See if the coils fire when you short the driver transistors from the SDU to the ground in the backbox. This is really easy to do by just connecting the big metal tab at the top of each transistor to the metal in the back box with a peice of wire. If the coil fires then you know you have good connections between the playfield and the SDU. If the coils do not fire, check the harness connector. I had a very similar issue on a SSII where the harness connector between the head and the playfield needed to be repinned. Another thing to check is the 74154 at U16 on the MPU. This is a hex decoder chip that can be tested in cicruit...

Check u16 74154 chip.

sorry i meant u14 74154. do not shotgun replace it before testing it. you can easily test this chip in circuit using the dmm diode function.

I just reread your thread from the beginning. Sorry I came late with the transistor grounding idea. Looks like you already did that. Did you check the switches that fire the coils by doing a switch test yet? I had the same issue on my SSII where I lost the coils. They would fire by grounding the transistors. I started poking around on the MPU like you are and I even replaced u14. Turned out to be the head/pf harness connector for the coil switches. 2 pins deactivated all the coils. I recommend trying a switch test and if the coil switches do not register try the following: remove the switch molex connector from the mpu (j5 i think?). Go into switch test mode and connect the pins on j5 that correspond to the switches that are out. If they register, then the issue is in the harness...

Hey don't fret! The wire colors should not change across the connectors. Some of the connectors are interchangeable too! So make sure you have them hooked up correctly...

hmmm yeah the yellow with black is the 24v coil voltage supply. interesting...

If you can get into the switch test, use this part of the playfield schematic to guide you. For example, jumping pins 1 to 13 on j5 should close the kickout switch. 1 to 10 should close the upper right bumper switch. Etc, etc. If this works then the 74154 is working properly... Before doing all that, just try depressing the bumper skirts in switch test to make sure they close...

Screenshot_20170301-220416 (resized).png

I think the score switch is activated when the pop drops so after the coil fires. For the others I am not sure I would check for you but my SSII is down at the moment...

Here is a SSII manual screenshot. It should ne the same since they both use the MPU2. You need the press the diagnostic button 15 times to get into switch test...

Screenshot_20170301-224202 (resized).png

Yes it sounds like a harness connector that the switches hook to. none of those coils will fire if the switch signals do not get to the mpu.

Look for the wire colors shown going to j5 on the schematic shot above. Those are the same color wires that go to the suspect connector on the playfield harness...

Are you getting 6 flashes on the MPU?

I think we are back to the wire harness. I thought before you said the coil switches scored though? Would be good to narrow the issue down to the board. Can you check continuity between the coil switches and the end of the wire cluster that hooks to MPU board connector J5?