Voltage discrepancy

So you're saying that replacing the green wire fixed the problem before? The ignition switch most likely isn't you're issue judging from what you've said here in this thread. Like I said, your ecu is being energized.

More often that not, an ignition switch failure is very obvious because most or all circuits it's responsible for supplying power to are effected. A good example of this would be having a working radio while the vehicle is started, but no radio power when the ignition key is turned to the auxiliary position.

Have you had your coil checked? Perhaps you have a bad connection to it or your flywheel position sensor? Any number of things could be creating your intermittent spark condition. My advice would be to continue investigating step-by-step with the help of your workshop manual's troubleshooting guide. It's a far more efficient and cost effective way to fix your car.

vze2jshn;50 wrote: I am troubleshooting my 1978 928 ignition system and have reached the point where I am checking and comparing the voltage at the ignition control unit plug Terminal 15 with the battery voltage. Shop manual trouble shooting says these voltages should be the same. My battery reads 12.03 volts while the ignition cotrol unit plug 15 reads 11.83 volts is this significant enough to warrant futher readings back to the ignition switch to determine where the voltage drop is occurring? Manual says they shoul be the SAME not an acceptable range. Any comments are greatly appreciated. Best, Greg

Well I would say 11.83v is absolutley sufficient to start the car... if its really there...

I say this because its only when you are trying to crank that matters - not just with the ignition on. 12.03v is very low for the battery anyway - I suspect you need to charge it.

20mV is also a big drop on that wire... I'd wonder what that voltage is when cranking - if its below 7-8V you have a problem - maybe there is excess resistance in the circuit.

so:

1. Charge or replace the battery until you get 12.6 or more steady state
2. Check the final stage voltage while cranking

Alan