Do you have pins 20 and 21 connected to the vss in your tranny?If not you will not get a reading on pin 50 .I connected pin 50 to the speedometer wire that came out of my drac box behind the glove box.I think the 1990 has a different setup.

Here is some good write ups that my help you out

http://www.ls1truck.com/forums/lsx-t...-wiring-4.html

http://www.ls1truck.com/forums/lsx-t...gmc-k1500.html

http://www.ls1truck.com/forums/lsx-t...eter-help.html

http://www.ls1truck.com/forums/lsx-t...eedometer.html

http://www.ls1truck.com/forums/lsx-t...eter-help.html