The fact that you viewed some terminal changing volyage when you went from EC to any other position confirms you have the "comp on" signal at the Klima.
The 87 terminal is probably the terminal that will be hot when the clutch is engaged. If you don't have a bad Klima then you have a bad signal from: engine speed, temp, voltage or throttle position.
Does the tach work?
Bosch Master, ASE Master, L1
33 years MB technician