During the process of trying to get the starter motor out of our 1991 Volvo 240 Wagon, we managed to break the seal that attached, to the top of the transmission, an electrical wiring harness coming from the gearshift lever.

Today, after putting everything back together except for that wiring harness, we found out what that wiring harness does.  At highway speeds (e.g., 60 mph or above) the car is in high gear with, presumably, the torque converter locked up.  A button on the side of the shift lever would, when pressed, make an orange arrow appear in the instrument cluster, and cause the engine rpm to increase as if the torque converter has stopped locking up (which is probably the entire purpose of the button).

Now that the cable is not longer attached to the transmission, the button just lights up the arrow. It doesn’t cause the engine rpm to increase. Probably it no longer telling the torque converter “don’t lock up.”

Aside from that, the transmission seems to be working perfectly.