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Working on my new-to-me 1971 BMW 2002 ITB race car, just trying to clean up some messy wiring and do a few other small things to make it more track-worthy.

The PO wired in a 6 pole kill switch, one of those that's designed to route any residual power from the alternator through a resistor to ground when it's switched off. I *thought* it was wired correctly, as turning the switch off did completely shut the car down. Then tonight I was working on installing alternator + oil pressure warning lights on the switch panel inside the car, I noticed that if I switch the elec fuel pump switch off, THEN switch the main power switch off, the car will continue to run!!

These are simply 2 toggle switches mounted inside the car, the main power switch replaces the ignition switch, and the fuel pump switch just turns the elec fuel pump off and on. The elec fuel pump switch is switched by the main power switch.

What happens is, I flip the fuel pump switch off, then flip the main power switch off, the car continues to run. The only way I found to shut the car down after this is to then flip the fuel pump switch ON, and the car shuts off (!!!). It does not matter if I flip the kill switch off, the car continues to run until I flip that fuel pump switch to the ON position.


So, something is backfeeding somehow, but I'm sorta burnt out thinking about it. I'll try to trace out the fuel pump wiring a bit more tomorrow and see if I can figure it out- but feel free to post up if you have any great ideas or thoughts on what to check.


Thanks!

Before I make a fool of myself, please confirm that there are 3 wires on the back side of your fuel pump switch. If so, then there should be an easy fix.
Charlie G

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1998 BMW Z3
1987 BMW 325is
2000 BMW
1996 F250 Turbo Diesel

The fuel pump switch is a simple single pole toggle, the pump itself is grounded about 6" away from the pump with a ground wire to the rear inner fender in the trunk. The pump is just a basic low pressure fuel pump to draw fuel from the fuel cell to the carb. The main power switch (NOT the kill switch) is also a simple single pole toggle, 12v+ comes in one side and then goes out to various things on the other side- including the fuel pump switch. So, the fuel pump switch is somewhat redundant since it won't work unless the main power switch is ON.


I futzed around with it a bit more tonight, but did not really have any revelations on why it's doing what it's doing. It works the way it is- as long as you know the procedure, but something is clearly not quite right.

I did manage to get nice bright oil pressure + alt warning lights installed + wired up properly.

OK, here's my nut ball theory...I've never known of a single pole switch that the center pole made contact with the output in one position, but contacted the housing (ground) in the other direction, but this almost has to be the case (in order to have more than one current path). If so, then I suspect that the grounding resistor is grounded to the same metal part to which the fuel switch is mounted. This way you could switch off the fuel switch which would connect the fuel pump "+" lead to the switch ground. Then switching the master switch would route power to ground through the resistor into the same metal part that holds the fuel switch, thereby providing a power source to the pump. Of course, this also requires that the metal surface used for ground is not particularly well connected to the chassis ground.
All in all this reminds me of my old days as an electronic tech and looking for ground loops in circuits.
P.S. I need a wheel cover plastic wrench for my wife's 528...got any?
Charlie G

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1998 BMW Z3
1987 BMW 325is
2000 BMW
1996 F250 Turbo Diesel

Just another thought to keep in mind, if there is any rotation on the teminals on the long acre or most of the 4 to 6 point kill switches the connection wires inside the switch then seem to cross over some how and the switch is useless. I haven't torn one apart yet to see why but I have had two go bad and a very good engineer tell me this is what is happening.

I have since switched to a 3 point switch and it seems to be a lot tougher and a lot more reliable.

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Marty Howard
2011 NASA SE Factory Five Challenge Champion
Track Events Logistics Coordinator - TZC/THSCC
2007 Factory Five Challenge Car.
http://www.mh-motorsports.com