A friend was working on a customer's 78, and there was an issue with the headlights. I took a quick look at it a couple of weeks ago when my friend was hosting a party (It was a great way to decrease the amount I had to socialize.), and it seemed like corrosion in the fusible link may have been the cause. The fusible links were pretty nasty. My friend put in a Maxi Fuse holder, but the problem remained.

I went out to take another look today. I saw voltage on both sides of the right fuse when I pulled the ground on the circuit. However, the was no voltage on the downstream side when I had the ground plugged in. I cleaned the fuse holders with a wire wheel, but no change. Neither headlight would light, either. I swapped the fuse for the right headlight with another that was in the fuse box, but that fuse wasn't in great shape. I got another 10A fuse from my friend, and a little while later, I detected that it had blown.

Getting nowhere fast in my diagnostics, I talked with @Captain Obvious, but I couldn't provide enough information for a true ah-ha moment. He did make a good suggestion to start using a test light instead of a meter because we both knew I had to get around an impedance issue if it existed.

I decided to focus on the left headlight next. I disconnected the headlight and rigged up a jumper between the engine harness connector and the headlight connector. I rigged up another jumper that I could connect to ground. I still couldn't get the left headlight to light up with the new ground. I checked things with the test light. I could get the positive side of the wiring to light up the test light, but the ground leg would not cause the test light to light up. (Hold on to this.)

At that point, my friend and I broke for lunch, and I discussed the next steps with him. We agreed that pulling off the headlights would be the course of action because I narrowed down the problem to that part of the circuit.

He got the front end on jackstands, and I got the left headlight out. This is what I found. Yikes!

Someone thought duct tape would make good insulation. You can see how hot and melty it got. That is the positive wire with all of the missing insulation. Unfortunately I didn't have my headlight connectors with me to replace the bad terminal. At least I found the impedance issue with the left headlight. I'm surprised it didn't pop the fuse.

I went over to the other side and pulled the right headlight. I found someone had overfilled it because all of the excess water came out. Okay, so the outer sheath had cracked and without inner liners, water got trapped inside the sheath. I cleaned it up and wrapped it with electrical tape.

I decided to put in a 15A fuse since my friend had those on hand and test just the right headlight with the left disconnected. Sure enough, it lit up nicely. I'm not sure what caused the 10A fuses to pop, but I'm going to install a 10A fuse when I go back next week.

With the right side operating, I connected the left headlight bulb (H4 housings with halogen bulbs) to the socket and turned on the headlights. The right still came on fine, and the left was dim. I'm thinking the left side was probably getting its power from the right side fuse backfeeding it.

Next week I'll return to my friend's place to fix the wiring on the left headlight bucket, and he can have the owner pick up the car.

Lessons learned:

1. Follow my own advice and ALWAYS remove the fuse to test it. I was fooling myself early on when I didn't test the fuse for continuity. I know that the headlights can back feed. With a car that you're not familiar with, you don't know what someone might have crossed wiring-wise. (Finding a wire nut in the engine bay is also a BAD SIGN.)
2. Have plenty of the right size fuses on hand when doing the testing. A fuse kit will become a regular part of my road kit for house calls. I keep them in my Z cars, but it wouldn't hurt to have a third for my tool kit.
3. Drill down to the component level to verify diagnostics when you don't have a smoking gun. I spent more time at the entire circuit level than I should have.

Nice Work friend.

Nice sleuthing!

Having not removed headlamps on a Z yet - based on your post, it all removes from the wheel well with the splash guard removed, obviously. I'm looking at Holley 7" 'retrobright' style LED lamps. Not a fan of LED's at all, however an acquaintance has them on his Fiat Spider, and they look good and project well. Having reduced current draw compared to converting to H4 80W's makes sense.

3 minutes ago, HusseinHolland said:

Nice sleuthing!

Having not removed headlamps on a Z yet - based on your post, it all removes from the wheel well with the splash guard removed, obviously. I'm looking at Holley 7" 'retrobright' style LED lamps. Not a fan of LED's at all, however an acquaintance has them on his Fiat Spider, and they look good and project well. Having reduced current draw compared to converting to H4 80W's makes sense.

I would advise against those, @HusseinHolland. 

1. That pigtail on the back of the light will make it where it won't fit in an S30 headlight housing. You would have to cut a hole in the housing to run the pigtail out. Then you would have to worry about water getting into the housings, and you would also have more issues integrating it. To me, it's more pain than it's worth.
2. I can't guarantee they are polarity independent. The Fiat Spider has 2 positives and one negative. The Z has 1 positive and two negatives. If the Holley headlights aren't polarity independent, you'll have even more work figuring out how you can integrate relays into the wiring harness.

Holley headlight:

Inside an S30 headlight housing: (Image from https://www.zeddsaver.com//collections/exterior/products/restored-headlight-bucket-set)

That is why I use Hella 7" housings or Koito housings. Paired with the Auxito H4 LED bulbs. It works. As an added bonus, the Auxito bulbs are not polarity dependent. 

I know you like to experiment, but please trust me on this one. Here are links to my recommended parts:

Hella H4 housings:       https://www.amazon.com/dp/B001G76Q2W
or
Koito H4 housings:       https://parts.toyotasouth.com/oem-parts/toyota-headlamp-assembly-8111060p70

Auxito H4 LED bulbs:       https://www.amazon.com/gp/product/B07TQLK6SH

By the way, Koito is an electrical parts supplier to the Japanese auto industry. You may still have some Koito bulbs in your car. Toshiba was another common supplier back in the day.

All together, it would be about $125 to go my route. With the Holley lights, you're starting out over $380, and then you still have the integration issues.

 

51 minutes ago, SteveJ said:

I would advise against those, @HusseinHolland. 

1. That pigtail on the back of the light will make it where it won't fit in an S30 headlight housing. You would have to cut a hole in the housing to run the pigtail out. Then you would have to worry about water getting into the housings, and you would also have more issues integrating it. To me, it's more pain than it's worth.
2. I can't guarantee they are polarity independent. The Fiat Spider has 2 positives and one negative. The Z has 1 positive and two negatives. If the Holley headlights aren't polarity independent, you'll have even more work figuring out how you can integrate relays into the wiring harness.

Holley headlight:

Inside an S30 headlight housing: (Image from https://www.zeddsaver.com//collections/exterior/products/restored-headlight-bucket-set)

That is why I use Hella 7" housings or Koito housings. Paired with the Auxito H4 LED bulbs. It works. As an added bonus, the Auxito bulbs are not polarity dependent. 

I know you like to experiment, but please trust me on this one. Here are links to my recommended parts:

Hella H4 housings:       https://www.amazon.com/dp/B001G76Q2W
or
Koito H4 housings:       https://parts.toyotasouth.com/oem-parts/toyota-headlamp-assembly-8111060p70

Auxito H4 LED bulbs:       https://www.amazon.com/gp/product/B07TQLK6SH

By the way, Koito is an electrical parts supplier to the Japanese auto industry. You may still have some Koito bulbs in your car. Toshiba was another common supplier back in the day.

All together, it would be about $125 to go my route. With the Holley lights, you're starting out over $380, and then you still have the integration issues.

 

 

Thank you for the caution Steve!

I have H4 7" housings from my X1/9 original pop up headlamps. So, I would just need to get the Auxito H4 LED bulbs to make this happen 🙂 - Any chance you have a pic of the lighting? 

 

That's the great thing. H4 housings are H4 housings. The ones from the X1/9 should work just fine.

I don't have any photos, but for the people who have that set up, I have never had a complaint. I have those bulbs in both of my Zs. As a matter of fact, I did that for a friend's 280Z, and he then brought his other over for me to convert that one, too.

Just to follow up on the initial post...

I returned to the scene of the crime today. I cut off the connector because it had some damaged wire and put a new 9003 connector on. Both headlights worked.

Here's how the connector looked. You can see missing insulation from the positive and burn marks on the insulation on the high beam wire.

My friend had some small BA15 bulbs so I replaced the side marker bulbs. The front left and rear right had corrosion, so I used a wire brush on a dremel to remove the corrosion. The front lights also were attached without the weatherstrip block, so I got those in place to protect the lights better.

Afterward I convinced the owner that LED bulbs, especially the headlight bulbs, would be a good idea. I hope he follows up with doing that. Yes, he does have my list.