280Z Parking Light Circuit

[SteveJ]

So I was working on a customer's 280Z today, and I managed to isolate/solve a couple of his electrical issues, but there is one with the parking light circuit that I haven't been able figure out quite yet.

I had to replace the headlight switch because his old switch had heat damage from the headlight circuit overheating. The pin that operates the rocker would no longer stay in place. I had a spare switch that I needed to re-solder the parking light wires to the switch body. The switch operates normally, but the parking lights won't come on.

I checked the resistance of the green/white wire to ground. It was about two ohms even with all of the side marker bulbs out, the right front marker and taillights unplugged. So it only had the license plate light, left front marker and dash lights plugged in.

Here is the weird part. I could see the voltage to ground at the switch go to zero. That's what I would expect to see in a circuit with a short. However, the fuse didn't blow. I even monitored with a clamp on ammeter (that was working when I found his "phantom draw" on the battery), but there was no indication of current flow. Yet the voltage always dropped to nothing. 

The next time he brings his car over, I plan to unplug the connector for the speedometer to try to take the dash lights out of the circuit. I also need to try to unplug the C-8 connector to isolate the dash harness from the body harness.

Unfortunately this car has been suffering from long-term benign neglect. It will be an effort to get all of the bugs worked out.

[Captain Obvious]

First of all, I got no problem whatsoever with this:

2 hours ago, SteveJ said:

I checked the resistance of the green/white wire to ground. It was about two ohms even with all of the side marker bulbs out, the right front marker and taillights unplugged. So it only had the license plate light, left front marker and dash lights plugged in.

License plate bulbs (two), the remaining marker light, bunch of dash lights, glove box, cigarette lighter, ash tray.... Probably others. All in parallel? I got no problem with a couple Ohms to ground on the G/W.

As for the readings you're seeing on the switch... I'm thinking you've got a high impedance connection somewhere upstream in that G/L wire.

When the switch is in the OFF position and there is nothing connected to drag the G/L wire down, it floats up to battery voltage and looks good. But as soon as you turn the switch on, all those filaments in parallel will drag that high impedance source to ground. I'm guaranteeing that there IS current flowing. Just not enough for your home wiring style clamp-on meter to detect.

So I'm thinking that's the issue. Finding it is a different story.

Fuse that looks good, but isn't really? Barely holding on by a tiny sliver of metal inside? End caps that have been overheated and mostly de-soldered themselves?
Corrosion on the fuse clips?
Corrosion in the headlight switch? (Unlikely because you would have probably seen that while you were messing with the switch).
Solder joint on the back of the switch that looks good, but really isn't. (I've never seen you solder. LOL)

Anyway, happy hunting.

[SteveJ]

Thanks, @Captain Obvious. I needed the reminder to look upstream. Corrosion at the fusebox could cause the behavior I'm seeing. I can create a jumper to bypass the switch the rule it out. That's easy enough to do.

[Captain Obvious]

Glad to help. And of course, but some of the potential culprits may depend on the year you're working on. For example, (quick look at some of the wiring diagrams indicate that) 75 and 76 have a connector (C-2) between the fuse and the multifunction switch, while 77 and 78 do not. So if you're working on a 75 or 76, that connector may be corroded as well.

But basically... Yeah. I would first start by putting a meter on both sides of the fuse and see what happens when you turn the switch on. Both sides should be solid battery voltage regardless of the switch position. And then work your way downstream from there if the problem isn't the fuse itself.

Also, I'm assuming the fusible link and wiring to the "high side" of all the fuses is good or you would have mentioned a whole host of other electrical issues. I'm assuming this problem is relatively isolated to the lights. I mean, if you had a problem with the fusible link, it would show up in a whoooooole bunch of other areas as well. If everything else pretty much works, you ought to be able to start at the fuse box and work your way down from there.

[SteveJ]

Yep, I documented the C-2 connector in my notes to my customer. I try to have a work plan ready for paying customers so I don't rack up a bunch of hours on random diagnostics.

The white/red wire feeds many fuses in the fuse box. My customer had another shop put in a disconnect switch due to a battery drain, so I used my ammeter to track that down. (Fix the problem, and don't use a bunch of band aids.) I traced it to that branch in the fuse box, and we removed fuses to see when the current draw ceased. That took a while because the draw would come and go somewhat randomly. Eventually I found that the passenger side door switch was wonky. There wasn't easy access to disconnect that switch, so we just pulled the fuse for now since he was not worried about the dome light or other lights on that circuit.

I'll have to look at the other circuits in C-2 to see if there are other indicators of corrosion. The terminal for the parking light circuit could have overheated at that connector. It's not like I haven't seen damage to a lot of fuse boxes and wiring for the parking lights.

The nice thing is that he is willing to get his hands dirty and does not mind providing assistance since that reduces the number of hours I charge. He seems willing to learn, too. He was thrilled that I was willing to inspect his car with him present and give him an idea of what issues he may have. What worries me, though, is that the rear brake MC reservoir was almost empty. I added fluid and bled the rear brakes, but the car is still HEAVILY biased toward the front brakes. I didn't have time to bleed at the MC, and I plan on looking at his front brakes on his next visit anyway. If it's the valve between the front and back brakes, that will be a challenge. The rubber hoses on the back need to be replaced, and I wouldn't be surprised if the front brake hoses are in the same condition. (Heck, ballooning in the rubber hoses could have caused the problems I felt in his brakes.

There are many signs of bad mechanics or idiot previous owners working on the car. It will take some money to get things right (and safe). I just want this guy to be able to enjoy his car.

Somehow I get the feeling that my retirement job will be fixing S30 electrical systems in the Atlanta area.

[Obie 280Z]

yea, seeing how I live near you Steve, I hoping it will be your retirement job!