MPI hum followed by loss of ECU signal (troubleshoot my work, possible ECU issue without the telltale signs)

[Wanted01]

Hello group. I don't post much here but long time member since early 2000 (my old account disappeared a few years ago most likely to inactivity)

Working on a 92 auto Talon (6 bolt) with ECMLink. The car was garage a couple of years ago while I was in Korea. When parked, the car ran. At the time it was on a evo8 MAF, 1000cc low impedance injectors and the stock alternator.

I decided I wanted to finally go SD and flex fuel. I added the ECMTuning plug and play harness to the stock MAF harness, swapped to 1200 low impedance injectors, and tried swapping to a Saturn alternator.

The later proved a mistake as I wired it wrong and the alternator wires got hot around the splice area. I've since went back to the stock alternator and repaired the wires.

I'm wondering if I hurt the ECU as when I'm trying to get the car to start and idle, after a few attempts I experience the MPI relay humming followed be a loss of ECU signal and the fuel pump not activating.

I swapped out the MPI relay twice with known good ones. Same pattern.

I swapped my battery. The mis-wired Saturn alternator drained the old one and the battery wasn't taking a charge. Same issue.

I've checked my anti-theft fuse, not popped.

Checked the 80 amp alternator fuse. Noted popped but replaced it anyway. Same.

Checked my engine grounds and starter wires. Cleaned them all, checked and ensured the grounding points were good. Redid 1 that was questionable. Same issue.

Verified ECU ground signals on pins 101, 106 and 104 were good. (104 is kinda moot as I bypassed the neutral safety switch some time ago). Same issue.

Key on engine off, ground pins 63 and 66 individually. The MPI relay clicks but the fuel pump does not kick on. Maybe fuel pump died, but that won't cause the hum or loss of ECU signal.

Inspected the ECU. No sign of capacitor caps leaking, burns or smell of any kind.

I'm stumped.

Frustrating this is the fact that I'm trying to dial in the ECU for startup while troubleshooting an electrical gremlin.

From what I've done so far, I'm almost thinking something is starting to fail in the ECU, but it doesn't physically look like it upon inspection.

What did I miss? What else could I try short of replacing my entire wiring harnesses? I'd like to get this car back on the road again as it's actually a fun car to drive.

Thanks in advance for your time and brains.

 

[steve]

So ECU pins 63 and 66 both go to activating the power side of the MPI relay. If you trying to activate the fuel pump side you need to ground ECU pin 56.

Can you get a log of this humming happening. I get that when it does you're likely to loose the connection.
What happens right after the ECU turns off? What happens if you switch the ignition off and then back on?

The relay doesn't usually make noise unless it can't hold the contacts in. So the coil voltage in the relay either isn't high enough on the battery side, low enough on the ground side (ECU driver isn't pulling it low enough) or the current is fluctuating causing it to sing.

ECU pictures and ECMLink log files can tell us more.