caspermj

Hi all,

1983 US auto, just installed a stereo, and a day before the CE warning light flashed (on the console, not instrument panel). Did the stereo install, tested, worked fine. Got in the car in the am, got out of the driveway and the interior filled with smoke. Quickly shut it off and took a look. In a nutshell, "some" of the black with blue tracer wires have melted the insulation off them. Unplugged the stereo, still got hot. Pulled the instrument light/cigar lighter fuses, still got hot. Found a melted bk/bl wire on the CE panel, position K, unplugged that connector and no more melting wires I have the Jim M CD and have been searching wiring diagrams but find many difficult or impossible to read the wiring colors. Makes it hard to search out my problem.

Can anyone point me in a direction? Here's another confusing thing for me, wires that were paired together of the same color, only one got hot and melted?! Perhaps the connection was acting like a heat sink? Any tips or hints would be appreciated

Alan

K7 Black/Blue dual wires feed the lamp control unit (footwell) and the seat belt relay in the console with ignition switched power (via ignition switch).

You likely shorted something in the console at the seat belt realy socket - These wires are not fused... They are thin - only 0.5mm^2 so likely whichever one shorted just burned out...?

Only one burned up because only one was shorted - all the current went that way... check the chime relay/socket in the console (chime relay has holes in the end for the sound...)

Alan

GlenL

It's surprising that there are so many unfused wires running around the car. How much current does the car draw? I'd like to put in a master fuse at the jump post. 100A? At least I wouldn't be fixing the back of the CE panel again.

dr bob

I'm afraid that the lighting wiring that the OP reports as melted would not be protected by a 100A fuse at the jump post.

I bought a 60A breaker that has manual trip and reset available, with the idea that I could put it in the feeder from the jump post, mounted above the CE panel someplace. Installing it is on my "someday" list. My thinking is that the jump post feeder carries a percentage of total draw, since the fuel pump, injection, ignition and cooling fan loads are fed directly from the battery positive. Lighting, wipers, and the fresh-air blower would be the major consumers through that feeder. The 60A rating was a SWAG number, based on the apparent ampacity of those feeders. I don't expect it to really "protect" anything unless there's some otherwise fatal short, more it looked like a handy way to isolate the common parasitic loads when the car is stored for extended periods. More on this when it gets closer to the top of my projects list.

Bmw635

Would a fusible link by the + battery give any protection on the accessory wires?

caspermj

The chime relay has melted wires to it. Can't tell which color they were I pulled the relay, still got hot. I will look for the seat belt relay socket. I had also unplugged the lamp control unit and it made no diference. Any idea what I'm running without by having K plug pulled?

As a side note, I've been preparing for my first pca event, a 3 hour tour and meeting some of the members. It's tomorrow morning and then I have this happen. Just my luck, lol! I'd love to go but it's not worth risking a flaming 928S unless I know it'll be ok with the K plug pulled!!!

caspermj

This is what I was thinking during that moment with a cabin full of smoke....why hasn't a fuse popped to save me!

Alan

The Seat Belt Relay is the Chime Relay (same thing).

No - it would just melt everytime you tried to start the car.

Any shorted 0.5mm^2 wire is likely to melt before any 30A fuse will - you'd likely need a 300A-500A fuse for the starter to operate reliably

There are many many unfused items - including in most cases the radio supplies - which given the carnage we have seen from "audio professionals" (they get paid..!) & POs alike, its just surprising there isn't more damage reported...

So a 100A fuse for CE at the Jump Post will protect you against some things - but there are many (most) unfused circuits that will fry well before that fuse would blow. I do actually think Porsche should have fused the CE supplies there at something between 75A & 100A - but it honestly affords only rather limited protection. More importantly they also should have individually fused the direct battery supplies near the battery (for EZK, LH, Injectors, Fuel Pump, ABS brain, Cooling Fans). All mine are fused there because I think what they did is just hideously bad practice. Only the Starter & ABS hydraulic units remain unfused.

Alan

caspermj

So do I need any of the wires that go to the seatbelt/chime relay? Can it be deleted easily, or at all?

Alan

You don't need the chime relay - not sure what else is on plug K...

Alan

dr bob

A fusible link is the same as a fuse with wires attached. Fig Newton's Third Law of Electrical Reciprocity applies perfectly here. The critical wiring, protected by a fuse, inevitably melts first, protecting the fuse. Fuses are sized for circuit protection based on the size of the conductors only. In this case, a 100A fusible link will be intact long after 18ga (equivalent) lighting signal conductors have melted.

Challenges are that the wiring is bundles in harnesses, where heat generated in a faulted circuit is shared with the neighboring wires. Collateral damage to insulation on those wires begets more heat as they short out, and it's a wiring death spiral from there.

Recommendation to the OP: Start by undoing whatever it is you did during the stereo install. A too-common practice is to stab wires blindly with a test light to find one that seems to be hot at the right times. The alarm chime is handily underneath the radio, so it is a common victim of either the stabbing or casual pinched contact with the radio chassis for instance. Radio lighting circuit and an always-on power feed, the aux feed to the rear antenna motor, each a chance to accidentally tap into the wrong wires if the stab-and-grab circuit detection method is used. There's no circuit diagram handy, hence the questions about other circuits through the K harness connector I guess.

Sorry to be so brutal.

dr bob

From what I can see in the WSM wiring diagrams:


K1 is unfused 15 circuit (on with ignition) labeled "to gearbox" (see I 24)
K1 is also shown in an unfused loop circuit from 15 circuit (via A5) for the kickdown relay coil (see "automatic gearbox" options detail at 4)
K2 is fused (21 15A) and feeds the cental locking motors at (see VI 25 & 26) via the power window relay VI (see VII 19)
K5 connects to the Central Warning Unit (see X 3) from the washer fluid levels switch looped from Q6 (see IX 17)
K6 Connects to the Front left brake pad wear sensor (see VII 30)
K7 is unfused 15 circuit labeled "to lamp control unit" bk/bl (The one you found melted) (see V 10)
K7 is unfused 15 circuit to the HVAC control head and the related vacuum solenoid bank forward of the radio, also bk/bl. (see V 11, VIII 19)
K8 is 30 bus unfused (see I 11)

Bmw635

So the 4g cable from + battery is not for starter and the 2-10g wires from battery are not accessories? Iirc, the main + cable from battery goes to starter directly while eveything else feed through fuses by the 10g accessory wires in my 635 and Land Cruiser by fusible links.

I am still learning how to read the Porsche schematic since it's unreadable in print and hard to follow on ipad since I have a lot of shorts. Thx.

Alan

I have no idea what you are talking about.. can we switch back to English instead?

Alan

GlenL

I'll interpret. He's spotted the big (4 gauge) and small (10 gauge) feeds for the electrical system on those cars. Similarly on the 928, there's a big (? gauge) to the starter and then the rest of the car is fed from there.

What the 928 doesn't have is any fusible links and there are wires that are both always hot and unfused. Not clever.

As Dr. Bob points out, there's a problem with the single-point fuse theory: the current needed to feed the car is more than enough to overheat and fry a single wire. Good point. You can't have a fuse big enough to run the car and small enough to prevent damage. But... Would a 100A prevent a fire?

I tell ya one thing I recently did: I found the feed for the rear wiper switch was blown out on the back of the CE panel. That's an always-hot wire. When I re-wired it I ran it through the previously-unused fuse #13. It didn't blow out so I don't know what the deal was, but I'm happy with the change.

On a similar note... The light holder for the cigar had come loose and the hot contact was touching a ground. Popped fuse #4 regularly. I dug around and found that, wrapping with electrical tape and remounted it.

Combine an old car with complex wiring and aftermarket stereo, phone and alarm systems and it's a big challenge to set things right.

Now let's all have a good time.