I finally understood how to read/build the home-made check engine LED switch. Many thanks to JimF, Arthur Dalton, and others who posted. However, I am not able to get any reading from the LED switch. Here is what I did:

Connected pin 1 (-)
Connected pin 3 (+)
Connected pin 19
Turn on ignition
Push switch for 3 sec

The LED light would only turn on/off whenever I push the switch, there are no blinks. What did I do wrong?

The car is a 95 S320 with 38 pin diagnostic connector.

Jack