MS1v3 was working fine on USB, switched to serial and now it won't start/run?
 

UPDATE: ISSUE RESOLVED! - The USB adapter had been installed with excessive solder on the 5v takeoff at the proto area, and apparently when I desoldered it, a glob on the backside dropped off, solidified, and found it's way somewhere onto the circuit board, causing a solder bridge. While double checking things, it dropped off the MS1 and into the palm of my hand. Car starts and runs just fine again. YAY!

Any ideas as to what I could have possibly screwed up?

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1997 NA with Rotrex supercharger
MS1v3, parallel install, dedicated IAT and CLT sensors, Joe's spark mod
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My MS1 has a DLP-TXRX usb adapter. When this was installed by the PO, he removed the U6 chip and half of the socket and soldered the RX and TX lines in place, then drew power and ground from the proto area. It all worked fine.

I recently picked up a serial-to-bluetooth adapter for logging with my phone, so I figured I'd restore the DB9 functionality and when needed use my serial-to-usb cable. I installed a new socket to U6, and temporarily reinstalled the RX and TX data lines into the socket. It all worked fine.

Today my replacement U6 chip arrived, so I removed the DLP-TXRX adapter (unplugged the TX and RX leads from the socket, and desoldered power and ground leads from the proto area). I installed the U6 chip, checked connectivity and it all looks good. MS1 properly connects with TunerStudio on the laptop or ShadowDash on the phone. Went to start the car and it will not start. It turns over, but does not catch.

Since I'm running in parallel, I tried swapping fuel and spark back to OBD2 ECU. The car starts and runs with OBD2 controlling everything. If MS1 is controlling either fuel or spark, it will not start, so it does not seem to be a problem limited to just fuel control or just spark control.

The only other thing that may have changed is software related. In my project properties on TunerStudio, I changed from manifold pressure displayed in PSI to KPA, and then back again. The values in the tune should not have changed though (since I reloaded my converted values directly from the controller rather than sending the new unconverted values).

So it seems like I must have somehow borked the hardware, but I can't think of how or what I might have messed up? Ideas? Or at least any ideas on how to troubleshoot it?