12v doesnt turn off, 6v draw when "off"

[JohnJL]

This is a completely new, bare bones electrical harness in a track car. I have wiring diagrams for everything, but its not porsche-specific. I wont attempt to put all the diagrams up just yet, I am hoping someone will hear the description of the symptoms and say "Its XXXX."

When turning on the main "kill switch" the ignition switch performs as normal. 12v is fed to the proper pins on the switch, and I see switched 12v at the ecu fuse panel (back seat area) and 12v at the engine compartment fuse/terminal block.

When I turn the ignition switch back "off" the ecu terminal block continues to show 6v and the LED on the MSD boxes stays lit. THe engine compartment block still gets 12v on the switched power supply.

When I turn off the main kill switch, completely disconnecting the battery, all the power to all fuse blocks is indeed off. THe MSD LEDs go off too.

WHen I turn on the main kill switch, everything behaves as normal. Switched 12v feeds are all off until I turn on the ignition switch and as normal all the right feeds get 12v. Also the Megasquirt also flashes its LEDs when initially powering on.

But again when I turn the ignition switch off, that ECU and MSD see 6v and the rear switched feed stills sees 12v.

I thought maybe a relay was sticking and not "unlatching" when its switched feed was turned off, but changing them all over didnt solve the problem.


I have done a few of these, but the first time I see this problem before. Anyone have an idea?

THanks!

[djpateman]

That is not a good thing. One thought is that the 6V indicates that something is wrong with the ignition switch or the wiring. It seems like power is finding an alternate path such that when the ignition switch is off the terminal block is in the middle of the path rather than direct to the battery.

[JohnJL]

Found it!

Took a deep breath, opened an adult soda, and got busy with the multimeter. It was indeed a relay sticking open, but because there was a short/feedback through a pair of switched pins on the idle valve. GRadually disconnecting components and circuits eliminated it.

I am using a stock Carrera idle valve, the pins arent labelled so I took a guess (and referenced someone else on the interweb with reportedly the same valve?) and connected switched 12v, FIdle signal to Megasquirt and ground (through a resistor.)

Switching them around a few times got me the combination that solves the ECU, Megasquirt, and ignition issues.

Electricity is hard.

[djpateman]

I'm very glad you found your alternate path. Electricity is insidious, and will take every path that it can create, many of them not paths that we want it to take. The multimeter is an essential tool.