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My 635 CS produced in 1980 (M90 engine) was previously driven in Germany where in 1997 was upgraded with GAT catalyst with lambda probe. Lambda is connected to the "black box" switched to wireing just nereby L-Jetronic control unit. Job was done at BMW's in Germany.

I want to remove the catalyst so I'll be greatful for advises:
- how to remove the black box - just plug it off or connect the cut wires again?
- has L-Jet control unit been also upgraded during catalyst adding?
- shall I replace catalyst with pipe or resonator?

It sounds like some of the strange things that were done to euro spec cars privately imported to the US back in the day. Maybe I only hear about the ones that were a problem, but they seemed to be a problem. What they did was change the fuel injection coolant temp input by means of input from the oxygen sensor. A good idea, but they didn't seem to have original equipment longevity. If you have to just cut wires or splice old ones together depends on how they did it, you'll have to troubleshoot that.

I've checked how the Lambda-Probe "overcontrol" unit (GAT-Tronic ST 25) is switched to the harness of L-Jetronic control unit. "Overcontrol" has 3 wires (WH, BLUE and YE) cut-in to the harness just after L-Jetronic control unit.

Do You have any idea how to wire-off the GAT-Tronic bug to get back to original functionality ? Is it enough to plug off 3 wires of black box and that's is? I'll be greatful for wireing diagrame of L-Jetronic control unit plug. My car is Euro 635 of 1980. Control unit is 0-280-001-114.