Other "missing" fuses to add to list?

[BarryJB]

On at least 1984 & 1985 cars it seems the dash instrument power is also NOT fused. Even the DME appears to be not fused either?

After a recent smoky exit by the headlight switch, all outstanding fuse/electrical mods described in several threads here - i.e., fusing dash lights and four blowers, and adding headlight relays - were added.

Shortly after this my new dash-lights fuse (3A)... fused. Everything else about the dash was fine, just no illumination.

When I pulled everything out, I found this mess:


Untaping and separating all the melted and bare wires (enlarge to see the gore):


The red/black 12V feed to tach from ignition switch is looped off and through to other instruments (except clock). It had severely melted insulation, & the brown gnd feed, that grounds instruments (including the clock) from the same harness branch (behind tach) was reduced to bare copper.

There were numerous doubled-back folds of both, but especially the gnd wire, inside the pvc tape wrap, and this had the effect of creating more heat in some spots, melting the insulation together of these and other wires touching them inside the wrap. I never smelled anything amiss, which seems hard to believe. It wasn't like this a week earlier (fiddling with the erratic clock).

Both red/blk and gnd wires were good for about 2-3" beyond where they exited the main harness - very lucky.

Anyway, took 3 hours and 7-10 ft of exact sized wire (chunk of old Porsche harness), reconstructing wires and terminals section by section in place. A few other wires needed repair where insulation was damaged.

Reconnected battery, turned on... a miracle, all good, including dash lights. No definitive cause found, but seems likely red/blk & brn shorted together initially at one spot, overheating both wire runs and not helped by all the folded loops of extra wire.

Odd thing was, even with this mess, everything had been working except illumination. Gnd wire had melted into blu/blk illumination wire from dimmer, so that new fuse did do its job perfectly...

BUT... WHY DIDN'T SOME OTHER FUSE BLOW?

1st thought, wrong value fuse... but no fuse listed for instrument power. Bentley for 1984 & 1985 (p970-2 & pp970-75, 970-76), shows NO fuse protecting the ignition-switched 12V feed to tach and other instruments.

So I'm thinking to add a fuse or breaker in to the red/blk wire between where it emerges from the harness and enters the tach. Fuse rating based on wire gauge should do... will look that up.

That there aren't also fuses/breakers ahead of ignition switch after (and near) battery - ahead of the permanently hot runs - seems like another weird oversight... and what about the DME?

Or is there some basis for it NOT being desirable to protect one or more of these?

[BarryJB]

Sort of a bump here to my original (20 Jan) post.

Does anyone else concur there does not seem to be a fuse protecting the wiring for the dashboard instrument power?

Maybe the post appeared to be about the dash instrument illumination circuit, which wasn't the intention! The dash illumination is a well-known candidate for an additional under-dash fuse, along with the various blower fuses and relay additions covered in several excellent threads here.

Looking at the Bentley wiring for '84 & '85, there appears not to be a dedicated—or any—fuse protecting the dashboard instrument power wiring circuit. It looks hot-wired all the way from the battery, albeit via the ignition switch, with nothing in the ground side either. And given I may be mistaken on this too, the DME wiring also appears unprotected.

The damage shown in my original post originated in the long power & ground wires that loop between all the instruments themselves, and power them. As they are long (perhaps to aid manufacturing assembly), they are doubled back on themselves in a few places under the taped wiring harness. Thus their resulting meltdown also melted the insulation of adjacent wires in the harness including the dash lighting circuit, so fortunately, the newly installed lighting fuse did blow, or I would have had an even worse mess (and smoke, flames, etc).

Somehow after the lighting fuse blew, the instruments still kept working, even if "in the dark", so, assuming it was just lighting circuit issue, it seemed reasonable it was something that could wait

I drove around a few days like that... It's most fortunate it did not catch fire before I pulled the instruments, frankly.

[timmy2]

Porsche never fused the instrument wires or ignition switch wires as you have noted.
They fused most circuits after they were energized or switched if they were routed from the fuse panel. The hi/lo lighting switch in the turn stalk is unfused until it gets back to the fuse panel.
This is the main reason for the addition of relays for the headlights.

The usual culprits to melt the dash harness are the clock circuit, interior lighting circuits, (especially the glove box lamp), gauge illumination circuit and the instrument power circuit. Occasionally the main feeds from the battery will melt down in the 6 pin connectors if the short to ground is bad enough.