No Parking Lights/Running Lights/Dash Lights

[KDMatt]

Short version:

My car has blown the same fuse three times in the last 2 days -- the fuse for the voltometer, map light and dome light. I believed the issue to be a short in the door-jamb switch, and I -thought- I'd rectified it.

New issue: No parking lights/running lights or dash lights. I'm digging around in my wiring diagram, absolutely blind. Stupid question: is there a fuse for this?

Long version:

I started having electrical troubles after I put in a new starter (the old one was starting to grind, so I figured $35 was worth the extra state of mind). The aforementioned fuse (voltometer/dome light/map light) blew for the first time, but only blew at night after running with the lights on -- this fuse has never blown in the daylight, only at night after parking the car, shutting off the lights, and opening the door.

I've replaced that fuse again for the third time (proper 10A fuse), and fiddled with the door-jamb switch for the dome-light, to make certain that it wasn't just a short in the switch.

So far so good until tonight, I flick on my lights and I've only got the headlights -- no running taillights or parking lights, and no dash illumination.

Seriously, what the hell.

Is there any way these two circuits share a connection? A ground wire perhaps? I'm assuming the fuse block itself has some kind of ground wire? Maybe?

I cant make heads or tails of the electrical diagram -- I hate dealing with wiring.

I'm losing my mind over this car, please help me

[FastWoman]

It's NOT a short in your doorjam switches. Your switches work by "shorting" to ground when the door opens, at least if they're like every other door switch I know, including on my '78. In other words, no matter how your switches might short to ground, all that would happen would be that the dome light would come on.

What I see in my '78 schematics is that these things (voltmeter, dome light, map light, dash lights) are all powered through different fuses. I'm sure it's the same with your '76. I'm guessing someone did some creative wiring "repairs." Even so, these things all seem to run through a fuse, which is good. Not all professionals are so courteous as to make sure their ad hoc repairs are fused.

Based on what you've described, I'm going to guess that there's a short in your dome light -- perhaps in the socket, or perhaps in the wiring that connects to the unit. When you open the door, the door switch grounds the dome lamp unit, which is shorted, such that current runs through the 10A fuse, through the dome lamp assembly (bypassing the bulb), through the door switch, and straight to ground. This of course would blow the fuse. I'd almost bet money on it!

Please let me know if I got it right.

Peace,
Sarah

EDIT: I just noticed the title, which includes parking lights and running lights. Those should be on an entirely different circuit -- and in fact the same circuit as the headlights. You might be having a wiring harness meltdown -- probably from some of the creative electrical "repairs" that have been made to the cars. You might want to start untaping the wiring harnesses to search for melted wiring. Insulate any melty wires with electrical tape. Melted wiring is like a cancer and can take down other circuits too. Honestly, from all you've described, you probably need to get your car to someone who can restore your electric to OEM design and condition. Unfortunately very few people know electric.

[KDMatt]

Hello Sarah,

Actually, the parking lights/dash lights all run off of a fuse that I did eventually track down in the early hours this morning. It's a 20A fuse, and I've replaced it ... so far so good.

The dome light/map light/voltometer do actually share a fuse -- that's one thing that I can make sense of in my wiring diagram for my '76 ... No doubts though that there is some, as you say "ad hoc" wiring afoot.

Good info about the door jamb switches, I'll investigate my dome-light itself now... like I said, it only happens at night when I would notice. :P

... say you don't suppose there's a wattage limit on that bulb do you? I replaced mine not long ago, and I could swear that it's brighter than the one I had before.

[SteveJ]

EDIT: I just noticed the title, which includes parking lights and running lights. Those should be on an entirely different circuit -- and in fact the same circuit as the headlights. You might be having a wiring harness meltdown -- probably from some of the creative electrical "repairs" that have been made to the cars. You might want to start untaping the wiring harnesses to search for melted wiring. Insulate any melty wires with electrical tape. Melted wiring is like a cancer and can take down other circuits too. Honestly, from all you've described, you probably need to get your car to someone who can restore your electric to OEM design and condition. Unfortunately very few people know electric.

Actually, the parking lights and running lights are on a circuit together, but they are NOT on the same circuit as the headlights. If they were, then he wouldn't have headlights, either.

Page BE-15 and BE-17 show a good simplification of how the fuse box handles power distribution and goes out to the parking lights and instrument lights. All of these lights are on the same circuit.
Page BE-18 shows the dome light, and BE-20 shows the map light. The dome light and map light are on the same circuit.

Both circuits are powered off the same bus in the fusebox. Before tearing apart all of the wiring bundles, it would be prudent to check the fusebox for corrosion. Check the condition of your fusible links, too. There are plenty of posts on this board warning about corrosion in the fusebox and at other points of the wiring harness. It happens with a 30+ year old car.

... say you don't suppose there's a wattage limit on that bulb do you? I replaced mine not long ago, and I could swear that it's brighter than the one I had before.

Ohm's Law applies here. You have a 12 volt system and a 10 amp fuse. You would need to draw more than 120 watts of power to blow the fuse in a healthy electrical system. How many watts were the bulbs rated for? Also, did the old bulbs have corrosion at the base? Corrosion causes voltage drops (and sometimes short circuits), and less voltage at the bulb means less light.

If you don't have the FSM, go to Xenon S30, find the reference section, and download the FSM. The breakouts in the BE section are probably easier to understand than the wiring diagram. If you're still having problems after that, send me a PM, and I'll give you a phone number to call me so I can talk you through things.

Steve

P.S.: Search the Electrical section for the word corrosion. It will produce about 3 pages of threads addressing this issue.

[FastWoman]

Ah, OK. I remembered wrongly regarding the fusing of the running lights. Yeah, the tail/clearance lights run off of a separate fuse on my '78 too, although it's a 15A fuse. The same fuse illuminates the instrument panel. When you talk about the fuse going to the voltmeter, are you talking about the illumination of the meter or the meter movement itself?

Dome lights and map lights are all apparently fused differently between the two years (because mine are on separate circuits), so I'll be making fewer assumptions in the future.

That said, the basic strategy of wiring a dome light is going to be the same. I'm still betting on a short in the dome light or its wiring. In answer to your question, I doubt you could put a high enough wattage bulb in that little fixture to blow the fuse. As Steve indicates, it would take 120W of load (actually more like 150W, since the 10A rating is only a nominal one) to blow that circuit. In my FSM, I have that bulb listed at 10W. It might not hurt to try another bulb, in the event yours was mislabeled or thrown in the wrong package.

I agree with Steve that corrosion is a BIG problem in the wiring of these old cars. You can get corrosion in the bulb socket, which can cause heat buildup and melting of the wires. You can also get corrosion at any of the connections, which can also cause melting. (My fuel pump wiring recently failed that way -- at the big wiring harness connector in the firewall.) Finally, there are crimp connections sprinkled throughout the wiring harnesses. They're all wrapped up, so you don't see them. They corrode too, and they can even get hot and melty.

I admit I'm veeery conservative when it comes to wiring, and I'm very quick to unwrap wiring harnesses for a look. However, it's just tape, and you can (and should) wrap it back up with new tape when you're done. Based on what I've seen in my own almost-completely-uncorrupted, dealer-maintained wiring harnesses, I'd honestly recommend that anyone with an S30 unwrap certain parts of their wiring harness and re-make the internal crimp connections, probably sweetening them with solder. There are parts of my own harnesses I've discarded and rebuilt. I've seen many cases (not in a Z, but often in better wired cars -- not too impressed with some of the wiring in the Z) where a hot/melty spot in a wire would take down an adjacent wire, which would take down an adjacent wire, etc., etc., until the entire wiring tree was toast.

That said, there's no reason to tear into your wiring harness if you don't see any evidence that circuits are combined that shouldn't be or that impact each other when they shouldn't. If you find that a circuit draws power from a fuse that it shouldn't, that's a bad sign. Either it was "repaired" by someone who didn't/couldn't/wouldn't make a proper repair, or it's shorted somehow. That's when you might go unwrapping the wiring harnesses for a look.

I do agree with Steve in general that it's prudent to check for corrosion, but I would find it very hard to imagine that the fuse-blowing problem is due to a crusty fusible link or corrosion in the fuse box (apart from a detached part), which would limit your current and have a sparing effect on the fuse. Instead, you've got an overcurrent situation, most likely a short somewhere. (But do check your bulb to make sure it's the appropriate type.)

[KDMatt]

Update:

Curiously enough folks, I cleaned up the chassis-ground wire in the engine bay/firewall off of the battery terminal and... fingers crossed... so far so good.

*knocks on some wood*

... just goes to show that corrosion really IS the enemy...

[SteveJ]

Don't forget to examine your fusebox, too. If you let corrosion build up there, it can melt down. I've had that happen to me.