79 911 SC Fuse Box System Failure

[OsoMoore]

I'm having an electrical problem with my '79 911 SC. This problem was initially discovered when I found a wire getting really hot, and my horn intermittently honking.

History!
I updated my fuse box ~6 years ago to change to ATO fuses from the old fuses. I did not change the layout of the fuses, but they weren't stock to start with and they did have a headlight relay kit already installed.
My update went without any issues and everything worked great for years.
About 3 years ago I added a relay to the running lights, so they could only turn on when the key was turned to Accessory, On, or Start. Again, it didn't have any issues.
They wire overheat may be related to a stalk issue I'm having, where often the stalk contacts for high beams and low beams are on simultaneously.

Where I started last week.
My horn started honking while driving, so I pulled over and the fuse box itself was warm to the touch. I checked at home and a particular wire was fairly hot, and had significant heat damage. Also, my fuel pump relay which was next door to it has concerning curves on the sides. But the car still ran.
My fuse box rebuild was done with ATO fuse blocks, so there was no fuse bus between any of the tops of fuses. The stock fuse blocks have these at various spots, so I had replicated those buses with short lengths of 10 gauge twisted copper wire. As a result, there were several "hops" to bus together different tops of fuses. The wire that was frying was a hop on the Accessory-Controlled bus. Specifically, this hop joined the positive voltage over to a fuse for the cigarette lighter plug (nothing plugged in) and something unknown. From there, there was another bus hop to get to the high and low beams and the running lights. So this wire had a LOT of current when both beams and lights and all were running.


What I did first to fix it.
This past week I resolved to fix the giant mass of hops leading the current fuse-to-fuse. I soldered 3 wires onto the incoming Accessory-Switched voltage. I sent one of those off to the high/low/running light section of the fuse box. I sent one to the cigarette lighter, and I sent another to a few other fuses which hadn't been in the path of the fried wire, but were part of the original bus. I got everything all tinned and heat-shrinked and installed snugly. But I discovered that the entire Accessory-Switched bus was misbehaving and sometimes shutting down entirely, sometimes taking other parts of the system with it. However after a few minutes, it seemed to recover (only to fail again when I did more things).



New image uploads seem broken, so here's an imgur link to the updated wires. https://imgur.com/a/2sUmnxt

The play-by-play video.
This video (https://youtu.be/__dZ-_F0Ges 10 minutes) shows my work trying to reproduce the system failure (audio gets way out of sync, will try to fix it). I have pulled out the fuses on the Accessory-switched bus so nothing should be drawing power there. Then I show I'm getting 12 volts at Always-Hot, and 12 at Accessory-switched when I turn the switch.
I re-insert some fuses and see the running lights come online. The low beams work once I have their fuse in. I put in the high beam fuse and turn them on, and everything goes down.
The Accessory-switched power is down at 2 volts, but the always-on is at 12 volts. This state persists after I remove all the fuses and relays.

Current state.
Regardless of fuse or relay presence on the accessory bus, I'm getting 12 volts at the always-on bus and 2 at Accessory-switched bus. I'm also seeing the speedometer bump up to 20 or 30 MPH when I turn on Accessory, but it goes to 0 when I turn on the lights. It also goes to 0 if I go to the On key position. Video of this (https://youtu.be/VWasYeKgtog 3 min).
Right now the system has "recovered" back to normal, although I have all the fuses on the Accessories-Switched bus removed, and all the high/low/running light fuses removed.

I'm not sure where to check next. I tested all the fuses and replaced all the relays. Maybe something somewhere in the harness fried and shorted? Maybe my heat-shrinked new wire is making partial contact and shorting.
I think I'm about to learn a whole lot more than I want to.

[snbush67]

It seems like you are getting power from somewhere that you shouldn’t be. Are fuses blowing? I think that the power to your running lights should come from light switch, not directly to a relay from the ignition switched power.

I think that the experts are going to tell you to get a wiring diagram for your car and check that every wire at the, light switch, ignition switch, and fuse block is in the right spot.

Also check under the dash to make sure all 5 plugs (hazard switch plug, headlight switch plug, ignition switch plug, turn signal plug, and wiper washer plug) are in the right spots.

And make sure the wires under the dash from the turn signal switch to the other switches are plugged in, on my car (78SC) it’s yellow to yellow, black/white to black/white and grey to grey.

[Jonny H]

The screw terminals on those type of fuses are meant to accept a forked crimp terminal. Stuffing bare ended wires in there doesn’t provide reliable contact surface.

The extra links (‘hops’) you have added are compounding the issue, making the wiring more messy and prone to shorting.

Get a proper fuse panel that has been designed for the car and doesn’t need the extra links.

[OsoMoore]

snbush67 - power is definitely going somewhere it shouldn't. No fuses blowing. The power for headlights goes via relay since the installation of the headlight relay kit many years ago. I'll check the colored wires under the dash.

JonnyH - I see what you mean. I am going to look at doing another rebuild with a better fuse panel.

This evening I did another test. I totally disconnected all the relays for the headlights and running lights, as well as pulling the power wire into the fusebox which is switched by the ignition switch (accessory position).

I connected the battery and saw 12 volts at the battery and the Always-On terminal in the fuse box. I then turned to the key Accessory and then On position and heard the little "whine" noise you expect. I pulled the headlight switch to both positions. The whine's pitch changed slightly, so something was being connected/disconnected.
When I went to attempt a start, there was a loud "click" noise and the "whine" stopped. I then checked voltage at the Always-On fuse box terminal and the battery and both had dropped to 3.2 volts.

Here's a video of the test: https://youtu.be/BnDSpdJg14g

What I'm considering doing next is checking the circuit for the starter, as that was what was engaged when the system went down. I do have a hot-start relay kit (courtesy of our host). I also have a new-ish solenoid.

[snbush67]

I think you should take the fuse panel off and check behind it to make sure there aren’t any wires grounding.

Do you have a diagram for your car? You have to be narrow down the issue. Is your horn still blowing? Do the flashers work? Turn signals?

The light switch makes the speedometer go to 30 mph? I think that the only thing that sends a signal to the speedometer is the sensor from the transmission. Check the wires on the speedometer for correct connection.

[OsoMoore]

snbush67 - I will take some good pics/notes, and disassemble the fuse panel and look behind the it. I do have the overall car wiring diagram somewhere, and I can also find it online with a bit of hunting.

The horn was only blowing intermittently while driving, and I disabled it by removing the horn relay. I can put the relay back in and turn the wheel a bit to see if it misbehaves. Of course, if the battery shorts again when I reconnect it, I'll not try the horn thing.

The speedometer was going to 30 when the key was turned to the Accessory position. It was going to 0 when I pulled the headlight switch to the On position. The speedometer wiring hasn't changed in the years I've had the car, but clearly something is shorting out.

[snbush67]

Look at the wiring to the instruments, the speedometer. Replace or repair any bare wires.

[OsoMoore]

Removed the fuse box and checked all wiring behind it. No apparent damage, melting, scrapes, crimped wires, etc.
Looked under the dash and didn't see anything out of the ordinary. It is a bit of a mess from 40 years of people doing modifications, but didn't see any damage.
Looked at the hot-start relay kit mod. There was a fair bit of road grit/oil tossed around down there, but I didn't see anything for sure wrong. The fuse down there was fine.

I'm pondering possible next steps.
- Start tracing wires from the fuse box to wherever they go. Keep notes and check for shorts to ground or other wires.
- Order a new fuse box and install it.
- Order a new wiring harness and install it.

The system is a little overwhelming, but I knot its not too complex to figure out. Just hard with 3 kids and a busy life. If I start taking things apart further, I'm going to want better labeling than my current masking tape method. Any suggestions?

[timmy2]

P-Touch label maker.
If you have routing questions for the wires coming out of the fuse panel hit me up.

Schematics for your car:
https://www.dropbox.com/s/yjfoxfvwlci6lys/911_electrical_78SC_USA.pdf?dl=0

[snbush67]

I think that a hot start relay indicates that there were probably earlier electrical issues. Check to see if it’s connected right, and that it isn’t shorting out, check and clean the wires at the starter.

A new fuse block would be a good start, but you could get by with the right bridges for the terminals, and the right ends for the wires, for the current fuse block you have.

You’re probably going to have to use your multimeter to chase down your electrical gremlins. I’ve can’t even pretend to know enough about troubleshooting further to help you with that. Dennis is the expert and he makes wiring harnesses.

[timmy2]

One thing to note about the wiring schematics is the fuses are marked as an S and the numbering starts at the fuse closest to the headlight.

[OsoMoore]

timmy2 - Thanks for the schematic, downloaded.

snbush67 - The hot start relay was put in by myself after the solenoid couldn't close on a very hot day after a long drive. Delivers better current to the solenoid and thus overcomes getting stuck due to temp expansion.

Sounds like time to chase some gremlins. I'm planning to start tracing/checking wires at the top of the fuse block, as it was shorting the battery to ground without blowing a fuse. Which seems to indicate the short isn't after a fuse.

[OsoMoore]

Update!
- Charged my battery and had it tested. It is great and isn't the issue. Might have been running low previously.
- Ordered the fuse box rebuild kit from our host. Arrives Friday.
- Ordered a DC current meter.

I'm planning to reassemble with the previously overheating circuit detached. I'll check for overcurrent with the meter. Should be able to figure out if anything is out of line, and track the problem.

I suspect the stalk's double-beam issue led to the overheating bus wire. The wire deteriorated and its resistance went up, and it got hotter. The battery drained quickly as a result, and thus as soon as I put load on it (lights or starter) it's voltage dived.

EDIT: Linking my old 2014 thread for its original pictures.
Sick Of It! Time to update my SC Fusebox