[Ben M.]

Okay. I am almost 100% positive this is a bad grounding issue, but I'm pretty stumped on where and need some suggestions to bounce off of the walls.

Car is a 1972 Lemans. No issues before, and I flew back home to drive it 500 miles. Get in car and the dome light doesn't come on. Assuming the bulb is blown, I go to replace it. Bulb is still good though... Check fuse box and find the stop light fuse is blown and as such, I don't have stop lights either. Replace fuse, all is well and good. Driving up, notice that dome light stops coming on after about 100 miles. Check and there are no stop lamps. Hazards do not work either but everything else does.

Get to the end of my journey, go to check fuse. Stop fuse is still good. Pull it anyways, clean off the contacts in the fuse box (a little rusty). Pull all of the other fuses one at a time, clean them up and put them back (no blown ones). Now my stop lights work just fine as do my hazards and turn signals, but I don't have parking lights, tail lights, OR dash lights (both worked fine before). When I run the car, the alternator charging light is dim (14.2V measured at battery and alternator) and when I push the parking brake in it really lights up the alternator light and dimly lights the brake light. I've tried running a good known ground to the back of my dash lights and it doesn't help any. I disconnected the tail light harness too to verify that it wasn't a short or poor ground there either. All of the bulbs in the harnesses are good and are making good connections.

So... I have no dash lights and no tail lamps or parking lamps (0V at the connections). Everything else appears to be working just fine (dome light, radio, headlights, hazards\turn signals, etc). Any idea where else in the harness to start looking? No evidence of critters chewing any of the harness in the engine compartment either. I'm really curious why it would start behaving like this all of the sudden. My next step is perhaps the headlight switch, but that's some involved work to get out of this car. It all screams "bad ground" but I have ruled out the easy things and would hate to start stripping the dash and interior out to try and locate a possibly pinched wire somewhere for a condition that just magically started happening after sitting for a few weeks.

[wrenchmen]

How is the positive connections at the starter?

Is the headlp. switch really that hard to remove on a 72?...on my 70 you just reach up behind the dash, push in a little button on the side of the switch and than you can remove the knob, lossen nut behind knob and the switch will come out.

You might try the "wiggle test", when the lights are not working and the fuse is good, wiggle the harnesses and see if you can find a spot where the lights come on. Sometimes a wire can break under the insulation which makes it hard to find.

Also I have seen Light Bulbs that can be internally shorted and pop a fuse.

[Ben M.]

Positive connection to the starter is good. 2ga battery cable and good clean wire to the alternator.

I've never had any luck trying to remove the headlight switch. Other years are easier and haven't ever given me problems.

I don't think it's a short at a bulb because I'm not blowing the fuse anymore, just not getting any voltage at them (combined with the odd alternator light behavior).

["QUICK-SILVER"]

Check voltage on both ends of the tail/dash fuse and on the fuse retainers/clips. Then you'll know for sure if there's power going to the headlight switch, or it at least should be going to it.
If it is: lights on, check for voltage at the dash light fuse. Power goes through it before going to the dash. Remember to twist the light knob.
This will at least get you pointed in a direction.

Need to look at a schematic of the printed circuit to see if the two bulbs share something in common.

Don't know what else to say till you verify voltage going to the headlight switch at the tail/dash fuse.

[Ben M.]

Troubleshooting day! Voltage comes in to the headlight switch plug when the switch is disconnected from the plug. When you plug it in, still have voltage. Turn headlights on and voltage drops to <1V. Continuity is good between the two lines. Disconnected tail light AND headlight harness from car. Problem persists. Obviously indicates something is dragging it down badly, most likely in the dash.

It gets interesting now. Put everything back together and start the car... The alternator measured at >15V on the meter. And the self-excitation circuit does not work anymore either (when the key is on and car is off, no GEN light). If I rev the engine past 1400RPM, the alternator starts to howl and the GEN light lights up very brightly. Sounds indicative of a failing alternator\voltage regulator. Since it's over 8 years old, probably time to change it. Once I do that, I'll see what happens to the rest of the car's dash light problem. While it may not be related completely, at least it is something that needs doing. I have seen failing voltage regulators do strange things in modern electronics, so I need to isolate that problem and fix it.

["QUICK-SILVER"]

Check voltage on both ends and retainer clips of the tail light fuse while doing the light check. Sounds more like something is barely making contact instead of a short dragging the voltage down. Seems like something getting all the voltage would've blown the fuse.

I'd check the excite wire for power with switch on and the wire un-plugged from alternator. There's something strange going on with that. Maybe a fuselink or something is about to go on the sensing wire AND it may have something to do with power to the tail lights. Maybe even where the main power supply wire starts splitting off to different things.

I may just be over thinking this but it sure sounds like you've got a power loss from a bad connection.
Also probe the hot wire where it goes through the bulk head connector.

I'll quit now

[Ben M.]

Quick-silver, the more I think about it, the more I think you are right. The fuse box is kind of crusty down inside since it's 37 years old now and the stop light, instrument panel, and courtesy lamp circuit are all green or brown (everything else is fine naturally). I did a screwdriver rub on them, but they probably need a little bit more. Any thoughts on how to clean a glass fuse holder without actually removing it from the car?

Engine harness at alternator looks good and I don't see any major issues with wires, plus disconnecting the pin from the alternator didn't do anything for my problem ;-)

["QUICK-SILVER"]

The way the terminals are made it's hard to scrape where the fuse actually rests, without using something crooked on the end. I've done pretty good with brass bore brushes. .22 & .25 caliber held as flat as you can against the fuse box and twist them. Like your sticking the brush in one end of the fuse holder from the opposite end.
Haven't tried any sprays, like contact cleaner, but that might be worth trying.

Terminals losing their spring tension has been a problem for me with the older fuse boxes. Then it's bend, twist, wedge, just whatever it takes to tighten the fuse short of replacing the fuse box.

Keep us updated on how things go.

[Ben M.]

Problem solved. Bad fuse to the tail light circuit. Not a blown one, but one that would work... and then 5s later stop. As it heated up the metallic piece inside was moving around slightly, causing a high impedance problem. Tested it by pulling the instrument panel fuse and the tail light fuse. Ran a wire from the battery to the other side of the terminals to find out that it wasn't the instrument panel fuse and it wasn't the instrument panel connection from the tail light fuse (isn't that something? The tail light fuse is first in series with the instrument panel fuse). Only left the fuse. Cleaned the terminals as best as I could, put a new fuse in and now the lights work in all positions without issue.

Alternator is still dying (GEN light glows and the alternator HOWLS above 1400RPM). Only a matter of time until that dies, so I'm going to replace it later this week.