84 Carrera: Melted wiring at fuse panel on license plate circuit - stumped

[johnnythunderr]

So I've been chasing an issue with my '84 Carrera's headlight circuit - the yellow wire under the dash has been getting hot to the touch when the lights are on. Figured it was time for a headlight relay kit to reduce the load on the switch circuit.

But here's where it gets interesting: pulled the front fuse panel cover to install the relay, and what do I find? Completely bare wire on the license plate lamp circuit, plus melted sheathing on other wires in that same circuit. Not great.

Double-checked the bulbs - they're correct spec (12v 5w). Lamp housings and wiring at the rear are clean, no corrosion or moisture issues. The wire insulation is only melted within an inch or two of the fuse panel itself, rest of the loom looks good from what I can see.

Here's what's puzzling me: this is all isolated by the fuse panel, and both the fuse and wire connections looked solid. What would cause localized heat damage right at the panel like this? Drawing too much current somewhere? Bad connection creating resistance even though it looked tight?

Anyone else dealt with something similar?

(Zip ties in second photo are so that I could group wires before pulling panel to inspect behind it)

[EC900]

Just looking at the fuse terminals in your photo, they look a bit corroded (unless photo isn’t focusing) but they’re even a different color, maybe even the fuse ends too. Resistance increased due to corrosion can cause an overheating situation.
Brass brush or very fine sandpapering the terminals and fuses ends or better yet replace defective fuse(s).

You may have some work ahead of you to get fresh wire ends reconnected.

Maybe somewhere there’s a aftermarket device tied into that circuit (?) increasing the amp draw without blowing fuse, multimeter can check the amperage on that circuit.
Radio or component (aftermarket?) may be wired wrong if Yellow wire (constant power) is heating up.
Many wire a radio to yellow for radio lighting rather than using a switched circuit.

[76FJ55]

that is very interesting. based on the wiring diagram, assuming the power inters the top of the fuses and goes out the bottom of the fuses, that melted portion is unfused. There should only be a single wire connected to the top of fuse #7 of the 8 fuse block. it should be a size 0.5 grey wire. coming from the light switch. the bottom should have 2 wires going out a 0.5 grey to the license plate lights and a 1.0 grey going to the headlight washer switch. I can't tell if you have 2 wires coming out the bottom in the pic, but regardless the insulation on the wire coming out the bottom that is visible looks to be in poor condition.

[gerakroo]

This is a typical problem for the Carrera 3.2 models, and in fact, it’s something everyone should check. The wires are bundled together with waxed lacing cord. Over the years, Porsche started using thinner wires with thinner insulation. In these model years, this often causes an issue at this exact point: the wires are too tightly bundled, cutting into the (very thin) insulation. As a result, they make contact with each other, which leads to a short circuit.
Also, the installed fuses are not correct!

[johnnythunderr]

Originally Posted by gerakroo

Also, the installed fuses are not correct!

Oh man! I never noticed this before.

You're right in that the waxed thread had the wires bundled super tightly together. However, I think more of an issue is the resistance I'm getting across that fuse block. I finally got that section pulled and measured 2.7ohm across that specific circuit. All the surrounding circuits measure 0.3-0.5ohm across the fused contacts.

[wazzz]

The copper strands inside the wire are probably cooked. No good.

[Johnr352]

Apologies for the related thread jack, but to the comments above, is this a reason to move to an updated fuse panel from CR or similar? Is the problem the tightly bundled wires, the fuse block's resistance over time or both, such that replacing the panel with modern blocks and fuses would help reduce this risk?