wrote:

www.ws6transam.org/ALDL.html

www.ws6transam.org/aldl_28.html

I should add: when my car is tested, it is connected to the test
equipment in three ways.

1) The rear (driving) tires are on the treadmill, providing speed
information.
2) An exhaust gas analyzer probe is inserted into the tail pipe,
providing exhaust gas information.
3) An inductive pickup is clipped over one of the spark plug wires,
providing engine rpm information.

There is no connection made between the ALDL port and the test
equipment.