Engine dies when headlights are turned on.

[waveygravey]

My car is a 1974 Spider completely stock with the exception of a gear reduction starter and a headlight relay kit installed several years ago.

The car dies when I turn on the headlights. Seems simple. Where do I begin?

Thanks

[GeorgeT]

Check your alternator and battery. Autozone and similar stores can check your alternator for output and your battery for health. Also, the headlight circuit runs through the ignition switch so there could be a problem there - if you haven't installed headlight relays you should.

[samakijoe]

Weird stuff like this usually turn out to be a bad ground somewhere. Electricity always seeks the easiest path. You might pull and clean/lubricate all the connects you can find and clean and tighten all the ground points.

[zachmac]

You stated you have a headlight relay kit and otherwise have stock wiring and that the engine dies when you turn on the headlights. So, what do we know?

1) you are not trying to pass large current through the ignition switch as you are only using that circuit to trigger relays.
2) the headlight switch has two sources of power going to it, an unswitched and a switched.
3) power to the ignition (via the tachometer) doesn't share the same circuit as the headlight switch power supply. The former is from the #1 fuse via the pink wire off the #15 ignition switch post, and the latter direcetly from the BLUE/blk wire off the #INT ignition switch post. There is no physical ignition switch fault that can cause those two to make contact.

I suspect the problem is a faulty headlight switch. If turning it on is grounding out the power that will kill the engine. But let's try some things to diagnose it.

I assume the car starts and runs fine as long as the headlights are off? (yes or no?)

Assuming yes, you say when you turn on the headlights, the engine dies. Does it die when you turn on the parking lights (first position of the headlight switch) or only when you go to the second position, headlights on?

Assuming the answer to above is stays running in parking light position but dies in headlight position, does it do it on both high and low beams or just one or the other? To figure this out you might have to start the car, make sure low beams are selected and turn on the headlights. Assuming this kills the engine turn off the headlights, restart the car and make sure the selector if down for the high beams and then turn on the headlights. Same result or not?

Please answer these questions / try these tests and report back the results and we'll go from there.

[waveygravey]

Thank you for the responses. Much appreciated. I have not had the opportunity to get to your recommendations. But I will soon and I will get back to you folks.

My car is an occasional use car (weekend nice weather). Hopefully I can get to it soon.

[zachmac]

As you progress if you want some real time phone help contact me via a PM. If time zones and times allow I'll be happy to participate in some real time remote troubleshooting. Might be cool to see if it can be done / useful.

[DieselSpider]

The headlight relays kits do not usually address the other running lights so many times when this occurs it can be the load from the running lights. For me it was the brake lights and it was voltage drop from the always on power feed to the brake light switch. When you turn on the headlights there are also tail lights, side marker lights and license plate lights that are running in parallel with the headlamps.

I ended up quickly switching the feed to the brake light switch which resolved my stalling issue. I have since added relays to the tail, stop and rear turn signals to make my trailer lights brighter.

[zachmac]

Diesel Spider - which is exactly why I asked him if it occurs in the parking light switch position versus the headlight on position of the light switch. That position powers all the stuff you mention without powering the headlights. If the problem is in one of the non-headlight lighting circuits it will kill the engine in either on position. If it only does it in the headlights on position then most likely it is a headlight issue or a headlight switch issue specific to only that position. When diagnosing electrical problems the first task is always trying to narrow down the part of the harness that the problem exists in.

If it kills the engine in the parking lights position then we proceed to isolate the fault there. If only in the headlight on position we pursue a different troubleshooting fault tree path. I'm just trying to gather enough info to know which way to proceed from the first node in the possible paths / fault tree process.

[ORFORD2004]

What about bad groud from the negative cable of the battery to the body of the car

[zachmac]

What about bad groud from the negative cable of the battery to the body of the car

Why would that only cause an issue when he turns on the headlights?

[ORFORD2004]

Why would that only cause an issue when he turns on the headlights?

Too much power for the poor connection.
When i bought my car and drive it back from Charlottesville my car start to act like misfire early in the morning. When i turn off light, the misfire disappear so i was thinking about an electrical problem with my light system. 5 minutes later the car was dead on the side of the highway. They tow me to a parts retail store and they test the alternator on the battery. The alternator was charging but not enought. I bought the bigger battery they got and recharge mine and i was able to go back home on the new battery. At home i remove the alternator and send it for rebuilt. The place told me that the alternator was OK but they change the regulator just in case. So i'm pretty shure that my problem was the connection of the ground to my body.

[samakijoe]

Naaaaa: I'm still thinking it has to be the light switch.

[zachmac]

Well gentlemen, IMHO guessing or replacing perfectly good parts (called Easter egging when poor mechanics do it) is not the way to go. It is relatively simple to systematically isolate an electrical issue / ground. Once the OP answers the first set of questions we'll have more knowledge on which to base the next step. Then by removing certain fuses we can further isolate the section of the wiring where the problem lies. You keep doing so until you are down to a relatively small part of the harness THEN you go find the problem in that part of the circuit.

If as I hope / suspect the answer is his car only dies when the headlights are on and not on the parking light position we will have eliminated a great deal of wiring. We'll basically be down to the switches (headlights switch and hi lo beam selector), the headlights, and the high beam indicator circuit. Next step will be to remove all four of the headlight fuses (#3/4/5/6) and try it. If turning on the headlights switch and toggling the hi / lo beams switch doesn't kill the engine we've eliminated the switches. If we then put one fuse at a time back we should be able to isolate the problem to a single part of the headlight circuit.

BUT, if the parking lights on kills it then most likely we are chasing a ground in the various auxiliary lights. Different fuses but same process follows.

All this doesn't mean it isn't a bad engine / battery ground but that should show up in other ways.

Finally, the fact he has a headlight relay kit installed may mean he simply has a bad relay. It also probably means two of his fuses are doing nothing as would be the case in 90% of the cars that have had the kit installed. Every kit I've seen marketed uses only the left or right side stock headlight feeds as relay triggers and uses one relay for both lo beams and the other for both high beams. Usually the driver's side, green/blk and grey/black feeds from fuses 3 and 5 (hi and lo beam triggers respectively). The feed to the passenger side is just left unplugged. This means the 4 / 6 fuses aren't sending power anywhere. I used this to create two new fuses locations for whatever I want to power in the future in the stock fuse box. I took out the input side contacts for the 3/4 fuses (they are ganged) and took a dremel and separated them. I then moved the lo beam trigger from the #5 fuse to the newly created independent #4 fuse (now electrically separate from the #3 fuse) thereby leaving me with the 5/6 fuse location to use in some other way in the future.

[waveygravey]

Want to post an update for all the folks who were kind enough to offer help and assistance.

This past weekend I got my car out for a drive up to Road Atlanta and the Walter Mitty.

So after an oil change and filter, top off all fluids, check inflation on tires, wash and wax, battery charger over night the car started up immediately. Then filled up the tank with ethanol free premium and a can of Sea Foam. The car ran perfectly all day.

When I returned safely home and half way into the garage I tried turning on the lights. And they worked perfectly and the engine did not die.

Can't explain it unless the battery was low and the overnight charge fixed it.

[76was124]

How old it your battery? Could be weak cell (cause or symptom?) and possibly alternator. Next time problem comes back (and it likely will) have a volt Meter handy and check voltage at the battery, then also at the coil. Previous post also suggested stopping in at the local auto supply store to test both. Good advice it costs only your time.