Electrical Short in '82 SC

1982911SCTarga · 09-25-2005, 06:43 PM

I've got a short going on in my 1982 911SC Targa. With the ignition in the on position, there's a brown/red wire (BRN/RED in Porsche/Bentley manual parlance) joined by a plastic covered spade connector that is melting, along with the wire insulation near this connection. This wire is located near the luggage compartment fuse panel and the fuel tank and overflow connections.

Removal of the fuel pump relay does not change this shorting condition. According to the Bentley manual, the only circuits I can identify involving a brown/red wire is the speedometer sensor and RPM limiter circuits. Are there any common failure points in these two circuits or is there something else I'm missing?

For what it's worth, the speedo seems to work fine. Any thoughts on how to remedy this? Thank you.

1982911SCTarga · 09-26-2005, 09:20 AM

As an update: I unplugged the RPM limiter harness at the RPM limiter under the dash and in front of the fuel and oil level gauge. This didn't change the situation. With the ignition key in the on position, the BRN/RED wire at the connection described above started heating up and melting.

Suggestions?

911mot · 911mot's Garage

Is everything switched off inside, i.e hrw,hfw,fans, radio, nothing in cig lighter etc
Can you disconnect it? If so see what stops working when you do. That will narrow your search. Sounds like something has high resistance somewhere

1982911SCTarga · 09-26-2005, 09:39 AM

Yes, everything is off.

Like I said in the first post, the only circuits I see where a BRN/RED wire is used is the speedometer sensor and the RPM limiter.

What gives? What's creating all that resistance and heat through the BRN/RED wire?

john walker's workshop · 09-26-2005, 10:06 AM

unplug it, find which side has 12V, and trace the other side to whatever is shorting to ground. see what doesn't work when it's unplugged for starters.

1982911SCTarga · 09-27-2005, 06:29 PM

Here's the latest on the shorting situation. I disconnected the BRN/RED wire at the junction where the wire was getting hot and the insulation was melting. I drove the car and everything seems fine and the speedometer works, which eliminates the speedo sensor circuit as the source of the shorting problem.

However, I no longer hear the buzz of what I thought was the fuel pump when I switch the ignition from "off" to "on." (Bear in mind, I removed the seatbelt warning relay some time ago to eliminate that separate annoying startup sound). Anyway, everything's totally quiet now as I turn the ignition from "off" to "on" to "start" and release back to "on" after starting the car.

And the car seems to run fine.

However, studying the Bentley manual wiring diagrams, there is a BRN/BLK wire from the airflow sensor contact that T's off into two BRN/RED wires. One branch of the BRN/RED runs to position 85 of the fuel pump relay; the second one goes to the alarm system.

I don't think the alarm system on my car is functional. That is, I've played with setting it and nothing happens, and I presumed the P.O. had it disabled since they tended to be problematic.

Next steps?

OSI930 · 09-27-2005, 08:23 PM

Did you see which side of the connection has 12 volts as John recommended? At the connection you are refering to, is one wire brown/red and the other wire brown/black?

RoninLB · 09-27-2005, 09:38 PM

There are inexpensive "short finder" kits available. It's a nice $50 toy.

Otherwise.
The reasons will have to be found or it should be considered a fire waiting to happen.. imo

1982911SCTarga · 04:33 AM

I'll start probing and tracing the disconnected wires this evening. As noted earlier, the shorting connection is in the front luggage compartment between two BRN/RED wires.

I suspect the airflow sensor contact BRN/BLK wire T's to the two BRN/RED wires back at the engine firewall.

I'm beginning to think that what I'm dealing with is related to the alarm/anti-theft system.

The Bentley page 970-9 shows the T from the airflow sensor to the two BRN/RED wires, with one of them going to the "alarm system."

However, when I flip to page 970-13 to view the schematic for the anti-theft system, there's no reference to a BRN/RED wire. Is "alarm system" on page 970-9 referring to something else?

1982911SCTarga · 10-01-2005, 03:02 PM

OK, I've isolated the segment of BRN/RED wire that's shorting. It runs from the eight-pin plug to the alarm module under the fresh air blower to the connection I mentioned earlier.

Now things get interesting. Neither the alarm plug nor the other two-pin plug is connected to the alarm module. The eight-pin alarm module has been jumpered (not necessarily a bad thing since the alarm system can be problematic).

Anyway the jumpering is as follows: One loop jumps the BRN/RED wire (at the pin that Porsche calls K1 on the alarm module box) to the BLK/RED wire (pin 15). A second loop jumps from BLU/RED (pin 61) to BLU (pin 61). Lord Bentley's diagrams don't show a pin position as K1, nor a BRN/RED wire running to the alarm module.

From cross checking my harness to the Bentley diagrams, it looks to me like the BRN/RED wire takes the place of the BLK wire connecting at what Bentley calls pin 87a, which runs from the alarm module to the fuel pump relay.

Questions for you guys that have BTDT (been there, done that). Is my alarm harness plug correctly jumpered? I mean, is the BRN/RED wire shorting because it's jumpered incorrectly?

Here's some eye candy for your help.

OSI930 · 10-01-2005, 03:44 PM

The only factory alarm disconnect I have read about is jumping 61 to 61 after the alarm module has been disconnected. I would disconnect the brn/red from the blk/ red and do what John Walker suggested and see which one is 12v.

1982911SCTarga · 10-01-2005, 04:20 PM

Agreed. Other threads describe disconnecting the alarm and jumpering as simply one loop connecting the pin 61's. I'm just curious regarding why mine was done this way (two jumpers). The P.O. was an attorney who never worked on the vehicle, so a professional wrench somewhere along the line did the deed.

I'm also curious why I'm dealing with a BRN/RED wire instead of the BLK described in the diagrams. Is that a year-to-year change or something? It would have made this easier to track down.

Next up is the fresh air blower. I've got juice to the blower at the wire connection but the blower doesn't run. How easy are the blower units to take apart and service?

1982911SCTarga · 10-01-2005, 05:50 PM

OK, I did some more checking and the source of 12v is the BLK/RED wire at pin 15 in the alarm harness connector, which then passed on to the BRN/RED wire through the second jumper.

Removing the jumper and leaving the correct one in place (between the two 61 pins) solves the problem. The question remains regarding why the second jumper was there in the first place.