HELP! Error code #54 and weird behaviour, (after an engine swap) Options

[Hardy]

Hello,
I'm in desperate need for help!

Those who know error code #54 will know how troublesome it can be.
Here's the situation:

JDM ST205 3S-GTE Gen. 3 with a European ST205 ECU integrated into a Corolla E11. All five wires are connected to the pump like Toyota did it. Three wires connected to the other two - one on red, two on black. It seems to be the right combination because in other combinations the problem is much worse. (No pump control at all I think.)

When the engine is started, the ECU can control the pump, but it doesn't simply switch it off after 30 seconds without throttle activity. What happens is this:
- Engine started
- Pump runs for very short time or not at all (0-2 seconds)
- Pump switches off
- Now when the throttle is opened (once or for a longer time, doesn't matter), the pump is activated for 4-5 seconds
- Pump switches off and engine control light is on -> error code #54
- Light goes away after engine is stopped and restarted

By now the mechanic will also have tried to bridge the connector to the water level sensor to ensure that this is not the cause. He used two different pumps (Toyota OEM and a compact one), but that didn't change anything.
I hope my ECU wasn't damaged during testing, but it's said ECUs don't break that easily.

[Hardy]

By the way... Here's a picture - in case you wondered what it might look like.