermal overload switch as it self resets once the overload clears) in that section to prevent high current overloads. It's the ) at the red BAT wire.

The dome light switch shows at the right hand end of the switch drawing. All it does is ground the white wire D. GD and complete the dome light circuit when the headlight **** is turned all the way left. It is not electrically connected to the rest of the headlight switch. Many GM cars prior to 1964 did not even have this feature as the dome light had it's own slide switch on the lamp housing or sometimes on the B pillar.

The park/tail circuit feeds from the TAIL fuse in the fuse block. This fuse is hot at all times as it's supplied from the battery thru the main junction block underhood. This part of the switch is fed thru the brown/white stripe wire on the diagram to the TL.FD & PARK terminal in the switch. When the headlight switch is pulled to park or headlight position, the brown/white stripe wire connects to the brown TL terminal to feed tail, park and marker lights, and also to the green PNL terminal to the dash lighting thru the dimmer rheostat.

 

There IS another possibility. The feed for the underhood lamp is on the same circuit. Even if you don't have the underhood lamp, the feed wire is there in the harness. If this wire gets pinched or shorted, it will blow the tail light fuse. My first 1968 442 had exactly this problem. The feed wire was loose and would hang down and lay on the header, melting the insulation. I would go over a bump and the bare spot would ground out, blowing the fuse. Needless to say, by the time I got home, the wire had moved and there was no short to be found. It took several months of scouring the wiring diagram and tracing ever single wire in the harness until I found that.

 

With a good fuse and the headlights off do the brake lights, and signal lights work?

Will the fuse blow without any bulbs in the rear sockets?

 

Thanks to everyone. I did put in a new H/L switch an tested it without the switch in place. So far so good, the tail light fuse did not blow when the headlights were on.
Thanks again. Will let you know if once I get the new switch in if my luck continues.
Hugh

 

I had this exact problem as well in my '73 Custom Cruiser. Signal lights, brake lights, 4-way flashers all work, but turn on the headlight switch and the taillight fuse would blow immediately. Although it appears that you have a different cause, mine turned out to be a bad ground in one of the rear side marker lights. I never really did figure out why a bad ground would cause a short and not just make that side light not work, but then I've learned over the years that when a ground is bad, the electricity will find some other path, and it's usually not the one you want it to take.

 

And my '57 was blowing tail light fuses also and it was the disconnected under hood light wire that was shorting on the heater case!!!

 

I'm starting over. I thought I'd solved the problem w/the new switch but when I drove back to storage (thru a strong rainstorm) the fuse blew again%$@#??? I will start by checking for any loose or frayed wire(s) and then pull all bulbs that could effect the tail light fuse. I'm trying to do it this weekend.

Just a side note. I have two other electrical problems: 1-My hearter/A/C fan runs intermitently (mostly off as it is now) and 2-My engine temp light is on all the time. I have a new blower switch and engine temp switch on order but after my stupid guess on the tail light problem, I asking first, "Could there be any connection between these three situations??".
Thanks for the help so far.
Hugh

 

I am fairly certain that your fan is supposed to run on LO when in the OFF position

the engine temp light could be a loose or broken wire @ the temp sender, or a bad sender itself

My $$$ is on NOT related to lights

 

Hey Hugh, have been reading this thread with keen interest.
Did you solve the problem, and if so, how?
Ya, I know it has been 4 years.

 

My car is stored at my daughters house. I will check with her as soon as I can run her down. To be honest, we have had no reasons to drive it with the H/Ls on. I'll let you know. Thanks for your interest. Hugh