electrical problem, lights knob turns on fuel pump

[rentadate]

I have a 1975 911s Targa with a 83SC engine in it. My problem is the fuel pump turns on with the light knob on the dashboard.

I can turn the car on and off like normal with the key. But when the lights are on the car somehow manages to idle for a while longer when I pull the key out and leave the lights on.

Not sure what is going on here. I'm thinking maybe a cross connection in the dashboard.

I recently had to "ghetto" fix the rear taillight section, because I couldn't get the damn wiring from the new taillight to come out so i can insert the boot with the light off...I ghetto fixed it by splicing the wires to where they are supposed to go. the only problem with me doing that is I managed to switch the reverse light with the light for the one that goes to the license plate. Easy fix and not too hard. I fail to see why that would activate the fuel pump.

Also I finished installing a new Pioneer CD deck into the dashboard. Maybe I managed to cross something when I wasn't supposed to? I followed the direction on the thing to the tooth, and even ran a new wire to the battery so that the stereo wouldn't lose it's memory.

When I have the car off and I turn on the lights the fuel pump starts to run, and I don't know why. The wires look fine under the dashboard. Maybe I had this problem when I bought the car and never noticed. I'm still leaning towards the fact that the car developed this problem in the last couple of days.

I don't really know what is going on with the vehicle. Any help and or advice would be really helpful.

Thanks,

-Nick

[buster73]

Fuel pump should only get power when ignition/starter switch is switched on. Check if the supply wire is connected to the correct fuse (Green/red wire on Fuse 12 for fuel pump). Light switch would be Fuse 17).

[Rex Walter]

The headlight switch has two large red wires going to it for power. One is used for all the marker lights, turn signals, dash lights etc. The other red wire is used for the headlights. Fuse 6 and 8 should control the low and high beam (yellow and white wires respetcively). It's possible that the PO spliced the fuel pump into the red headlight wire going to the headlight switch from the ignition switch. Stick your head under the dash and see if this is true.

Rex

[zweistein]

Hello Nick,

I have exaclty your problem on my 1972 911s :

With ignition key OFF:

Turning the light knob on will switch on the fuel pump (and also the ignition and instruments)

Also: Indicator left or right will also turn on fuel pump and (normal indicators) and not parking light left/right.

Also: with lights on engine will keep running with ignition switch turned off. (many people investigate the ignition switch , but it is not the problem)


I investigated ignition switch andf light knob and my radio wiring, but all correct.

Finally I found the solution, very simple:

12 V were fed into the system via the brake light switch.

For this to happen there must be two conditions:
1. brake light switch must be in switch postition. (This happens erroneously when the lever has slipped off the washer)

2. In the tail light (left or right) there is a wrong bulb (e.g. single filament instead of 5/21W double filament)) that makes contact between the brake light and position light 12V wires. A single filament bulb will do this.

Solution: replace wrong bulb to the 5/21W one or fix the interconnection if there is one and adjust brake switch.


I hope that helps