Need a little ECU help

Is the plug that you disconnected the plug running to the wiring loom from the cat ecu, or the one from the thermocouple screwed into the cat? All you should need to unplug is the connection running the wiring loom from the cat ecu. Once that is disconnected there isn't any way for the Motronic to get a signal from the cat ecu. So it wouldn't matter if you still had the thermocouple still screwed into the cat. Be sure you didn't disconnect the O2 sensor on the cat.

The diagram below is for a 2.7, which is what your car should be. You only need to unplug the connection on part #69 going to the wiring loom. You can leave #35 plugged into the cat ecu, that's the thermocouple. You must leave #34 plugged in because that is for the O2 sensor and the car MUST have the signal from the O2 sensor to run properly. What you are looking for is the connection on the back of #69 that runs to the loom, and not to the cat.
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This diagram is for the 5.2 Motronic and it has two O2 sensors one in the inlet of the cat, and one at the exit. #'s 35, 72, and 73 all stay plugged in. You are again unplugging #69 from the wiring loom.
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On your '95 you shoud be able to leave everything connected and not have a problem. The motronic needs read values from those ecu's.

Do your test pipes have fittings to mount the temp. probes?

Don't forget, after you reconnect the components, you need to cycle the battery kill switch off & on, to clear the memory in the 2.7 motronic ecu's. When you start the car you need to let it run for approx 10 minutes before turning it off, to establish a baseline in the ecu mapping.

 

Hmmm?

I'm not sure if they made changes to the 355 2.7's but in my 348 the cat ecu signal is a silent code and I get no slow down lights when they are disconnected. Try pulling the codes out of the Motronic, and then clearing them. You should have a code that says you are missing the signal coming from the cat ecu. Maybe once you clear the codes the Motronic will shut off the slow down light?